BIOMOLECULES AND ENZYMES

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DISCLAIMER

This educational material is prepared for Biology learning and NEET-UG preparation. Questions are intended for practice and revision. The year and examination name should be included only when the question’s appearance in that examination can be established from a reliable source. Some questions may be adapted from previous-year question patterns and should not be treated as verbatim official questions unless verified against the original paper. The author and website are not responsible for discrepancies in unofficial question compilations. Refer to the official NTA/NEET question papers and answer keys for confirmation.

Part 1: Complete Theory

1. INTRODUCTION TO BIOMOLECULES

Biomolecules are chemical compounds found in living organisms. They form cellular structures, store and release energy, regulate metabolism and carry genetic information.

The major biomolecules include carbohydrates, proteins, lipids, nucleic acids, and several smaller molecules such as amino acids, nucleotides, vitamins and metabolic intermediates.

1.1 CHEMICAL COMPOSITION OF LIVING ORGANISMS

Living organisms contain both inorganic and organic substances.

Inorganic constituents: Water, mineral salts and dissolved ions.

Organic constituents: Carbohydrates, proteins, lipids, nucleic acids and other carbon-containing compounds.

Water is generally the most abundant chemical component of cells. Organic compounds are essential for cell structure, energy storage, catalysis and heredity.

1.2 ANALYSIS OF CHEMICAL CONSTITUENTS

Biologists study cellular composition by analysing living tissues and their chemical components.

1. Elemental analysis: Determines the elements present in biological material, such as carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur.
2. Compound analysis: Identifies individual compounds such as glucose, amino acids, fatty acids and nucleotides.
3. Acid-soluble fraction: Generally contains small molecules, including sugars, amino acids, nucleotides and organic acids.
4. Acid-insoluble fraction: Generally contains macromolecules and cellular structures, including proteins, nucleic acids, polysaccharides and membrane-associated lipids.

NEET concept: Lipids have relatively low molecular masses compared with many macromolecules, but they are commonly recovered in the acid-insoluble fraction because they occur largely in membranes and other aggregates.

2. CARBOHYDRATES

Carbohydrates are organic compounds composed mainly of carbon, hydrogen and oxygen. They are important energy sources and structural materials.

2.1 Classification of carbohydrates

Monosaccharides

The simplest carbohydrates; they cannot be hydrolysed into smaller carbohydrate units.

Examples: glucose, fructose, ribose and deoxyribose.

Oligosaccharides

Contain a small number of monosaccharide units. Disaccharides are particularly important.

Examples: sucrose, maltose and lactose.

Polysaccharides

Long chains of monosaccharide units that serve storage or structural functions.

Examples: starch, glycogen, cellulose and chitin.

2.2 IMPORTANT MONOSACCHARIDES

Monosaccharide	Importance
Glucose	Major cellular respiratory substrate
Fructose	Common fruit sugar
Ribose	Component of RNA and ATP
Deoxyribose	Component of DNA
Galactose	Component of lactose

Monosaccharides can be classified by the number of carbon atoms. For example, glucose is a hexose, while ribose is a pentose.

2.3 DISACCHARIDES

Disaccharides consist of two monosaccharides joined by a glycosidic bond.

Disaccharide	Components
Sucrose	Glucose + fructose
Maltose	Glucose + glucose
Lactose	Glucose + galactose

Sucrose is a non-reducing sugar, whereas maltose and lactose are reducing sugars.

2.4 POLYSACCHARIDES

Starch: The principal storage carbohydrate in plants. It contains amylose and amylopectin.

Glycogen: The principal storage polysaccharide in animals; it is highly branched and is also present in fungi.

Cellulose: A structural polysaccharide of plant cell walls, composed of β-glucose units.

Chitin: A structural polysaccharide found in arthropod exoskeletons and fungal cell walls.

NEET distinction: Humans can digest starch but lack the enzyme cellulase needed to digest cellulose.

3. LIPIDS

Lipids are a diverse group of hydrophobic or amphipathic compounds. They are important for energy storage, membrane formation, insulation and cell signalling.

3.1 TYPES OF LIPIDS

Fats and oils: Composed mainly of glycerol and fatty acids; serve as energy reserves.

Phospholipids: Major components of biological membranes, with hydrophilic heads and hydrophobic tails.

Waxes: Provide protective, water-resistant coatings.

Steroids: Include cholesterol and several steroid hormones.

3.2 FATTY ACIDS

Fatty acids contain a carboxyl group and a hydrocarbon chain.

Saturated fatty acids: Contain no carbon–carbon double bonds in the hydrocarbon chain.

Unsaturated fatty acids: Contain one or more carbon–carbon double bonds.

Triglycerides contain one glycerol molecule linked to three fatty acids by ester bonds. Unlike proteins and nucleic acids, most lipids are not true polymers composed of repeating monomer units.

NEET focus: Phospholipids are amphipathic, whereas triglycerides are primarily energy-storage molecules.

4. PROTEINS

Proteins are biological macromolecules composed of amino acids joined by peptide bonds. They perform structural, catalytic, transport, regulatory, defensive and contractile functions.

4.1 AMINO ACIDS

A typical amino acid contains:

An amino group (–NH₂)

A carboxyl group (–COOH)

A hydrogen atom

A variable side chain (R group)

These groups are attached to the alpha carbon in most protein-forming amino acids.

There are 20 standard amino acids commonly used to build proteins. Their different R groups determine their chemical properties.

4.2 PEPTIDE BOND

A peptide bond forms between the carboxyl group of one amino acid and the amino group of another, with the elimination of water in the conventional condensation representation.

Two amino acids form a dipeptide; additional amino acids form longer peptides and polypeptides.

4.3 LEVELS OF PROTEIN STRUCTURE

Primary structure

The specific sequence of amino acids in a polypeptide chain.

Secondary structure

Local folding, especially the alpha helix and beta-pleated sheet, stabilised mainly by hydrogen bonds between backbone groups.

Tertiary structure

The overall three-dimensional folding of a single polypeptide chain.

Quaternary structure

The arrangement of multiple polypeptide subunits in a functional protein, as in haemoglobin.

4.4 FIBROUS AND GLOBULAR PROTEINS

Feature	Fibrous proteins	Globular proteins
General shape	Long and fibre-like	Compact and rounded
Solubility	Often poorly soluble	Often water-soluble
Main role	Structural support	Diverse functional roles
Examples	Collagen, keratin	Haemoglobin, many enzymes

4.5 DENATURATION OF PROTEINS

Denaturation is the disruption of a protein’s native conformation by factors such as heat, extreme pH or certain chemicals.

It commonly affects secondary, tertiary and quaternary structures without breaking all the peptide bonds of the primary structure.

Example: Heating egg white causes its proteins to denature and coagulate.

5. NUCLEIC ACIDS

Nucleic acids store, transmit and help express genetic information. The two principal types are DNA and RNA.

5.1 NUCLEOSIDES AND NUCLEOTIDES

Nitrogenous base + pentose sugar = nucleoside.

Nucleoside + phosphate group(s) = nucleotide.

The nitrogenous bases are classified into two groups:

Purines: Adenine (A) and guanine (G).

Pyrimidines: Cytosine (C), thymine (T) and uracil (U).

5.2 DNA AND RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, G, C, T	A, G, C, U
Usual structure	Double-stranded	Often single-stranded
Major role	Genetic information storage	Gene expression and other cellular functions

DNA nucleotides are connected through 3′–5′ phosphodiester bonds. In double-stranded DNA, adenine pairs with thymine and guanine pairs with cytosine.

RNA includes messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA), among other functional types.

5.3 ATP

Adenosine triphosphate (ATP) consists of adenine, ribose and three phosphate groups. It is a major energy-transfer molecule in cells.

ATP supports biosynthesis, active transport, muscle contraction and numerous other cellular processes.

6. OTHER IMPORTANT BIOMOLECULES

Vitamins

Vitamins are organic compounds required in small quantities for normal physiological functions.

Vitamin	General importance
A	Vision and epithelial health
B₁	Carbohydrate metabolism
B₉	DNA synthesis and cell division
B₁₂	Red blood cell formation and nervous system function
C	Collagen synthesis and antioxidant functions
D	Calcium and phosphate homeostasis
E	Antioxidant protection
K	Blood clotting

MINERALS AND COFACTORS

Mineral ions such as magnesium, iron, zinc and calcium participate in biological processes. Some enzymes require non-protein components called cofactors.

Cofactor: A non-protein component required for the activity of certain enzymes.

Coenzyme: An organic cofactor, often derived from vitamins.

Prosthetic group: A cofactor that is tightly bound to its associated protein.

Apoenzyme: The protein portion of an enzyme without its required cofactor.

Holoenzyme: The complete, active enzyme system consisting of the apoenzyme and its required cofactor.

7. ENZYMES

Enzymes are biological catalysts that increase the rate of chemical reactions by lowering the activation energy. Most enzymes are proteins, although some RNA molecules, called ribozymes, also catalyse reactions.

Enzymes accelerate reactions without being consumed in the overall process. They do not change the overall free-energy difference between reactants and products or the equilibrium constant.

7.1 PROPERTIES OF ENZYMES

1. Enzymes are highly efficient biological catalysts.
2. They often show specificity for their substrates and reactions.
3. Their activity depends on conditions such as temperature and pH.
4. Many require metal ions or organic cofactors.
5. They can be regulated by inhibitors, activators and cellular conditions.
6. They do not change the final equilibrium position of a reversible reaction.

7.2 ENZYME–SUBSTRATE COMPLEX

An enzyme binds to its substrate at a specific region called the active site.

The reaction takes place in the following steps:

E + S ⇌ ES → E + P

Where:

E = Enzyme

S = Substrate

ES = Enzyme-substrate complex

P = Product

At the end of the reaction, the product is released, and the enzyme remains unchanged. It can be used again for another reaction.

7.3 LOCK-AND-KEY AND INDUCED-FIT MODELS

Lock-and-key model: The active site is described as complementary to the substrate.

Induced-fit model: Substrate binding promotes a change in enzyme conformation that helps achieve an effective catalytic arrangement.

The induced-fit model better captures the flexibility of many enzyme active sites.

7.4 FACTORS AFFECTING ENZYME ACTIVITY

Temperature

Activity generally increases with temperature up to an optimum. Beyond this range, many enzymes lose activity because their structures are disrupted.

pH

Each enzyme has a suitable pH range. Changes in pH can alter the ionisation of active-site residues and the enzyme’s structure.

Substrate concentration

When the substrate concentration is low, increasing the substrate concentration usually increases the reaction rate.

When the substrate concentration becomes high, most of the enzyme active sites become occupied. The enzyme reaches saturation, and the reaction rate approaches its maximum value, called Vmax.

7.5 MICHAELIS–MENTEN EQUATION

For a simple enzyme-catalysed reaction, the reaction rate can be calculated using the Michaelis-Menten equation:

v = (Vmax x [S]) / (Km + [S])

Where:

v = Initial reaction rate

Vmax = Maximum reaction rate under specific conditions

[S] = Substrate concentration

Km = Michaelis constant

Important points:

1. When the substrate concentration ([S]) is equal to Km, the reaction rate (v) is half of Vmax.
2. A lower Km value can indicate greater apparent affinity between an enzyme and its substrate under the simple Michaelis-Menten model.
3. Km is not always a direct measure of binding affinity because it depends on the rates of the enzyme-catalysed reaction.

Top of Form

Bottom of Form

7.6 ENZYME INHIBITION

Types of Enzyme Inhibition

1. Competitive Inhibition: The inhibitor competes with the substrate for the active site of the enzyme. In this type of inhibition, the apparent Km value increases, but Vmax remains unchanged. Increasing the substrate concentration can reduce the effect of competitive inhibition.

2. Pure Non-Competitive Inhibition: The inhibitor reduces the effective activity of the enzyme. In this type of inhibition, Vmax decreases, but Km remains unchanged.

3. Uncompetitive Inhibition: The inhibitor binds only to the enzyme-substrate complex. In this type of inhibition, both the apparent Km and Vmax values decrease.

7.7 CLASSIFICATION OF ENZYMES

The traditional NCERT/NEET presentation commonly uses six major classes:

Class	Main reaction type	Example
Oxidoreductases	Oxidation–reduction	Dehydrogenases
Transferases	Transfer of functional groups	Transaminases
Hydrolases	Bond cleavage using water	Lipases
Lyases	Addition or removal of groups without hydrolysis or oxidation	Decarboxylases
Isomerases	Intramolecular rearrangements	Phosphoglucose isomerase
Ligases	Joining molecules, often coupled to ATP hydrolysis	DNA ligase

Modern enzyme nomenclature also recognises a seventh class, translocases, which catalyse the movement of ions or molecules across membranes or their separation within membranes. For NEET questions, follow the classification specified in the relevant NCERT edition or examination syllabus.

7.8 ENZYMES IN DIGESTION

Enzyme	Substrate	Major product(s)
Salivary amylase	Starch	Smaller carbohydrates
Pepsin	Proteins	Smaller peptides
Trypsin	Proteins and peptides	Smaller peptides
Pancreatic lipase	Triglycerides	Fatty acids and monoacylglycerols, with further products depending on conditions
Maltase	Maltose	Glucose
Lactase	Lactose	Glucose and galactose

Important distinction: Enzymes are not limited to digestion. They also function in DNA replication, transcription, respiration, photosynthesis, cell signalling and metabolic regulation.

PART 2: NEET PREVIOUS-YEAR QUESTIONS (PYQS) WITH ANSWERS AND EXPLANATIONS

Question 1. Cofactor of carboxypeptidase

NEET-UG 2024

The cofactor required by the enzyme carboxypeptidase is:

A. Flavin

B. Haem

C. Zinc

D. Niacin

Correct answer: C. Zinc

Explanation: Carboxypeptidase is a zinc-containing metalloenzyme. Zinc ions help the enzyme catalyse the hydrolysis of peptide bonds. Flavin and haem participate in other biological processes, while niacin is a vitamin that contributes to coenzyme formation.

Repeated-question note: The prosthetic-group/cofactor concept has also appeared in NEET-UG 2019 and NEET-UG 2025, although the exact wording and enzyme

Question 2. Lecithin

NEET-UG 2024

Lecithin, an organic compound found in living tissues, is an example of:

A. Amino acids

B. Phospholipids

C. Glycerides

D. Carbohydrates

Correct answer: B. Phospholipids

Explanation: Lecithin is a phospholipid and an important component of biological membranes. Its hydrophilic and hydrophobic regions help it participate in membrane structure.

Question 3. Enzyme inhibition

NEET-UG 2024

Inhibition of succinate dehydrogenase by malonate is a classical example of:

A. Enzyme activation

B. Competitive inhibition

C. Cofactor inhibition

D. Feedback inhibition

Correct answer: B. Competitive inhibition

Explanation: Malonate resembles succinate structurally and competes for the enzyme’s active site. It inhibits the conversion of succinate to fumarate. Increasing substrate concentration can reduce the effect of a reversible competitive inhibitor.

Question 4. Protein, hormone and transporter matching

NEET-UG 2024

Match List I with List II.

List I	List II
A. GLUT-4	I. Hormone
B. Insulin	II. Enzyme
C. Trypsin	III. Intercellular ground substance
D. Collagen	IV. Enables glucose transport into cells

Choose the correct sequence.

A. A–IV, B–I, C–II, D–III

B. A–I, B–II, C–III, D–IV

C. A–II, B–III, C–IV, D–I

D. A–III, B–IV, C–I, D–II

Correct answer: A. A–IV, B–I, C–II, D–III

Explanation:

GLUT-4 transports glucose across cell membranes.

Insulin is a hormone that regulates blood glucose.

Trypsin is a protein-digesting enzyme.

Collagen is a structural protein found in connective tissue.

Question 5. Bonds hydrolysed by enzymes

NEET-UG 2024

Match the enzymes with the bonds they act upon.

List I	List II
A. Lipase	I. Peptide bond
B. Nuclease	II. Ester bond
C. Protease	III. Glycosidic bond
D. Amylase	IV. Phosphodiester bond

A. A–IV, B–II, C–III, D–I

B. A–III, B–II, C–I, D–IV

C. A–II, B–IV, C–I, D–III

D. A–IV, B–I, C–III, D–II

Correct answer: C. A–II, B–IV, C–I, D–III

Explanation: Lipases hydrolyse ester bonds in lipids, nucleases cleave nucleic acids by breaking phosphodiester linkages, proteases hydrolyse peptide bonds, and amylases act on glycosidic bonds in starch.

Question 6. Effect of substrate concentration

NEET-UG 2024 Re-Examination

Which graph represents the effect of substrate concentration on the velocity of an enzyme-catalysed reaction?

A. A curve that increases and then approaches a plateau

B. A straight line that decreases continuously

C. A curve that remains at zero at every concentration

D. A curve that decreases continuously as substrate concentration increases

Correct answer: A. A curve that increases and then approaches a plateau

Explanation: When the substrate concentration is low, increasing the substrate concentration increases the reaction rate because more substrate molecules bind to the available enzymes.

When the substrate concentration becomes high, most enzyme active sites become occupied. The enzymes become saturated, and the reaction rate approaches its maximum value, called Vmax.

Question 7. Enzyme class

NEET-UG 2024 Re-Examination

Enzymes that remove groups from substrates by mechanisms other than hydrolysis, often leaving a double bond, belong to which class?

A. Hydrolases

B. Lyases

C. Ligases

D. Isomerases

Correct answer: B. Lyases

Explanation: Lyases catalyse the removal or addition of groups without the usual mechanisms of hydrolysis or oxidation. Removal of a group can result in the formation of a double bond.

Question 8. Protein structure

NEET-UG 2024 Re-Examination

Match the levels of protein structure with the appropriate descriptions.

List I	List II
A. Primary structure	I. Human haemoglobin
B. Secondary structure	II. Disulphide bonds
C. Tertiary structure	III. Polypeptide chain sequence
D. Quaternary structure	IV. Alpha helix and beta sheet

A. A–III, B–IV, C–II, D–I

B. A–II, B–III, C–I, D–IV

C. A–IV, B–I, C–III, D–II

D. A–I, B–II, C–IV, D–III

Correct answer: A. A–III, B–IV, C–II, D–I

Explanation: Primary structure is the amino acid sequence. Secondary structure includes alpha helices and beta sheets. Disulphide bonds can stabilise tertiary structure, and haemoglobin is a familiar example of a protein with quaternary structure.

Question 9. Fatty acids

NEET-UG 2024 Re-Examination

Which of the following are not fatty acids?

A. Glutamic acid

B. Arachidonic acid

C. Palmitic acid

D. Lecithin E. Aspartic acid

Choose the correct combination.

A. A and E only

B. B and C only

C. A, D and E only

D. B, C and D only

Correct answer: C. A, D and E only

Explanation: Glutamic acid and aspartic acid are amino acids. Lecithin is a phospholipid, not a fatty acid. Arachidonic acid and palmitic acid are fatty acids.

Question 10. Repeated concept: enzyme prosthetic groups

NEET-UG 2019

Consider the following statements about enzyme prosthetic groups:

1. A coenzyme or metal ion tightly bound to an enzyme protein may act as a prosthetic group.
2. Every enzyme functions independently of cofactors.

Which statement is correct?

A. Statement 1 only

B. Statement 2 only

C. Both statements

D. Neither statement

Correct answer: A. Statement 1 only

Explanation: A prosthetic group is a tightly bound non-protein component of an enzyme. Some enzymes require cofactors for catalytic activity, although not all enzymes need an additional cofactor. Therefore, the second statement is incorrect.

Repeated-question note: The cofactor/prosthetic-group concept was tested again in NEET-UG 2024 and NEET-UG 2025, with a related question on the haem prosthetic group in 2025. These are related concepts, not necessarily identical questions.

PART 3: ADDITIONAL NEET-LEVEL PRACTICE MCQS

Question 11. Reducing sugars

Which of the following is a non-reducing sugar?

A. Glucose

B. Maltose

C. Lactose

D. Sucrose

Answer: D. Sucrose

Explanation: In sucrose, the anomeric carbons of glucose and fructose are involved in the glycosidic bond. It therefore lacks a free anomeric carbon for the usual reducing-sugar reaction.

Question 12. Storage carbohydrate in animals

The principal storage polysaccharide in animals is:

A. Cellulose

B. Starch

C. Glycogen

D. Chitin

Answer: C. Glycogen

Explanation: Glycogen is a highly branched glucose polymer stored especially in liver and skeletal muscle.

Question 13. Bond between amino acids

Amino acids in a polypeptide are linked by:

A. Glycosidic bonds

B. Peptide bonds

C. Phosphodiester bonds

D. Hydrogen bonds only

Answer: B. Peptide bonds

Explanation: A peptide bond forms between the carboxyl group of one amino acid and the amino group of another.

Question 14. RNA sugar

Which sugar is present in RNA?

A. Deoxyribose

B. Ribose

C. Glucose

D. Fructose

Answer: B. Ribose

Explanation: RNA contains ribose, whereas DNA contains deoxyribose.

QUESTION 15. PURINE BASES

Which pair consists entirely of purine bases?

A. Adenine and guanine

B. Cytosine and thymine

C. Thymine and uracil

D. Cytosine and adenine

Answer: A. Adenine and guanine

Explanation: Adenine and guanine are purines. Cytosine, thymine and uracil are pyrimidines.

Question 16. Enzyme specificity

The region of an enzyme that binds its substrate and participates directly in catalysis is the:

A. Peptide backbone

B. Active site

C. Phosphate group

D. Cell membrane

Answer: B. Active site

Explanation: The active site contains residues that help recognise the substrate and facilitate the reaction.

Question 17. Competitive inhibition

In simple reversible competitive inhibition, which parameter remains unchanged?

A. Apparent Km

B. Vmax

C. Substrate affinity under all conditions

D. Enzyme active-site sequence

Correct Answer: B. Vmax

Explanation: In competitive inhibition, the inhibitor competes with the substrate for the enzyme’s active site. The apparent Km increases, but Vmax remains unchanged. The maximum reaction rate can still be reached by increasing the substrate concentration sufficiently.

Question 18. Biological catalysts

Which statement about enzymes is correct?

A. They change the equilibrium constant of a reaction.

B. They are consumed permanently during each reaction.

C. They lower the activation energy of a reaction.

D. They make every endergonic reaction occur without energy input.

Answer: C. They lower the activation energy of a reaction.

Explanation: Enzymes accelerate reactions by providing a lower-activation-energy pathway. They do not alter the equilibrium constant or remove the need for energy coupling in energetically unfavourable reactions.

Question 19. Cofactors

The protein portion of a conjugated enzyme without its required cofactor is called the:

A. Holoenzyme

B. Apoenzyme

C. Substrate

D. Product

Answer: B. Apoenzyme

Explanation: The apoenzyme is the protein component. When combined with the required cofactor, it forms the complete holoenzyme.

Question 20. Cellulose

Cellulose is composed primarily of:

A. Alpha-glucose units joined by alpha linkages

B. Beta-glucose units joined by beta-1,4 linkages

C. Fructose units joined by peptide bonds

D. Amino acids joined by glycosidic bonds

Answer: B. Beta-glucose units joined by beta-1,4 linkages

Explanation: Cellulose forms long, unbranched chains that provide structural strength to plant cell walls.

PART 4: IMPORTANT REVISION POINTS FOR NEET

Sucrose is non-reducing; maltose and lactose are reducing sugars.

Starch is a major storage polysaccharide in plants; glycogen is the principal storage polysaccharide in animals.

Cellulose contains beta-1,4-linked glucose units.

Peptide bonds link amino acids; glycosidic bonds link sugar units; phosphodiester bonds link nucleotides in nucleic acids.

DNA contains thymine, while RNA contains uracil.

Adenine and guanine are purines.

Carboxypeptidase requires zinc as a cofactor.

Malonate competitively inhibits succinate dehydrogenase.

In competitive inhibition, the apparent Km value increases, but Vmax remains unchanged. This occurs because the inhibitor competes with the substrate for the enzyme’s active site. Increasing the substrate concentration can overcome the effect of the inhibitor.

Enzymes lower activation energy but do not change reaction equilibrium.

Apoenzyme plus its required cofactor forms the holoenzyme.

Lyases catalyse certain eliminations or additions without hydrolysis or oxidation.

Protein primary structure is its amino acid sequence; quaternary structure concerns multiple polypeptide subunits.

Lipids are not generally true polymers of repeating monomers.

SECTION A: BIOMOLECULES — PREVIOUS-YEAR QUESTION REVIEW

Question 21. Protein portion of an enzyme

NEET-UG 2025

The protein portion of an enzyme is called:

A. Apoenzyme

B. Prosthetic group

C. Cofactor

D. Coenzyme

Correct answer: A. Apoenzyme

Explanation: The apoenzyme is the protein component of an enzyme that requires a non-protein cofactor. The complete functional enzyme, when its required cofactor is present, is called a holoenzyme.

Related PYQ concept: The enzyme cofactor/prosthetic-group concept was also tested in NEET-UG 2019 and NEET-UG 2024. These are related questions, not exact duplicates.

Question 22. Haem as a prosthetic group

NEET-UG 2025

Which of the following enzymes contains haem as its prosthetic group?

A. RuBisCO

B. Carbonic anhydrase

C. Succinate dehydrogenase

D. Catalase

Correct answer: D. Catalase

Explanation: Catalase contains haem, which participates in the breakdown of hydrogen peroxide into water and oxygen. Carbonic anhydrase contains zinc, while RuBisCO is involved in carbon fixation.

Repeated concept: NEET-UG 2019, NEET-UG 2024 and NEET-UG 2025 tested the broader concept of enzyme cofactors or prosthetic groups, using different question formulations.

Question 23. Enzyme class for transfer of a group

NEET-UG 2025

An enzyme catalyses the transfer of a group other than hydrogen from one substrate to another. It belongs to which class?

A. Transferases

B. Ligases

C. Hydrolases

D. Lyases

Correct answer: A. Transferases

Explanation: Transferases catalyse the transfer of functional groups between molecules. Examples include aminotransferases, which transfer amino groups.

Exam detail: This question is listed in chapter-wise NEET-UG 2025 Biology question collections.

Question 24. Sequence of enzyme action

NEET-UG 2024

Consider these stages of an enzyme-catalysed reaction:

1. Substrate binds to the active site.
2. An enzyme–substrate complex forms.
3. Bonds are altered and products are formed.
4. Products are released, leaving the enzyme available for another catalytic cycle.

Which sequence is correct?

A. 1 → 2 → 3 → 4

B. 2 → 1 → 4 → 3

C. 3 → 2 → 1 → 4

D. 4 → 3 → 2 → 1

Correct answer: A. 1 → 2 → 3 → 4

Explanation: Substrate binding leads to the enzyme–substrate complex. Catalysis then produces products, which are released so that the enzyme can catalyse another reaction.

Note: This is a study-friendly formulation of the enzyme catalytic-cycle concept, not a claim that the wording is identical to the original paper.

Question 25. Nature of enzymes

NEET-UG 2024

Which statement about enzymes is incorrect?

A. Enzymes generally show substrate specificity.

B. Some enzymes require metal ions.

C. All enzymes are proteinaceous in nature.

D. Some enzymes of thermophilic organisms function at high temperatures.

Correct answer: C. All enzymes are proteinaceous in nature.

Explanation: Most enzymes are proteins, but some RNA molecules, known as ribozymes, possess catalytic activity. Therefore, the statement that all enzymes are proteins is incorrect.

Related concept: NEET-UG 2025 also tested enzyme classification and enzyme-related concepts. These questions are related but are not necessarily repetitions of this particular statement-based question.

Question 26. Zwitterion

NEET-UG 2024

Which of the following can exist as a zwitterion?

A. Fatty acid

B. Monosaccharide

C. Amino acid

D. Triglyceride

Correct answer: C. Amino acid

Explanation: Amino acids can contain a positively charged amino group and a negatively charged carboxylate group in the same molecule. At a suitable pH, both charges coexist, producing a zwitterion.

Question 27. Non-protein biological catalyst

NEET-UG 2024

A catalytic RNA molecule is called a:

A. Ribozyme

B. Ribosome

C. Nucleosome

D. Lysosome

Correct answer: A. Ribozyme

Explanation: Ribozymes are RNA molecules that catalyse biochemical reactions. Their existence demonstrates that catalytic activity is not restricted to proteins.

Question 28. Cofactor and coenzyme relationship

NEET-UG 2024

Which statement correctly describes a coenzyme?

A. It is always a protein.

B. It is an organic non-protein component that assists an enzyme.

C. It is always the substrate of an enzyme.

D. It permanently destroys the enzyme after one reaction.

Correct answer: B. It is an organic non-protein component that assists an enzyme.

Explanation: Coenzymes are organic cofactors that participate in enzyme-catalysed reactions. Many are derived from vitamins and assist in transferring electrons or chemical groups.

SECTION B: ADDITIONAL BIOMOLECULES MCQS

Question 29. Adenosine and adenylate

NEET-UG 2025

Match the following compounds with their correct descriptions.

List I	List II
A. Adenosine	I. Nitrogenous base
B. Adenylic acid	II. Nucleotide
C. Adenine	III. Nucleoside
D. Alanine	IV. Amino acid

Choose the correct option.

A. A–III, B–II, C–I, D–IV

B. A–II, B–III, C–IV, D–I

C. A–I, B–IV, C–II, D–III

D. A–IV, B–I, C–III, D–II

Correct answer: A. A–III, B–II, C–I, D–IV

Explanation:

Adenosine consists of adenine and ribose, so it is a nucleoside.

Adenylic acid, or AMP, is a nucleotide containing adenine, ribose and phosphate.

Adenine is a purine nitrogenous base.

Alanine is an amino acid.

Question 30. Most abundant protein in the biosphere

NEET-UG 2023

Which of the following is considered the most abundant protein in the biosphere?

A. Collagen

B. RuBisCO

C. Haemoglobin

D. Insulin

Correct answer: B. RuBisCO

Explanation: Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) participates in carbon fixation during photosynthesis. It is generally described as the most abundant protein in the biosphere.

Question 31. Denaturation of proteins

NEET-UG 2023

Protein denaturation generally results in:

A. The complete breakdown of all peptide bonds

B. Loss of the primary structure in every case

C. Disruption of secondary and tertiary structures

D. Formation of new amino acids from the protein

Correct answer: C. Disruption of secondary and tertiary structures

Explanation: Denaturation disrupts the native folding of a protein. The primary amino acid sequence generally remains intact because denaturation does not ordinarily hydrolyse peptide bonds.

Question 32. Enzyme prosthetic groups — repeated concept record

Repeated concept: 2019, 2024 and 2025

Which enzyme contains haem as a prosthetic group?

A. Carbonic anhydrase

B. Catalase

C. Carboxypeptidase

D. RuBisCO

Correct answer: B. Catalase

Explanation: Catalase contains haem, which enables the enzyme to catalyse the decomposition of hydrogen peroxide.

Question 33. Glycosidic linkage

Exam: NEET-UG syllabus-based practice, 2026

Which linkage joins glucose units in the linear portion of amylose?

A. β-1,4-glycosidic linkage

B. α-1,4-glycosidic linkage

C. Peptide linkage

D. Phosphodiester linkage

Correct answer: B. α-1,4-glycosidic linkage

Explanation: Amylose is a largely unbranched component of starch. Its glucose units are joined by α-1,4-glycosidic linkages. Amylopectin also contains α-1,6 linkages at branch points.

Question 34. Essential amino acids

Exam: NEET-UG syllabus-based practice, 2026

Which amino acid is essential in the human diet?

A. Alanine

B. Aspartic acid

C. Lysine

D. Glutamic acid

Correct answer: C. Lysine

Explanation: Humans cannot synthesise sufficient lysine to meet their requirements, so it must be obtained from dietary sources.

Question 35. DNA nucleotide

Exam: NEET-UG syllabus-based practice, 2026

Which components are present in a DNA nucleotide?

A. Ribose, uracil and phosphate

B. Deoxyribose, a nitrogenous base and phosphate

C. Glucose, adenine and peptide

D. Glycerol, fatty acid and phosphate only

Correct answer: B. Deoxyribose, a nitrogenous base and phosphate

Explanation: DNA nucleotides contain deoxyribose, a nitrogenous base and one or more phosphate groups. The base may be adenine, guanine, cytosine or thymine.

Question 36. Enzyme activity and pH

Exam: NEET-UG syllabus-based practice, 2026

Why does a large change in pH often reduce enzyme activity?

A. It converts every enzyme into a carbohydrate.

B. It may alter ionisation of active-site groups and disrupt the enzyme’s structure.

C. It always increases substrate concentration.

D. It changes the genetic code of the enzyme immediately.

Correct answer: B. It may alter ionisation of active-site groups and disrupt the enzyme’s structure.

Explanation: Enzyme catalysis depends on suitable chemical conditions. A substantial pH change can alter the charges of amino acid side chains and interfere with substrate binding or catalysis.

Question 37. Maximum reaction velocity

Exam: NEET-UG syllabus-based practice, 2026

 In the Michaelis-Menten model, what happens when the substrate concentration becomes very high?

A. The reaction rate approaches Vmax.

B. The enzyme is permanently consumed.

C. The reaction rate falls to zero.

D. The amino acid sequence of the enzyme changes.

Correct Answer: A. The reaction rate approaches Vmax.

Explanation: At high substrate concentrations, most of the enzyme’s active sites become occupied. The enzyme approaches saturation, and the reaction rate approaches its maximum value, called Vmax.

Question 38. Cellulose structure

Examination: NEET-UG syllabus-based practice, 2026

Cellulose is composed of glucose units joined primarily by:

A. α-1,4-glycosidic bonds

B. β-1,4-glycosidic bonds

C. Peptide bonds

D. Phosphodiester bonds

Correct answer: B. β-1,4-glycosidic bonds

Explanation: Cellulose is a structural polysaccharide in plant cell walls. Its β-1,4-linked glucose chains form strong fibres through interactions between adjacent chains. Humans cannot digest cellulose because they lack cellulase.

Question 39. Structure of triglycerides

Examination: NEET-UG syllabus-based practice, 2026

A typical triglyceride consists of:

A. Three amino acids and one phosphate group

B. Glycerol and three fatty acids

C. Three glucose molecules and one nitrogenous base

D. Ribose and three phosphate groups

Correct answer: B. Glycerol and three fatty acids

Explanation: A triglyceride contains a glycerol molecule esterified with three fatty acids. Triglycerides serve primarily as energy-storage molecules.

Question 40. DNA and RNA bases

Examination: NEET-UG syllabus-based practice, 2026

Which nitrogenous base is normally present in RNA but absent from DNA?

A. Adenine

B. Guanine

C. Cytosine

D. Uracil

Correct answer: D. Uracil

Explanation: RNA contains uracil in place of thymine. DNA normally contains adenine, guanine, cytosine and thymine.

Question 41. Enzyme specificity

Examination: NEET-UG syllabus-based practice, 2026

The ability of an enzyme to recognise a particular substrate is primarily associated with its:

A. Active-site structure and chemical properties

B. Total number of atoms in the cell

C. Location of the organism’s chromosomes

D. Concentration of cellulose in the cell wall

Correct answer: A. Active-site structure and chemical properties

Explanation: The shape, charge and chemical environment of the active site help determine which substrates an enzyme recognises and which reactions it catalyses.

Question 42. Competitive inhibition

Examination: NEET-UG syllabus-based practice, 2026

In the simple reversible competitive-inhibition model, increasing the substrate concentration can overcome inhibition because:

A. The inhibitor permanently destroys the enzyme.

B. The substrate competes with the inhibitor for the active site.

C. The inhibitor becomes a peptide bond.

D. The enzyme’s primary structure is immediately restored.

Correct answer: B. The substrate competes with the inhibitor for the active site.

Explanation: A competitive inhibitor competes with the substrate for the active site of an enzyme. When the substrate concentration is sufficiently high, the substrate can overcome the effect of the inhibitor.

In competitive inhibition, the apparent Km value increases, but Vmax remains unchanged in the simple model.

Question 43. Coenzymes

Examination: NEET-UG syllabus-based practice, 2026

Which statement about coenzymes is correct?

A. All coenzymes are proteins.

B. Coenzymes are organic molecules that assist enzyme-catalysed reactions.

C. Coenzymes are always polysaccharides.

D. Coenzymes permanently destroy enzymes after catalysis.

Correct answer: B. Coenzymes are organic molecules that assist enzyme-catalysed reactions.

Explanation: Coenzymes are organic cofactors. Examples include NAD⁺ and coenzyme A, which participate in electron or chemical-group transfer.

Question 44. Protein denaturation

Examination: NEET-UG syllabus-based practice, 2026

Which treatment can cause denaturation of many proteins?

A. Exposure to sufficiently high temperatures

B. Addition of amino acids under every condition

C. Formation of a peptide bond

D. Replication of DNA

Correct answer: A. Exposure to sufficiently high temperatures

Explanation: High temperatures can disrupt interactions that maintain protein folding. Denaturation usually affects secondary, tertiary or quaternary structure, without necessarily breaking the peptide bonds of the primary structure.

Question 45. Purine bases

Examination: NEET-UG syllabus-based practice, 2026

Which pair consists of purine bases?

A. Cytosine and thymine

B. Adenine and guanine

C. Uracil and cytosine

D. Thymine and uracil

Correct answer: B. Adenine and guanine

Explanation: Purines have a two-ring structure. Adenine and guanine are purines, whereas cytosine, thymine and uracil are pyrimidines.

Question 46. Enzyme classification

Examination: NEET-UG syllabus-based practice, 2026

An enzyme that joins two molecules using energy from ATP generally belongs to which traditional enzyme class?

A. Hydrolases

B. Isomerases

C. Ligases

D. Oxidoreductases

Correct answer: C. Ligases

Explanation: Ligases catalyse the joining of molecules, often coupled to ATP hydrolysis. DNA ligase joins DNA fragments by forming phosphodiester bonds.

Question 47. Michaelis constant

Examination: NEET-UG syllabus-based practice, 2026

In a simple Michaelis-Menten enzyme model, when is the reaction rate equal to half of Vmax?

A. When substrate concentration is zero

B. When substrate concentration is equal to Km

C. When substrate concentration is equal to 2 times Vmax

D. When enzyme concentration is zero

Correct Answer: B. When substrate concentration is equal to Km

Explanation: According to the Michaelis-Menten equation:

v = (Vmax x [S]) / (Km + [S])

When the substrate concentration ([S]) is equal to Km, the reaction rate (v) becomes half of Vmax. Therefore, v = Vmax/2.

Question 48. Inulin — carbohydrate composition

NEET-UG 2023 (Manipur)

Inulin is a polymer of which monosaccharide?

A. Fructose

B. Galactose

C. Amino acids

D. Glucose

Correct answer: A. Fructose

Explanation: Inulin is a fructan, a polysaccharide composed mainly of fructose units. It occurs in the storage tissues of several plants. Unlike starch, which is made of glucose units, inulin is primarily a polymer of fructose.

Question 49. Primary and secondary metabolites

NEET-UG 2023 (Manipur)

Which of the following is classified as a primary metabolite rather than a secondary metabolite?

A. Curcumin

B. Morphine

C. Anthocyanin

D. Lecithin

Correct answer: D. Lecithin

Explanation: Lecithin is a phospholipid involved in membrane structure and normal cellular function. Curcumin, morphine and anthocyanin are commonly classified as secondary metabolites in the NCERT discussion.

Question 50. Iodine test and cellulose

NEET-UG 2023

Why does cellulose fail to produce the characteristic blue colour with iodine?

A. It is a helical molecule.

B. It lacks the complex helical structure needed to accommodate iodine.

C. Iodine breaks cellulose into monosaccharides.

D. Cellulose is a disaccharide.

Correct answer: B. It lacks the complex helical structure needed to accommodate iodine.

Explanation: Starch, particularly amylose, can form a helical structure that accommodates iodine and produces a characteristic colour. Cellulose consists of straight, β-1,4-linked glucose chains and does not form the same iodine-binding complex.

Question 51. Enzyme activity and inhibition

NEET-UG 2023

Consider the following statements.

Statement I: Low temperatures can temporarily reduce enzyme activity, whereas sufficiently high temperatures can denature many enzymes.

Statement II: A competitive inhibitor competes with the substrate for enzyme binding, often because it resembles the substrate.

Choose the correct option.

A. Both statements are false.

B. Statement I is true, but Statement II is false.

C. Statement I is false, but Statement II is true.

D. Both statements are true.

Correct answer: D. Both statements are true.

Explanation: Low temperatures generally slow molecular reactions without necessarily denaturing the enzyme. High temperatures can disrupt the protein’s structure. Competitive inhibitors occupy the active site and reduce substrate binding.

Question 52. Prosthetic group of a peroxide-degrading enzyme

NEET-UG 2022 Phase 2

The enzyme responsible for decomposing hydrogen peroxide into water and oxygen contains which prosthetic group?

A. Niacin

B. Nicotinamide adenine dinucleotide

C. Haem

D. Zinc

Correct answer: C. Haem

Explanation: Catalase is an enzyme that contains haem with iron. It breaks down hydrogen peroxide into water and oxygen.

Reaction:

2H2O2 -> 2H2O + O2

Haem contains iron, which helps catalase carry out this reaction.

Question 53. Formula of maltose

NEET-UG 2022 Phase 1

Two glucose molecules combine by removing one water molecule to form maltose. What is the molecular formula of maltose?

A. C12H20O10

B. C12H24O12

C. C12H22O11

D. C12H24O11

Correct Answer: C. C12H22O11

Explanation: Two glucose molecules combine and release one water molecule to form maltose.

Glucose formula: C6H12O6

Reaction:

C6H12O6 + C6H12O6 – H2O = C12H22O11

Maltose is a disaccharide made up of two glucose units joined by a glycosidic bond.Top of Form

Bottom of Form

Question 54. Glycosidic and peptide bonds

NEET-UG 2020 Phase 1

Which option contains, respectively, a molecule with glycosidic bonds and a molecule with peptide bonds?

A. Glycerol and trypsin

B. Cellulose and lecithin

C. Inulin and insulin

D. Chitin and cholesterol

Correct answer: C. Inulin and insulin

Explanation: Inulin contains fructose units linked by glycosidic bonds. Insulin is a protein, and its amino acids are joined by peptide bonds. Trypsin also contains peptide bonds, but glycerol does not contain glycosidic bonds.

Question 55. Most abundant protein in animals

NEET-UG 2020 Phase 1

Which is the most abundant protein in the animal world?

A. Collagen

B. Lectin

C. Insulin

D. Haemoglobin

Correct answer: A. Collagen

Explanation: Collagen is the most abundant protein in animals. It is a fibrous structural protein found extensively in connective tissues and the extracellular matrix.

Remember the distinction: RuBisCO is generally described as the most abundant protein in the biosphere, while collagen is the most abundant protein in the animal world.

Question 56. Matching biological molecules with their bonds

Examination: NEET-UG 2021

Match List I with List II.

List I	List II
(a) Protein	(i) C=C double bonds
(b) Unsaturated fatty acid	(ii) Phosphodiester bonds
(c) Nucleic acid	(iii) Glycosidic bonds
(d) Polysaccharide	(iv) Peptide bonds

Choose the correct answer.

A. (a)–(iv), (b)–(i), (c)–(ii), (d)–(iii)

B. (a)–(i), (b)–(iv), (c)–(iii), (d)–(ii)

C. (a)–(ii), (b)–(i), (c)–(iv), (d)–(iii)

D. (a)–(iv), (b)–(iii), (c)–(i), (d)–(ii)

Correct answer: A

Explanation:

Proteins contain amino acids joined by peptide bonds.

Unsaturated fatty acids contain one or more carbon–carbon double bonds.

Nucleotides in nucleic acids are connected through phosphodiester bonds.

Monosaccharide units in polysaccharides are joined by glycosidic bonds.

Question 57. Concanavalin A

Examination: NEET-UG 2019

Concanavalin A is:

A. A pigment

B. An alkaloid

C. An essential oil

D. A lectin

Correct answer: D. A lectin

Explanation: Concanavalin A is a carbohydrate-binding protein classified as a lectin. Lectins bind specific carbohydrate structures and are useful in studies of cell recognition and interactions between biomolecules.

Question 58. Pantothenic acid and biotin

Examination: AIPMT 1999

Pantothenic acid and biotin are associated with which vitamin group?

A. Vitamin D

B. Vitamin

B complex

C. Vitamin K

D. Vitamin E

Correct answer: B. Vitamin B complex

Explanation: Pantothenic acid is vitamin B5, while biotin is vitamin B7. Both belong to the B-complex group and participate in metabolic processes, including reactions involving enzyme cofactors.

Question 59. Cellulose structure

Examination: AIPMT 1998

Which statement correctly describes the cellulose molecule?

A. It has β-1,4 linkages and is unbranched.

B. It has β-1,4 linkages and is branched.

C. It has α-1,4 linkages and is branched.

D. It has β-1,6 linkages and is unbranched.

Correct answer: A. It has β-1,4 linkages and is unbranched.

Explanation: Cellulose is composed of linear chains of β-D-glucose units linked by β-1,4-glycosidic bonds. Hydrogen bonding between neighbouring chains contributes to its strength and structural role in plant cell walls.

Question 60. Which of the following is NOT a complex polysaccharide?

Exam: NEET-UG 2024

A. Chitin

B. Inulin

C. Cellulose

D. Starch

Correct Answer: B. Inulin

Explanation: Inulin is a polysaccharide made mainly of fructose units. In the context of this question, it is distinguished from complex structural or storage polysaccharides such as chitin, cellulose and starch. Chitin occurs in arthropod exoskeletons and fungal cell walls; cellulose forms plant cell walls; starch is a major storage carbohydrate in plants.

Correction to Question 60: The original option set reported in the question bank is: (A) Inulin,

(B) Chitin,

(C) Glucosamine, and

(D) N-acetyl galactosamine.

The correct answer remains A. Inulin.

Question 61. Which secondary structure of a protein is associated with the lipid layer and lines the pores of the cell membrane?

A. Alpha-helix

B. Beta-strand

C. Beta-chain

D. Random structure

Correct Answer: B. Beta-strand

Explanation: Beta-strands can associate to form beta-sheets and beta-barrel structures. Certain membrane proteins use beta-barrels to form channels or pores through the lipid bilayer. This question tests the relationship between protein secondary structure and membrane architecture.

Question 62. Which factor is responsible for feedback inhibition of an enzymatic process?

Exam: AIPMT 2000

A. Substrate

B. Enzyme

C. End product

D. Temperature

Correct Answer: C. End product

Explanation: In feedback inhibition, the final product of a metabolic pathway inhibits an enzyme that acts earlier in that pathway. This prevents unnecessary product formation and conserves cellular resources. It is a form of metabolic regulation.

Question 63. ATP is classified as a:

Exam: AIPMT 2000

A. Nucleotide

B. Nucleoside

C. Nucleic acid

D. Vitamin

Correct Answer: A. Nucleotide

Explanation: ATP (adenosine triphosphate) consists of adenine, ribose sugar and three phosphate groups. A nitrogenous base + sugar + phosphate group(s) forms a nucleotide. A nucleoside contains only a nitrogenous base and sugar, without phosphate.

Question 64. Which of the following is an essential amino acid?

Exam: AIPMT 2000

A. Phenylalanine

B. Glycine

C. Aspartic acid

D. Serine

Correct Answer: A. Phenylalanine

Explanation: Essential amino acids cannot be synthesised by the human body in sufficient amounts and must be obtained through the diet. Phenylalanine is an essential amino acid. Glycine, aspartic acid and serine are generally classified as non-essential amino acids.

Question 65. Which protein is found in the maximum amount in the animal kingdom?

Exam: AIPMT 2002

A. Collagen

B. Haemoglobin

C. Insulin

D. Albumin

Correct Answer: A. Collagen

Explanation: Collagen is a fibrous structural protein and is the most abundant protein in the animal kingdom. It is a major component of connective tissues, including skin, tendons, ligaments and cartilage.

Important distinction: This question asks about the animal kingdom. RuBisCO, by contrast, is commonly identified as the most abundant protein in the biosphere because of its widespread role in photosynthesis. These are different facts and should not be confused.

Question 66. What is the role of an enzyme in a chemical reaction?

Exam: AIPMT 2000

A. It decreases the activation energy.

B. It increases the activation energy.

C. It acts as an inorganic catalyst.

D. It changes the final equilibrium of the reaction.

Correct Answer: A. It decreases the activation energy.

Explanation: Enzymes accelerate biochemical reactions by providing an alternative reaction pathway with lower activation energy. They are biological catalysts and are not consumed permanently during the reactions they catalyse. Enzymes do not change the equilibrium position of a reaction.

Question 67. Which of the following has a carbohydrate component as part of its structure?

Exam: AIPMT 2000

A. Glycoprotein

B. Chromoprotein

C. Lipoprotein

D. Nucleoprotein

Correct Answer: A. Glycoprotein

Explanation: Glycoproteins contain carbohydrate chains covalently attached to proteins. They are found in cell membranes, mucus and many biological recognition systems. Chromoproteins contain a coloured prosthetic group, lipoproteins contain lipid components, and nucleoproteins are associated with nucleic acids.

Question 68. Enzymes are composed of which of the following?

Exam: AIPMT 2002

A. Edible proteins

B. Proteins with specific structures

C. Nitrogen-containing carbohydrates

D. Carbohydrates

Correct Answer: B. Proteins with specific structures

Explanation: Most enzymes are proteins with specific three-dimensional structures that enable catalysis. However, some RNA molecules, called ribozymes, also possess catalytic activity. Therefore, it is inaccurate to state that every enzyme is proteinaceous.

Question 69. Which of the following enzymes hydrolyses triglycerides into fatty acids and glycerol?

Exam: AIPMT 2004

A. Lipase

B. Zymase

C. Pepsin

D. Maltase

Correct Answer: A. Lipase

Explanation: Lipases catalyse the hydrolysis of fats. Triglycerides contain glycerol linked to three fatty acids by ester bonds. Complete hydrolysis releases glycerol and fatty acids.

Question 70. Which hormone facilitates the conversion of glucose into glycogen?

Exam: AIPMT 2004

A. Bile acids

B. Adrenaline

C. Insulin

D. Cortisone

Correct Answer: C. Insulin

Explanation: Insulin is secreted by the beta cells of the pancreas. It promotes glucose uptake in insulin-sensitive tissues and supports glycogen synthesis, particularly in the liver and skeletal muscles. Adrenaline generally promotes the mobilisation of stored energy.

Question 71. Which vitamin is correctly matched with its chemical name?

Exam: AIPMT 2001

A. Vitamin E — Tocopherol

B. Vitamin D — Riboflavin

C. Vitamin B — Calciferol

D. Vitamin A — Thiamine

Correct Answer: A. Vitamin E — Tocopherol

Explanation: Vitamin E is commonly known as tocopherol. Vitamin D is associated with calciferols, vitamin B2 is riboflavin, and vitamin B1 is thiamine.

Question 72. The most abundant organic compound on Earth is:

Exam: AIPMT 2001; also reported in AIPMT 2004

A. Protein

B. Cellulose

C. Lipids

D. Steroids

Correct Answer: B. Cellulose

Explanation: Cellulose is a structural polysaccharide composed of glucose units linked by beta-1,4 glycosidic bonds. It is the major structural component of plant cell walls and is widely described in biology textbooks as the most abundant organic compound on Earth.

Repeated appearance: The same question is reported for AIPMT 2001 and AIPMT 2004. It is included only once here, with both years listed.

Question 73. Which of the following is a basic amino acid?

Exam: NEET-UG 2020

A. Serine

B. Alanine

C. Tyrosine

D. Lysine

Correct Answer: D. Lysine

Explanation: Lysine has an additional basic amino group in its side chain. It is classified as a basic amino acid. Serine, alanine and tyrosine are generally classified as neutral amino acids.

Question 74. The helical structure of a protein is stabilised by:

Exam: AIPMT 2004

A. Hydrogen bonds

B. Ether bonds

C. Peptide bonds alone

D. Dipeptide bonds

Correct Answer: A. Hydrogen bonds

Explanation: The alpha-helix is a secondary structure of proteins. It is stabilised mainly by hydrogen bonds between the carbonyl oxygen of one peptide bond and the N–H group of another peptide bond farther along the polypeptide chain.

Question 75. Which functional group participates in disulphide bond formation in proteins?

Exam: AIPMT 2005

A. Thioether

B. Thiol

C. Thioester

D. Thiolactone

Correct Answer: B. Thiol

Explanation: The thiol group (–SH) of cysteine can be oxidised to form a disulphide bond (–S–S–) between two cysteine residues. These bonds help stabilise the three-dimensional structure of proteins.

Question 76. A typical fat molecule is made up of:

Exam: NEET-UG 2016 Phase 1

A. Three glycerol molecules and one fatty acid molecule

B. One glycerol molecule and three fatty acid molecules

C. One glycerol molecule and one fatty acid molecule

D. Three glycerol molecules and three fatty acid molecules

Correct Answer: B. One glycerol molecule and three fatty acid molecules

Explanation: A typical fat, or triglyceride, consists of one glycerol molecule esterified with three fatty acid molecules. The ester bonds form through condensation reactions. Triglycerides serve primarily as energy-storage molecules.

Question 77. Which of the following is a nucleotide?

Exam: NEET-UG 2025

A. Adenine

B. Adenosine

C. Adenylic acid

D. Alanine

Correct Answer: C. Adenylic acid

Explanation: A nucleotide contains a nitrogenous base, a pentose sugar and one or more phosphate groups. Adenylic acid, also called AMP (adenosine monophosphate), contains adenine, ribose and phosphate. Adenine is a nitrogenous base, adenosine is a nucleoside, and alanine is an amino acid.

Quick revision

Concept	Key fact
Basic amino acid	Lysine
Alpha-helix	Stabilised by hydrogen bonds
Disulphide bond	Formed from cysteine thiol groups
Triglyceride	One glycerol + three fatty acids
Nucleotide	Base + sugar + phosphate

Question 78. Which of the following is the non-essential amino acid?

Exam: NEET-UG 2019

NEET Previous Year Questions

A. Valine

B. Leucine

C. Lysine

D. Alanine

Correct Answer: D. Alanine

Explanation: Alanine is a non-essential amino acid because the human body can synthesise it. Valine, leucine and lysine are essential amino acids and must be obtained through the diet.

Question 79. Sucrose on hydrolysis gives:

Exam: NEET-UG 2020, Phase 1

A. Beta-D-glucose + alpha-D-fructose

B. Alpha-D-glucose + beta-D-glucose

C. Alpha-D-glucose + beta-D-fructose

D. Alpha-D-fructose + beta-D-fructose

Correct Answer: C. Alpha-D-glucose + beta-D-fructose

Explanation: Sucrose is a disaccharide composed of glucose and fructose. Hydrolysis breaks its glycosidic bond and releases glucose and fructose. The conventional structural description of sucrose contains alpha-D-glucose and beta-D-fructose units.

Question 80. Which statement regarding enzymes is incorrect?

Exam: NEET-UG 2022, Phase 1

A. Enzymes are specific for particular reactions and substrates.

B. Enzymes act as biological catalysts.

C. Enzymes lower the activation energy of biochemical reactions.

D. Enzymes are polysaccharides.

Correct Answer: D. Enzymes are polysaccharides.

Explanation: Most enzymes are proteins, while some RNA molecules also have catalytic activity. Enzymes accelerate reactions by lowering activation energy; they are not polysaccharides.

Question 81. Which of the following is a basic amino acid?

Exam: NEET-UG 2020

A. Serine

B. Alanine

C. Tyrosine

D. Lysine

Correct Answer: D. Lysine

Explanation: Lysine contains an additional amino group in its side chain, giving it basic properties. Serine, alanine and tyrosine are generally classified as neutral amino acids.

Question 82. Which of the following pairs is correctly matched?

Exam: NEET-UG 2021

A. Protein — Glycosidic bonds

B. Unsaturated fatty acid — C=C double bonds

C. Nucleic acid — Peptide bonds

D. Polysaccharide — Phosphodiester bonds

Correct Answer: B. Unsaturated fatty acid — C=C double bonds

Explanation: Unsaturated fatty acids contain one or more carbon-carbon double bonds. Amino acids in proteins are joined by peptide bonds, nucleotides in nucleic acids by phosphodiester bonds, and monosaccharide units in polysaccharides by glycosidic bonds.

Revision table

Topic	Important fact
Non-essential amino acid	Alanine
Sucrose hydrolysis	Glucose + fructose
Enzymes	Mostly proteins; some catalytic RNA
Basic amino acid	Lysine
Unsaturated fatty acid	Contains C=C double bond
Protein linkage	Peptide bond
Nucleic acid linkage	Phosphodiester bond
Polysaccharide linkage	Glycosidic bond

Question 83. During digestion, proteins are hydrolysed into polypeptides and then into amino acids. The enzymes responsible, respectively, are:

Exam: AIPMT 2006

A. Amylase and maltase

B. Diastase and lipase

C. Pepsin and trypsin

D. Invertase and zymase

Correct Answer: C. Pepsin and trypsin

Explanation: Pepsin acts in the acidic environment of the stomach and breaks proteins into smaller peptides. Trypsin acts in the small intestine and helps digest proteins and peptides further. Complete protein digestion involves several enzymes, including other pancreatic and intestinal peptidases.

Question 84. Which statement about amino acids and proteins is NOT correct?

Exam: AIPMT 2004

A. The alpha-carbon of an alpha-amino acid is asymmetric except in glycine.

B. Human proteins are composed predominantly of L-amino acids.

C. The human body can synthesise all the proteins it needs without obtaining essential amino acids from food.

D. At pH 7, amino and carboxyl groups can exist in ionised forms.

Correct Answer: C. The human body can synthesise all the proteins it needs without obtaining essential amino acids from food.

Explanation: The body requires essential amino acids from dietary sources because it cannot synthesise them in sufficient quantities. These amino acids are used to build proteins. Glycine is achiral because its alpha-carbon has two hydrogen atoms.

Question 85. DNA and RNA are chiral molecules mainly because of their:

Exam: AIPMT 2007

A. L-sugar component

B. Chiral nitrogenous bases

C. Chiral phosphate group D.

D-sugar component

Correct Answer: D. D-sugar component

Explanation: The sugars in naturally occurring DNA and RNA are D-deoxyribose and D-ribose, respectively. Their stereochemistry contributes to the chirality of nucleic acid molecules.

Question 86. Which of the following compounds does NOT reduce Fehling’s solution?

Exam: AIPMT 1999

A. Glucose

B. Fructose

C. Sucrose

D. Maltose

Correct Answer: C. Sucrose

Explanation: Sucrose is a non-reducing sugar because both anomeric carbons are involved in its glycosidic bond. Glucose and maltose are reducing sugars. Fructose also gives a positive Fehling’s test because it can undergo tautomerisation under the alkaline test conditions.

Question 87. Which of the following are NOT secondary metabolites in plants?

Exam: NEET-UG 2021

A. Morphine and codeine

B. Amino acids and glucose

C. Vinblastine and curcumin

D. Rubber and gums

Correct Answer: B. Amino acids and glucose

Explanation: Amino acids and glucose are primary metabolites involved in fundamental processes such as protein synthesis and cellular respiration. Morphine, codeine, vinblastine and curcumin are examples of secondary metabolites. Rubber and many plant gums are also commonly discussed in the context of secondary products.

QUICK REVISION: QUESTIONS 83–87

Topic	Key fact
Protein digestion	Pepsin and trypsin participate in protein breakdown
Essential amino acids	Must be obtained through the diet
Nucleic acid chirality	Associated with D-ribose and D-deoxyribose
Fehling’s test	Sucrose is non-reducing
Primary metabolites	Include glucose and amino acids

Question 88. The two polypeptide chains of human insulin are linked together by:

Exam: NEET-UG 2016 Phase 1

A. Hydrogen bonds

B. Phosphodiester bonds

C. Covalent bonds

D. Disulphide bridges

Correct Answer: D. Disulphide bridges

Explanation: Mature human insulin contains two polypeptide chains, A and B, connected by two interchain disulphide bridges. An additional disulphide bond occurs within the A chain. These bonds help maintain insulin’s functional structure.

Question 89. Which of the following is a non-proteinaceous enzyme?

Exam: NEET-UG 2016 Phase 2

A. Pepsin

B. Trypsin

C. Ribozyme

D. Amylase

Correct Answer: C. Ribozyme

Explanation: A ribozyme is an RNA molecule that catalyses a biochemical reaction. Pepsin, trypsin and amylase are protein enzymes. Ribozymes demonstrate that catalytic activity is not restricted to proteins.

Question 90. The chitinous exoskeleton of arthropods is formed by the polymerisation of:

Exam: AIPMT 2015

A. N-acetyl glucosamine

B. Lipoglycans

C. Keratan sulphate and chondroitin sulphate D.

D-glucosamine

Correct Answer: A. N-acetyl glucosamine

Explanation: Chitin is a structural polysaccharide made of repeating N-acetylglucosamine units joined by beta-1,4 glycosidic linkages. It provides mechanical strength to the exoskeletons of arthropods and is also found in fungal cell walls.

Question 91. Which of the following biomolecules contains phosphodiester bonds?

Exam: AIPMT 2015

A. Triacylglycerol

B. Protein

C. Nucleic acid

D. Starch

Correct Answer: C. Nucleic acid

Explanation: Phosphodiester bonds connect adjacent nucleotides in the sugar-phosphate backbone of DNA and RNA. Proteins contain peptide bonds, starch contains glycosidic bonds, and triacylglycerols contain ester bonds.

Question 92. Which interaction is LEAST likely to be involved in establishing the three-dimensional folding of most proteins?

Exam: NEET-UG 2016 Phase 2

A. Hydrogen bonds

B. Electrostatic interactions

C. Hydrophobic interactions

D. Ester bonds

Correct Answer: D. Ester bonds

Explanation: Protein tertiary structure is stabilised by interactions such as hydrogen bonds, ionic or electrostatic interactions, hydrophobic interactions and van der Waals forces. Ester bonds are not a typical major interaction responsible for the folding of most proteins.

QUICK REVISION TABLE

Question	Topic	Key answer
88	Insulin structure	Disulphide bridges
89	Non-protein enzyme	Ribozyme
90	Arthropod exoskeleton	N-acetylglucosamine
91	Phosphodiester bonds	DNA and RNA
92	Protein folding	Ester bonds are least likely

Question 93. Which one of the following is a non-reducing carbohydrate?

Exam: AIPMT 2014

A. Maltose

B. Sucrose

C. Lactose

D. Ribose 5-phosphate

Correct Answer: B. Sucrose

Explanation: Sucrose is a non-reducing disaccharide because the anomeric carbons of glucose and fructose are both involved in its glycosidic bond. Maltose and lactose are reducing sugars.

Question 94. Which statement about enzyme inhibition is correct?

Exam: AIPMT 2014

A. Malonate is a competitive inhibitor of succinate dehydrogenase.

B. Malonate is a non-competitive inhibitor of succinate dehydrogenase.

C. Malonate permanently destroys succinate dehydrogenase.

D. Malonate is the normal product of the reaction catalysed by succinate dehydrogenase.

Correct Answer: A. Malonate is a competitive inhibitor of succinate dehydrogenase.

Explanation: Malonate resembles succinate, the normal substrate of succinate dehydrogenase. It competes with succinate for the enzyme’s active site. Increasing the substrate concentration can reduce the effect of a competitive inhibitor.

Question 95. Chitin is classified as a:

Exam: AIPMT 2013

A. Phosphorus-containing polysaccharide

B. Sulphur-containing polysaccharide

C. Simple sugar

D. Nitrogen-containing polysaccharide

Correct Answer: D. Nitrogen-containing polysaccharide

Explanation: Chitin is a structural polysaccharide made of repeating N-acetylglucosamine units. It occurs in arthropod exoskeletons and fungal cell walls. Its monomers contain nitrogen.

Question 96. The essential chemical components of many coenzymes are:

Exam: AIPMT 2013

A. Nucleic acids

B. Carbohydrates

C. Vitamins

D. Proteins

Correct Answer: C. Vitamins

Explanation: Many coenzymes are derived from vitamins, especially B-complex vitamins. Examples include NAD, derived from niacin, and coenzyme A, which contains a component derived from pantothenic acid. Coenzymes assist enzymes in transferring electrons or chemical groups during reactions.

Question 97. During an enzyme-catalysed reaction, the transition-state structure of the substrate is:

Exam: AIPMT 2013

A. Permanent but unstable

B. Transient and unstable

C. Permanent and stable

D. Transient but stable

Correct Answer: B. Transient and unstable

Explanation: The transition state is a short-lived, high-energy arrangement of atoms formed as reactants are converted into products. Enzymes accelerate reactions by stabilising the transition state and lowering the activation energy required to reach it.

QUICK REVISION: QUESTIONS 93–97

Topic	Key fact
Non-reducing carbohydrate	Sucrose
Competitive inhibition	Malonate inhibits succinate dehydrogenase
Chitin	Nitrogen-containing polysaccharide
Coenzymes	Many are derived from vitamins
Transition state	Transient and high-energy

Question 98. Which one of the following biomolecules is correctly characterised?

Exam: AIPMT 2012 Prelims

A. Palmitic acid — an unsaturated fatty acid with 18 carbon atoms

B. Adenylic acid — adenosine with a glucose phosphate molecule

C. Alanine — an amino acid containing amino and acidic groups

D. Lecithin — a phosphorylated glyceride found in cell membranes

Correct Answer: D. Lecithin — a phosphorylated glyceride found in cell membranes

Explanation: Lecithin is a phospholipid found in biological membranes. It contains glycerol, fatty acids, phosphate and a polar head group. Palmitic acid is a saturated 16-carbon fatty acid, while adenylic acid contains ribose, adenine and phosphate—not glucose.

Question 99. Which of the following is NOT a correct statement about proteins?

Exam: AIPMT 2002

A. The alpha-carbon of an alpha-amino acid is asymmetric except in glycine.

B. Proteins in the human body are composed predominantly of L-amino acids.

C. The human body can synthesise all the proteins it needs without essential amino acids from food.

D. At pH 7, amino and carboxyl groups can exist in ionised forms.

Correct Answer: C. The human body can synthesise all the proteins it needs without essential amino acids from food.

Explanation: The body needs essential amino acids from the diet because it cannot synthesise them in sufficient quantities. Glycine is achiral because its alpha-carbon has two hydrogen atoms. At physiological pH, amino acids commonly occur as zwitterions, with a positively charged amino group and a negatively charged carboxyl group.

Question 100. Which one of the following does NOT exhibit mutarotation?

Exam: AIPMT 2010

A. Sucrose

B. Lactose

C. Maltose

D. Fructose

Correct Answer: A. Sucrose

Explanation: Mutarotation is the change in optical rotation caused by interconversion between the alpha and beta anomeric forms of a sugar in solution. Sucrose does not undergo mutarotation because the anomeric carbons of both glucose and fructose participate in its glycosidic bond.

Question 101. Fructose reduces Tollens’ reagent because of:

Exam: AIPMT 2010

A. Its primary alcohol group

B. Its secondary alcohol group

C. Enolisation followed by conversion into an aldehyde under alkaline conditions

D. Its asymmetric carbon atoms

Correct Answer: C. Enolisation followed by conversion into an aldehyde under alkaline conditions

Explanation: Fructose is a ketose, but under alkaline conditions it can isomerise through an enediol intermediate to aldoses such as glucose and mannose. These forms can reduce Tollens’ reagent, producing a silver mirror.

Question 102. Which vitamin deficiency causes beriberi?

Exam: AIPMT 2012

A. Vitamin B1

B. Vitamin B2

C. Vitamin C

D. Vitamin D

Correct Answer: A. Vitamin B1

Explanation: Vitamin B1, also called thiamine, is essential for normal carbohydrate metabolism and nervous system function. Its deficiency can cause beriberi, which may affect the cardiovascular system and nerves.

QUICK REVISION TABLE

Topic	Key fact
Lecithin	Phospholipid in cell membranes
Essential amino acids	Must be obtained from the diet
Mutarotation	Absent in sucrose
Fructose and Tollens’ reagent	Positive reaction under alkaline conditions
Vitamin B1 deficiency	Beriberi

Question 103. Which properties make starch suitable as a storage material in plants?

Exam: AIPMT 2008

1. It is chemically non-reactive.
2. It is osmotically inactive.
3. It is easily translocated as an intact polymer.
4. It can be stored in plant organs.

A. 1 and 2 only B. 1, 2 and 4 only C. 2 and 3 only D. 1, 3 and 4 only

Correct Answer: B. 1, 2 and 4 only

Explanation: Starch is a large, relatively insoluble polysaccharide. It is osmotically inactive compared with an equivalent quantity of free glucose molecules and is suitable for long-term energy storage. It is not transported intact through plants as readily as soluble sugars such as sucrose.

Question 104. Modern detergents contain enzyme preparations derived from organisms adapted to alkaline conditions. These organisms are called:

Exam: AIPMT 2008

A. Thermophiles

B. Acidophiles

C. Alkaliphiles

D. Halophiles

Correct Answer: C. Alkaliphiles

Explanation: Alkaliphiles thrive in alkaline environments. Alkaline proteases produced by such organisms are useful in detergents because they help break down protein-based stains. Some detergent enzymes are also selected for their stability under the alkaline conditions of washing.

Question 105. Malonate is a competitive inhibitor of which enzyme?

Exam: AIPMT 2008

A. Alpha-ketoglutarate dehydrogenase

B. Malate dehydrogenase

C. Succinate dehydrogenase

D. Oxaloacetate decarboxylase

Correct Answer: C. Succinate dehydrogenase

Explanation: Malonate resembles succinate and competes with it for the active site of succinate dehydrogenase. This enzyme participates in the citric acid cycle and also functions in the electron transport chain. Competitive inhibition can be overcome to some extent by increasing substrate concentration.

Question 106. Which organism listed below has cellulose as a major component of its cell wall?

Exam: AIPMT 2008

A. Pseudomonas

B. Saccharomyces

C. Pythium

D. Xanthomonas

Correct Answer: C. Pythium

Explanation: Pythium is an oomycete whose cell wall contains cellulose. Pseudomonas and Xanthomonas are bacteria, while Saccharomyces is a yeast whose cell wall contains glucans and mannoproteins, along with chitin in smaller amounts.

Question 107. A gene is best described as:

Exam: AIPMT 2009

A. A segment of DNA that carries information for a functional product

B. A single nucleotide

C. A sugar molecule present in DNA

D. A nucleoside without phosphate

Correct Answer: A. A segment of DNA that carries information for a functional product

Explanation: A gene is a DNA sequence containing information needed to produce a functional product, such as a protein or functional RNA. Nucleotides are the building blocks of DNA; ribose is a sugar found in RNA, and a nucleoside consists of a nitrogenous base attached to a sugar.

QUICK REVISION: QUESTIONS 103–107

Topic	Key fact
Starch	Chemically stable and osmotically inactive
Alkaliphiles	Thrive in alkaline conditions
Malonate	Competitive inhibitor of succinate dehydrogenase
Pythium	Cell wall contains cellulose
Gene	DNA sequence encoding a functional product

Question 108. Three statements about enzymes are correct and one is wrong. Identify the wrong statement.

Exam: AIPMT 2010 Mains

A. Enzymes require an optimum pH for maximum activity.

B. Enzymes may remain active at 80–90°C in certain thermophilic organisms.

C. Enzymes are highly specific.

D. Most enzymes are proteins, but some are lipids.

Correct Answer: D. Most enzymes are proteins, but some are lipids.

Explanation: Most enzymes are proteins. Some RNA molecules, called ribozymes, can also catalyse biochemical reactions. Lipids are not the recognised class of non-protein enzymes.

Question 109. Which one of the following does NOT exhibit mutarotation?

Exam: AIPMT 2010

A. Sucrose

B. Lactose

C. Maltose

D. Fructose

Correct Answer: A. Sucrose

Explanation: Mutarotation occurs when a sugar changes its optical rotation through interconversion between its alpha and beta anomeric forms. Sucrose does not show mutarotation because both anomeric carbons are involved in its glycosidic bond.

Question 110. Which of the following is NOT a fat-soluble vitamin?

Exam: AIPMT 2011

A. Vitamin A

B. Vitamin B complex

C. Vitamin D

D. Vitamin E

Correct Answer: B. Vitamin B complex

Explanation: Vitamins A, D, E and K are fat-soluble. B-complex vitamins and vitamin C are water-soluble. These differences affect their absorption, storage and excretion.

Question 111. Which statement about lactose is incorrect?

Exam: AIPMT 2011

A. Lactose is a reducing sugar.

B. Hydrolysis of lactose produces glucose and galactose.

C. Lactose has the molecular formula C12H22O11.

D. Lactose is a non-reducing sugar and does not exhibit mutarotation.

Correct Answer: D. Lactose is a non-reducing sugar and does not exhibit mutarotation.

Explanation: Lactose is a reducing disaccharide composed of glucose and galactose. Its free anomeric carbon allows mutarotation. Its molecular formula is C12H22O11.

Question 112. Which of the following best describes the linkage between nucleotides in a DNA strand?

Exam: NEET/AIPMT

A. Peptide bond

B. Glycosidic bond alone

C. Phosphodiester bond

D. Hydrogen bond

Correct Answer: C. Phosphodiester bond

Explanation: Adjacent nucleotides in a DNA strand are joined through 3′–5′ phosphodiester bonds in the sugar-phosphate backbone. Hydrogen bonds primarily connect complementary nitrogenous bases between the two strands. Glycosidic bonds connect each nitrogenous base to its sugar.

QUICK REVISION TABLE

Topic	Key fact
Non-protein enzyme	Ribozyme (RNA)
Mutarotation	Sucrose does not show it
Fat-soluble vitamins	A, D, E and K
Lactose	Reducing sugar; glucose + galactose
DNA backbone	Phosphodiester bonds

Question 113. Which of the following statements is incorrect regarding enzyme action?

Exam: AIPMT 2014

A. A non-competitive inhibitor binds at a site other than the active site.

B. Competitive inhibition can be reduced by increasing substrate concentration.

C. Enzymes increase the activation energy of a reaction.

D. Enzymes can form an enzyme–substrate complex.

Correct Answer: C. Enzymes increase the activation energy of a reaction.

Explanation: Enzymes lower the activation energy by providing an alternative reaction pathway. They bind substrates to form enzyme–substrate complexes. A competitive inhibitor competes for the active site, whereas a non-competitive inhibitor binds elsewhere and reduces enzyme activity.

Question 114. Uridine, present in RNA, consists of:

Exam: NEET-UG 2013 (Karnataka)

A. Uracil and ribose

B. Uracil and deoxyribose

C. Uracil, ribose and phosphate

D. Uracil and phosphate

Correct Answer: A. Uracil and ribose

Explanation: Uridine is a nucleoside formed from the nitrogenous base uracil and the pentose sugar ribose. When a phosphate group is attached, it forms a nucleotide such as uridine monophosphate (UMP).

Question 115. A phosphoglyceride is always made up of:

Exam: NEET-UG 2013

A. Only glycerol and fatty acids

B. Glycerol, fatty acids, phosphate and a polar group

C. Glycerol and three fatty acids

D. Amino acids and phosphate

Correct Answer: B. Glycerol, fatty acids, phosphate and a polar group

Explanation: A typical phosphoglyceride contains a glycerol backbone, two fatty acid chains, a phosphate group and a polar head group. These molecules are amphipathic, with a hydrophilic head and hydrophobic tails, making them important components of cell membranes.

Question 116. Which option correctly describes the formation of sucrose?

Exam: AIPMT 2012

A. Alpha-D-galactopyranose and alpha-D-glucopyranose

B. Alpha-D-glucopyranose and beta-D-fructofuranose

C. Beta-D-galactopyranose and alpha-D-fructofuranose

D. Alpha-D-galactopyranose and beta-D-fructopyranose

Correct Answer: B. Alpha-D-glucopyranose and beta-D-fructofuranose

Explanation: Sucrose consists of alpha-D-glucose and beta-D-fructose joined by a glycosidic bond between their anomeric carbons. Because both anomeric carbons participate in this bond, sucrose is a non-reducing sugar.

Question 117. Which one of the following is the most abundant protein in the animal world?

Exam: AIPMT 2012 Prelims

A. Insulin

B. Collagen

C. Trypsin

D. Haemoglobin

Correct Answer: B. Collagen

Explanation: Collagen is the most abundant protein in the animal kingdom. It provides structural support to connective tissues such as skin, tendons, ligaments and cartilage.

QUICK REVISION: QUESTIONS 113–117

Topic	Key fact
Enzyme action	Lowers activation energy
Uridine	Uracil + ribose
Phosphoglyceride	Glycerol + fatty acids + phosphate + polar group
Sucrose	Alpha-D-glucose + beta-D-fructose
Most abundant animal protein	Collagen

Question 118. The diagram represents a small organic molecule found in living tissues. Which category and component correctly identify it?

Exam: AIPMT 2012 Prelims

A. Cholesterol — Guanine

B. Amino acid — NH2

C. Nucleotide — Adenine

D. Nucleoside — Uracil

Correct Answer: D. Nucleoside — Uracil

Explanation: A nucleoside consists of a nitrogenous base attached to a pentose sugar. It does not contain a phosphate group. Uridine, for example, consists of uracil and ribose. A nucleotide contains a nitrogenous base, a pentose sugar and at least one phosphate group.

Question 119. Which one of the following structural formulae represents a basic amino acid?

Exam: AIPMT 2012 Prelims

A. An amino acid with a hydroxyl-containing side chain

B. An amino acid with a single methyl side chain

C. An amino acid with a carboxyl-containing side chain

D. An amino acid with an additional amino group in its side chain

Correct Answer: D. An amino acid with an additional amino group in its side chain

Explanation: Basic amino acids, such as lysine and arginine, have basic groups in their side chains. These groups can accept protons and often carry a positive charge at physiological pH. This question was originally presented using structural diagrams; the options here describe those structures in words.

Question 120. Which of the following statements about enzymes is incorrect?

Exam: NEET-UG 2013 (Karnataka)

A. Enzymes are denatured at sufficiently high temperatures.

B. Enzymes are mostly proteins, but some are lipids too.

C. Enzymes are highly specific.

D. Enzymes require optimum pH and temperature for maximum activity.

Correct Answer: B. Enzymes are mostly proteins, but some are lipids too.

Explanation: Most enzymes are proteins, while some RNA molecules act as catalysts and are called ribozymes. Lipids are not recognised as a class of catalytic enzymes. Enzyme activity is influenced by temperature and pH, and sufficiently extreme conditions can denature protein enzymes.

Question 121. Which of the following is the most abundant protein in the whole biosphere?

Exam: NEET-UG 2023

A. Collagen

B. RuBisCO

C. Haemoglobin

D. Insulin

Correct Answer: B. RuBisCO

Explanation: RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyses carbon dioxide fixation in the Calvin cycle of photosynthesis. Because it occurs widely in photosynthetic organisms, it is commonly described as the most abundant protein in the biosphere.

Question 122. Which of the following statements about enzymes is wrong?

Exam: NEET-UG 2013

A. Enzymes are denatured at sufficiently high temperatures.

B. Enzymes are highly specific.

C. Enzymes require suitable pH and temperature for maximum activity.

D. Enzymes are mostly proteins, but some are lipids too.

Correct Answer: D. Enzymes are mostly proteins, but some are lipids too.

Explanation: This is a rephrased version of Question 120 and should not be counted as a separate unique PYQ. Keep one version in the final ebook.

REVISION SUMMARY: QUESTIONS 118–122

Topic	Key fact
Nucleoside	Nitrogenous base + pentose sugar
Basic amino acid	Has a basic side chain
Ribozymes	Catalytic RNA molecules
Most abundant protein in biosphere	RuBisCO
Most abundant protein in animal kingdom	Collagen

Question 123. The curve shows enzymatic activity in relation to temperature. What do the x-axis and y-axis represent?

Exam: AIPMT 2011 Prelims

A. X-axis: Enzymatic activity; Y-axis: Temperature

B. X-axis: Enzymatic activity; Y-axis: pH

C. X-axis: Temperature; Y-axis: Enzymatic activity

D. X-axis: Substrate concentration; Y-axis: Enzyme concentration

Correct Answer: C. X-axis: Temperature; Y-axis: Enzymatic activity

Explanation: Enzyme activity generally increases as temperature rises until the optimum temperature is reached. Above the optimum, activity often declines because the enzyme’s structure may become disrupted. The exact response depends on the enzyme and its source.

Question 124. Which statements regarding denaturation are correct?

Exam: AIPMT 2011

(a) Denaturation of proteins causes loss of secondary and tertiary structures.

(b) Denaturation can convert double-stranded DNA into single strands.

(c) Denaturation normally breaks the primary structure of proteins.

A. (a), (b) and (c)

B. (b) and (c) only

C. (a) and (c) only

D. (a) and (b) only

Correct Answer: D. (a) and (b) only

Explanation: Protein denaturation disrupts secondary and tertiary structure but normally leaves the primary amino acid sequence intact. DNA denaturation separates its two strands by disrupting the interactions that hold them together, particularly hydrogen bonding between complementary bases.

Question 125. Which of the following is NOT a fat-soluble vitamin?

Exam: AIPMT 2011

A. Vitamin A

B. Vitamin B complex

C. Vitamin D

D. Vitamin E

Correct Answer: B. Vitamin B complex

Explanation: Vitamins A, D, E and K are fat-soluble. B-complex vitamins and vitamin C are water-soluble.

Question 126. Which one of the following structural formulae of two organic compounds is correctly identified along with its related function?

Exam: AIPMT 2011 Prelims

A. Adenine — a nucleotide that makes up nucleic acids

B. Triglyceride — a major source of energy

C. Uracil — a component of DNA

D. Lecithin — a component of the cell membrane

Correct Answer: D. Lecithin — a component of the cell membrane

Explanation: Lecithin is a phospholipid found in cell membranes. Adenine is a nitrogenous base, not a complete nucleotide. Uracil is normally found in RNA rather than DNA. Triglycerides are an important energy reserve, but the original question uses structural diagrams to identify the compounds, so the answer depends on the diagram as well as the labels.

Question 127. Which one of the following enzymes carries out the initial step in the digestion of milk protein in adult humans?

Exam: AIPMT 2011 Prelims

A. Trypsin

B. Pepsin

C. Rennin

D. Lipase

Correct Answer: B. Pepsin

Explanation: Pepsin is a stomach enzyme that begins protein digestion in adults. It breaks proteins into smaller peptides in the acidic environment of the stomach. Rennin (chymosin) is particularly important in milk coagulation in young mammals.

Question 128. Which of the following statements about enzymes is wrong?

Exam: NEET-UG 2013 (Karnataka)

A. Enzymes are highly specific.

B. Enzymes generally have an optimum pH.

C. Some RNA molecules can act as enzymes.

D. Some enzymes are lipids.

Correct Answer: D. Some enzymes are lipids.

Explanation: Most enzymes are proteins, and some RNA molecules called ribozymes are catalytic. Lipids are not recognised as a class of enzymes. Enzyme activity depends on factors such as temperature, pH and substrate concentration.

Question 129. Which one of the following biomolecules is correctly characterised?

Exam: AIPMT 2012 Mains

A. Palmitic acid — an unsaturated fatty acid with 18 carbon atoms

B. Adenylic acid — adenosine with a glucose phosphate molecule

C. Alanine — contains amino and acidic groups

D. Lecithin — a phosphorylated glyceride found in cell membranes

Correct Answer: D. Lecithin — a phosphorylated glyceride found in cell membranes

Explanation: Lecithin is a phosphoglyceride present in biological membranes. Palmitic acid is a saturated fatty acid with 16 carbon atoms. Adenylic acid contains adenine, ribose and phosphate. Alanine contains both an amino group and a carboxyl group, but the original option’s wording is imprecise; the intended answer in the cited paper is lecithin.

Question 130. Which of the following is NOT a fat-soluble vitamin?

Exam: AIPMT 2011 Prelims

A. Vitamin A

B. Vitamin B complex

C. Vitamin D

D. Vitamin E

Correct Answer: B. Vitamin B complex

Explanation: Vitamins A, D, E and K are fat-soluble. B-complex vitamins and vitamin C are water-soluble.

REVISION TABLE

Topic	Key fact
Lecithin	Phospholipid in cell membranes
Initial digestion of adult milk protein	Pepsin
Catalytic RNA	Ribozyme
Palmitic acid	Saturated fatty acid with 16 carbon atoms
Fat-soluble vitamins	A, D, E and K

Question 131. The figure shows a hypothetical tetrapeptide portion of a protein with parts labelled A–D. Which option correctly identifies the labelled components?

Exam: NEET 2013 (Karnataka)

A. A — peptide bond; B — amino group; C — side chain; D — carboxyl group

B. A — glycosidic bond; B — phosphate group; C — sugar; D — nitrogenous base

C. A — ester bond; B — fatty acid; C — glycerol; D — phosphate group

D. A — hydrogen bond; B — nucleotide; C — ribose; D — nitrogenous base

Correct Answer: Cannot be assigned reliably without the original labelled diagram.

Explanation: A tetrapeptide consists of four amino acid residues joined by three peptide bonds. Its structure contains a terminal amino group, a terminal carboxyl group and amino acid side chains. Since the question depends on labels in a diagram, the original figure must be checked before selecting an answer.

Question 132. Macromolecule chitin is:

Exam: NEET 2013

A. A nitrogen-containing polysaccharide

B. A phosphorus-containing polysaccharide

C. A lipid

D. A protein

Correct Answer: A. A nitrogen-containing polysaccharide

Explanation: Chitin is a structural polysaccharide made of repeating N-acetylglucosamine units joined by beta-1,4 glycosidic bonds. It occurs in arthropod exoskeletons and fungal cell walls.

Question 133. Which one of the following statements is incorrect about competitive enzyme inhibition?

Exam: AIPMT 2015

A. The inhibitor resembles the substrate structurally.

B. The inhibitor competes with the substrate for the active site.

C. Increasing substrate concentration can reduce inhibition.

D. The inhibitor must bind permanently to the enzyme.

Correct Answer: D. The inhibitor must bind permanently to the enzyme.

Explanation: Competitive inhibitors commonly bind reversibly to an enzyme’s active site. Because the inhibitor and substrate compete for the same site, increasing substrate concentration can reduce the inhibitor’s effect. Permanent binding is not a defining feature of competitive inhibition.

Question 134. Which one of the following is the least likely to be involved in stabilising the three-dimensional folding of most proteins?

Exam: NEET 2016 Phase 2

A. Hydrogen bonds

B. Ester bonds

C. Hydrophobic interactions

D. Electrostatic interactions

Correct Answer: B. Ester bonds

Explanation: Protein folding is stabilised by hydrogen bonds, hydrophobic interactions, electrostatic interactions, van der Waals forces and, in some proteins, disulphide bonds. Ester bonds are not a typical major stabilising interaction in protein folding.

Question 135. Which of the following statements about enzymes is incorrect?

Exam: AIPMT 2015 Cancelled Paper

A. Enzymes lower the activation energy of reactions.

B. Enzymes are highly specific in their action.

C. Enzyme activity may be affected by temperature and pH.

D. Enzymes change the equilibrium constant of a reaction.

Correct Answer: D. Enzymes change the equilibrium constant of a reaction.

Explanation: Enzymes increase the rate at which equilibrium is reached, but they do not change the equilibrium constant or the overall free-energy difference between reactants and products. They accelerate the forward and reverse reactions by lowering the activation-energy barrier.

QUICK REVISION TABLE

Topic	Key fact
Tetrapeptide	Four amino acid residues
Chitin	Nitrogen-containing polysaccharide
Competitive inhibition	Substrate and inhibitor compete for the active site
Protein folding	Stabilised by several non-covalent interactions
Enzymes	Do not change the equilibrium constant

Question 136. Enzyme catalysis

Exam: AIPMT 2012 Prelims

Which one of the following statements is incorrect about enzyme catalysis?

A. Enzymes are least reactive at optimum temperature.

B. Enzymes are mostly proteinous in nature.

C. Enzyme action is specific.

D. Enzymes are denatured by ultraviolet rays and at high temperature.

Correct Answer: A. Enzymes are least reactive at optimum temperature.

Explanation: Enzymes generally show maximum activity under their optimum temperature and pH conditions. At temperatures that are too high, their three-dimensional structure may be disrupted, causing denaturation and loss of activity.

Question 137. Correctly characterised biomolecule

Exam: AIPMT 2012 Mains

Which one of the following biomolecules is correctly characterized?

A. Lecithin — a phosphorylated glyceride found in the cell membrane

B. Palmitic acid — an unsaturated fatty acid with 18 carbon atoms

C. Adenylic acid — adenosine with a glucose phosphate molecule

D. Alanine — contains amino and acidic groups that may occur anywhere in the molecule

Correct Answer: A. Lecithin — a phosphorylated glyceride found in the cell membrane.

Explanation: Lecithin is a phospholipid found in cell membranes. Palmitic acid is a saturated fatty acid containing 16 carbon atoms. Adenylic acid, or AMP, contains adenine, ribose and phosphate—not glucose phosphate. In standard amino acids such as alanine, the amino and carboxyl groups are attached to the alpha carbon.

Question 138. Essential components of coenzymes

Exam: NEET 2013

The essential chemical components of many coenzymes are:

A. Proteins

B. Nucleic acids

C. Carbohydrates

D. Vitamins

Correct Answer: D. Vitamins

Explanation: Many coenzymes are derived from vitamins. For example, NAD and NADP contain a component derived from niacin (vitamin B3), while FAD is derived from riboflavin (vitamin B2). Coenzymes assist enzymes in transferring electrons or chemical groups during reactions.

Question 139. Most abundant protein in the animal world

Exam: AIPMT 2012 Prelims

Which one is the most abundant protein in the animal world?

A. Trypsin

B. Haemoglobin

C. Collagen

D. Insulin

Correct Answer: C. Collagen

Explanation: Collagen is a fibrous structural protein abundant in connective tissues, including skin, tendons, cartilage and bone. Remember the distinction: collagen is the most abundant protein in the animal world, whereas RuBisCO is commonly described as the most abundant protein in the biosphere.

Question 140. Enzyme specificity and high temperature

Exam: AIPMT 2010 Mains

Three of the following statements about enzymes are correct and one is wrong. Which one is wrong?

A. Enzymes require an optimum pH for maximal activity.

B. Enzymes are denatured at high temperatures, but some enzymes in exceptional organisms remain active at 80–90°C.

C. Enzymes are highly specific.

D. Most enzymes are proteins, but some are lipids.

Correct Answer: D. Most enzymes are proteins, but some are lipids.

Explanation: Most enzymes are proteins, but some catalytic molecules are RNA molecules called ribozymes. Lipids are not the recognised non-protein class of enzymes. Certain thermophilic organisms possess enzymes adapted to high temperatures.

Question 141. Carboxypeptidase cofactor

Exam: AIPMT 2012 Mains

For its activity, carboxypeptidase requires:

A. Iron

B. Zinc

C. Niacin

D. Copper

Correct Answer: B. Zinc

Explanation: Carboxypeptidase is a zinc-containing metalloenzyme. Zinc acts as a metal-ion cofactor and helps the enzyme catalyse the hydrolysis of peptide bonds near the carboxyl end of a polypeptide chain.

Question 142. Enzyme action and activation energy

Exam: AIPMT 2010

Which statement best describes the effect of an enzyme on a chemical reaction?

A. It increases the activation energy.

B. It lowers the activation energy.

C. It changes the overall free-energy change of the reaction.

D. It changes the equilibrium constant of the reaction.

Correct Answer: B. It lowers the activation energy.

Explanation: Enzymes provide an alternative reaction pathway with lower activation energy. They do not change the overall free-energy difference between reactants and products or the equilibrium constant. This is a concept-based question; I could not independently verify this exact wording as the original AIPMT 2010 question, so it should not be published as a verbatim PYQ without checking the original paper.

Question 143. Protein portion of an enzyme

Exam: NEET 2025

The protein portion of an enzyme is called:

A. Coenzyme

B. Apoenzyme

C. Prosthetic group

D. Substrate

Correct Answer: B. Apoenzyme

Explanation: The protein portion of a conjugated enzyme is called the apoenzyme. It requires its appropriate non-protein component, such as a cofactor or coenzyme, to become fully functional. The complete active enzyme is called a holoenzyme.

Question 144. Haem as a prosthetic group

Exam: NEET 2025

Which one of the following enzymes contains haem as a prosthetic group?

A. Catalase

B. Trypsin

C. Lipase

D. Urease

Correct Answer: A. Catalase

Explanation: Catalase is a haem-containing enzyme that catalyses the breakdown of hydrogen peroxide into water and oxygen.

Reaction:

2H2O2 → 2H2O + O2

Trypsin and lipase are digestive enzymes, while urease is a nickel-containing enzyme.

Question 145. Histones

Exam: NEET 2025

Histones are enriched with which amino acids?

A. Lysine and arginine

B. Aspartic acid and glutamic acid

C. Glycine and alanine

D. Tryptophan and methionine

Correct Answer: A. Lysine and arginine

Explanation: Histones are basic proteins rich in lysine and arginine. Their positive charges help them associate with negatively charged DNA, facilitating DNA packaging into chromatin.

Question 146. Bonds in biomolecules

Exam: NEET 2021

Match the biomolecules with their characteristic bonds.

Biomolecule	Characteristic bond
A. Protein	I. C=C double bonds
B. Unsaturated fatty acid	II. Phosphodiester bonds
C. Nucleic acid	III. Glycosidic bonds
D. Polysaccharide	IV. Peptide bonds

Choose the correct option.

A. A–IV, B–I, C–II, D–III

B. A–I, B–IV, C–III, D–II

C. A–III, B–II, C–IV, D–I

D. A–II, B–III, C–I, D–IV

Correct Answer: A. A–IV, B–I, C–II, D–III

Explanation:

Proteins contain peptide bonds linking amino acids.

Unsaturated fatty acids contain one or more carbon–carbon double bonds.

Nucleic acids contain phosphodiester bonds in their sugar-phosphate backbone.

Polysaccharides contain glycosidic bonds linking monosaccharides.

Question 147. Lipids

Exam: NEET 2021

Consider the following statements about lipids:

1. Lipids containing only single bonds are called unsaturated fatty acids.
2. Lecithin is a phospholipid.
3. Trihydroxy propane is glycerol.
4. Palmitic acid contains 20 carbon atoms, including the carboxyl carbon.
5. Arachidonic acid contains 16 carbon atoms.

Which statements are correct?

A. 1 and 2 only

B. 2 and 3 only

C. 1, 3 and 5 only

D. 2, 4 and 5 only

Correct Answer: B. 2 and 3 only

Explanation: Lecithin is a phospholipid, and glycerol is propane-1,2,3-triol. Fatty acids with only single carbon–carbon bonds are saturated, not unsaturated. Palmitic acid has 16 carbon atoms, while arachidonic acid has 20.

Question 148. Glycosidic and peptide bonds

Exam: NEET 2020 Phase 1

Identify the substances having glycosidic bonds and peptide bonds, respectively.

A. Cellulose and insulin

B. Insulin and cellulose

C. DNA and starch

D. Glycogen and DNA

Correct Answer: A. Cellulose and insulin

Explanation: Cellulose is a polysaccharide whose glucose units are connected by glycosidic bonds. Insulin is a protein made of amino acids linked by peptide bonds.

Question 149. Basic amino acid

Exam: NEET 2020 Phase 1

Which of the following is a basic amino acid?

A. Glutamic acid

B. Aspartic acid

C. Lysine

D. Serine

Correct Answer: C. Lysine

Explanation: Lysine has an additional basic amino group in its side chain. Arginine and histidine are also classified as basic amino acids. Glutamic acid and aspartic acid are acidic amino acids, while serine is polar and uncharged.

Question 150. Concanavalin A

Exam: NEET 2019

Concanavalin A is:

A. A lectin

B. A steroid hormone

C. A nucleic acid

D. A structural polysaccharide

Correct Answer: A. A lectin

Explanation: Concanavalin A is a plant lectin, a carbohydrate-binding protein obtained from jack beans. Lectins bind particular carbohydrate structures and are useful in biological research.

Question 151. Prosthetic group and apoenzyme

Exam: NEET 2019

Consider the following statements:

A. A coenzyme or metal ion that is tightly bound to an enzyme protein is called a prosthetic group.

B. A complete catalytically active enzyme with its bound prosthetic group is called an apoenzyme.

Select the correct option.

A. Both A and B are correct.

B. Both A and B are incorrect.

C. A is correct, but B is incorrect.

D. A is incorrect, but B is correct.

Correct Answer: C. A is correct, but B is incorrect.

Explanation: A tightly bound non-protein component may be called a prosthetic group. The complete, catalytically active enzyme is the holoenzyme, not the apoenzyme. The apoenzyme is the protein portion alone.

Question 152. Functional groups of sugars

Exam: NEET 2018

The two functional groups characteristic of sugars are:

A. Aldehyde and ketone

B. Alcohol and ether only

C. Amine and carboxyl

D. Thiol and phosphate

Correct Answer: A. Aldehyde and ketone

Explanation: Monosaccharides are polyhydroxy aldehydes or ketones. For example, glucose is an aldohexose, while fructose is a ketohexose. Both also contain multiple hydroxyl (–OH) groups.

Question 153. Which of the following are not polymeric?

Exam: NEET 2017

A. Proteins

B. Polysaccharides

C. Lipids

D. Nucleic acids

Correct Answer: C. Lipids

Explanation: Proteins are polymers of amino acids, polysaccharides are polymers of monosaccharides, and nucleic acids are polymers of nucleotides. Lipids are not true polymers made of repeating monomer units, although some are formed from components such as glycerol and fatty acids.

Question 154. Correct statement regarding enzymes

Exam: NEET 2017

Which of the following statements is correct with reference to enzymes?

A. Holoenzyme = Apoenzyme + Coenzyme

B. Coenzyme = Apoenzyme + Holoenzyme

C. Holoenzyme = Coenzyme + Cofactor

D. Apoenzyme = Holoenzyme + Coenzyme

Correct Answer: A. Holoenzyme = Apoenzyme + Coenzyme

Explanation: The apoenzyme is the protein part of an enzyme. The holoenzyme is the complete, active enzyme. More generally, a holoenzyme consists of an apoenzyme plus its required cofactor, which may be a metal ion, a coenzyme or a tightly bound prosthetic group. Option A is correct when the required cofactor is a coenzyme.

Question 155. Non-proteinaceous enzyme

Exam: NEET 2016 Phase 2

A non-proteinaceous enzyme is:

A. Lysozyme

B. Ribozyme

C. Ligase

D. Deoxyribonuclease

Correct Answer: B. Ribozyme

Explanation: A ribozyme is an RNA molecule that catalyses a chemical reaction. Its catalytic activity demonstrates that not all biological catalysts are proteins. Ribozymes are found in biological systems, including the catalytic centre of the ribosome.

Question 156. Bonds stabilising protein structure

Exam: NEET 2016 Phase 2

Which of the following is least likely to be involved in stabilising the three-dimensional folding of most proteins?

A. Hydrogen bonds

B. Ionic interactions

C. Hydrophobic interactions

D. Ester bonds

Correct Answer: D. Ester bonds

Explanation: Protein folding is stabilised by hydrogen bonds, ionic interactions, hydrophobic interactions and, in many proteins, disulphide bonds. Ester bonds are not a typical major interaction responsible for stabilising the three-dimensional folding of most proteins.

Question 157. Two polypeptide chains of insulin

Exam: NEET 2016 Phase 1

The two polypeptide chains of human insulin are linked together by:

A. Hydrogen bonds

B. Phosphodiester bonds

C. Covalent bonds

D. Glycosidic bonds

Correct Answer: C. Covalent bonds

Explanation: The two polypeptide chains of insulin are linked by disulphide bridges, which are covalent bonds formed between the sulfur atoms of cysteine residues. Therefore, option C is the best answer among the given options.

Question 158. Enzyme inhibition by malonate

Exam: AIPMT 2014

Select the option that is not correct with respect to enzyme action.

A. The substrate binds to the enzyme at its active site.

B. Adding a large amount of succinate does not reverse the inhibition of succinate dehydrogenase by malonate.

C. A non-competitive inhibitor binds to a site distinct from the substrate-binding site.

D. Malonate is a competitive inhibitor of succinate dehydrogenase.

Correct Answer: B. Adding a large amount of succinate does not reverse the inhibition.

Explanation: Malonate resembles succinate and competes with it for the active site of succinate dehydrogenase. Increasing the succinate concentration can overcome this competitive inhibition. Therefore, option B is incorrect.

Question 159. Non-reducing carbohydrate

Exam: AIPMT 2014

Which one of the following is a non-reducing carbohydrate?

A. Maltose B. Sucrose C. Lactose D. Ribose 5-phosphate

Correct Answer: B. Sucrose

Explanation: Sucrose is a non-reducing disaccharide because the anomeric carbons of both glucose and fructose participate in the glycosidic bond. It has no free anomeric carbon to act as a reducing group. Maltose and lactose are reducing sugars.

Question 160. Incorrect statement about competitive inhibition

Exam: AIPMT 2015

Which statement about competitive inhibition is incorrect?

A. A competitive inhibitor can reversibly bind to an enzyme.

B. The inhibitor is not chemically changed by the enzyme.

C. A competitive inhibitor competes with the substrate for the active site.

D. The presence of a competitive inhibitor decreases the Km of the enzyme for its substrate.

Correct Answer: D. The presence of a competitive inhibitor decreases the Km of the enzyme for its substrate.

Explanation: In the standard Michaelis–Menten model, competitive inhibition increases the apparent Km because a higher substrate concentration is needed to reach half the maximum reaction rate. Vmax remains unchanged when sufficient substrate is available.

Question 161. Transition state in enzyme catalysis

Exam: AIPMT 2013

The transition-state structure formed during an enzyme-catalysed reaction is:

A. Permanent and unstable

B. Transient and unstable

C. Permanent and stable

D. Transient but stable

Correct Answer: B. Transient and unstable

Explanation: The transition state is a short-lived, high-energy arrangement of atoms formed as reactants are converted into products. Enzymes lower the activation energy by stabilising the transition state relative to the uncatalysed reaction.

Question 162. Identification of a nucleotide

Exam: NEET 2024 Re-Examination

Which of the following is a nucleotide?

A. Adenine

B. Adenosine

C. Adenylic acid

D. Ribose

Correct Answer: C. Adenylic acid

Explanation: A nucleotide consists of a nitrogenous base, a pentose sugar and one or more phosphate groups. Adenylic acid (AMP) contains adenine, ribose and phosphate. Adenine alone is a nitrogenous base, while adenosine is a nucleoside containing adenine and ribose.

Question 163. Enzyme class: Lyases

Exam: NEET 2024 Re-Examination

Enzymes that catalyse the removal of groups from substrates by mechanisms other than hydrolysis, often leaving double bonds, are called:

A. Hydrolases

B. Lyases

C. Ligases

D. Isomerases

Correct Answer: B. Lyases

Explanation: Lyases remove groups without hydrolysis or oxidation, often producing a double bond. Hydrolases catalyse bond cleavage using water; ligases join molecules; and isomerases rearrange atoms within a molecule.

Question 164. Protein structural organisation

Exam: NEET 2024 Re-Examination

Match the levels of protein structure with their descriptions.

List I	List II
A. Primary structure	I. Alpha helix and beta-pleated sheet
B. Secondary structure	II. Arrangement of multiple polypeptide subunits
C. Tertiary structure	III. Amino acid sequence
D. Quaternary structure	IV. Overall three-dimensional folding of one polypeptide

Choose the correct option.

A. A–III, B–I, C–IV, D–II

B. A–I, B–III, C–II, D–IV

C. A–IV, B–II, C–III, D–I

D. A–II, B–IV, C–I, D–III

Correct Answer: A. A–III, B–I, C–IV, D–II

Explanation: Primary structure is the amino acid sequence. Secondary structure includes alpha helices and beta sheets. Tertiary structure describes the overall folding of one polypeptide, while quaternary structure describes the association of multiple polypeptide chains.

Question 165. Inulin

Exam: NEET 2023 Manipur

Inulin is a polymer of:

A. Glucose

B. Fructose

C. Galactose

D. Ribose

Correct Answer: B. Fructose

Explanation: Inulin is a storage polysaccharide composed mainly of fructose units. It occurs in the roots and underground storage organs of certain plants, such as chicory.

Question 166. Biological functions of biomolecules

Exam: NEET 2022 Phase 1

Match the biomolecules with their biological functions.

Biomolecule	Function
A. Glycogen	I. Hormone
B. Globulin	II. Biocatalyst
C. Steroids	III. Antibody
D. Thrombin	IV. Storage product

Choose the correct option.

A. A–IV, B–III, C–I, D–II

B. A–III, B–IV, C–II, D–I

C. A–I, B–II, C–IV, D–III

D. A–II, B–I, C–III, D–IV

Correct Answer: A. A–IV, B–III, C–I, D–II

Explanation: Glycogen stores glucose, certain globulins function as antibodies, some steroids function as hormones, and thrombin is an enzyme involved in blood clotting.

Question 167. Formula of maltose

Exam: NEET 2022 Phase 1

A dehydration reaction links two glucose molecules to form maltose. If the formula of glucose is C6H12O6, what is the molecular formula of maltose?

A. C6H12O6

B. C12H24O12

C. C12H22O11

D. C12H20O10

Correct Answer: C. C12H22O11

Explanation: Two glucose molecules combine with the loss of one water molecule.

C6H12O6 + C6H12O6 − H2O = C12H22O11

Therefore, maltose has the molecular formula C12H22O11.

Question 168. Secondary metabolites

Exam: NEET 2022 Phase 2

Match the compounds with their respective categories.

Compound	Category
A. Adenine	I. Pigment
B. Anthocyanin	II. Polysaccharide
C. Chitin	III. Alkaloid
D. Codeine	IV. Purine

Choose the correct option.

A. A–IV, B–I, C–II, D–III

B. A–I, B–II, C–III, D–IV

C. A–III, B–IV, C–I, D–II

D. A–II, B–III, C–IV, D–I

Correct Answer: A. A–IV, B–I, C–II, D–III

Explanation: Adenine is a purine nitrogenous base, anthocyanins are plant pigments, chitin is a structural polysaccharide, and codeine is an alkaloid.

Question 169. Zwitterionic nature of amino acids

Exam: NEET 2022 Phase 2

Consider the following statements.

1. Amino acids have ionisable –NH2 and –COOH groups and can exist in different ionic forms at different pH values.
2. Amino acids can exist as zwitterions.

Choose the correct answer.

A. Both statements are correct.

B. Both statements are incorrect.

C. Statement 1 is correct, but Statement 2 is incorrect.

D. Statement 1 is incorrect, but Statement 2 is correct.

Correct Answer: A. Both statements are correct.

Explanation: Amino acids contain both acidic and basic groups. In a zwitterionic form, the amino group is protonated (–NH3+) and the carboxyl group is deprotonated (–COO−). The relative ionic forms depend on the pH of the surrounding solution.

Question 170. Lipids and cell membranes

Exam: NEET 2022 Phase 1

Which of the following statements about lipids is correct?

A. Lecithin is a glycolipid, and saturated fatty acids contain carbon–carbon double bonds.

B. Lipids are generally insoluble in water but soluble in certain organic solvents.

C. All lipids are polymers of repeating fatty acid units.

D. Glycerol contains two hydroxyl groups.

Correct Answer: B. Lipids are generally insoluble in water but soluble in certain organic solvents.

Explanation: Most lipids are hydrophobic or poorly soluble in water and dissolve in suitable organic solvents. Lecithin is a phospholipid, not a glycolipid. Saturated fatty acids have no carbon–carbon double bonds, and glycerol contains three hydroxyl groups.

Question 171. Enzyme class

Exam: NEET 2025

An enzyme that transfers a functional group from one substrate to another belongs to which class?

A. Hydrolases

B. Transferases

C. Lyases

D. Isomerases

Correct Answer: B. Transferases

Explanation: Transferases catalyse the transfer of a functional group from one molecule to another. Kinases, for example, transfer phosphate groups, commonly from ATP to a substrate.

Question 172. Why insulin cannot be given orally

Exam: NEET 2025

Why is insulin generally not administered orally to treat diabetes mellitus?

A. It is insoluble in water.

B. It is digested by enzymes in the gastrointestinal tract.

C. It cannot bind to receptors in the intestine.

D. It is a carbohydrate rather than a protein.

Correct Answer: B. It is digested by enzymes in the gastrointestinal tract.

Explanation: Insulin is a peptide hormone. When swallowed, it can be broken down by digestive enzymes and may not reach the bloodstream intact in a predictable, effective amount. Therefore, conventional insulin therapy is usually administered by injection or other specifically designed delivery systems.

Question 173. Alpha-helix

Exam: NEET 2026 (Cancelled Paper)

An alpha-helix is a feature of which level of protein structure?

A. Primary structure

B. Secondary structure

C. Tertiary structure only

D. Quaternary structure

Correct Answer: B. Secondary structure

Explanation: An alpha-helix is a regular, coiled arrangement of a polypeptide chain stabilised mainly by hydrogen bonds between backbone groups. A beta-pleated sheet is another common form of secondary protein structure.

Question 174. Correct statements about biomolecules

Exam: NEET 2026 (Cancelled Paper)

Consider the following statements:

1. Lipids are generally soluble in water.
2. Proteins are polypeptides.
3. Polysaccharides are polymers of monosaccharides.
4. Nucleic acids are polymers of nucleotides.

Which statements are correct?

A. 1 and 2 only

B. 2, 3 and 4 only

C. 1, 3 and 4 only

D. 1, 2, 3 and 4

Correct Answer: B. 2, 3 and 4 only

Explanation: Proteins consist of amino acid chains, polysaccharides consist of linked monosaccharides, and nucleic acids consist of nucleotides. Most lipids are poorly soluble in water, so statement 1 is incorrect.

Question 175. Amino acids

Exam: NEET 2026 (Cancelled Paper)

Which statement about amino acids is correct?

A. All amino acids have aromatic side chains.

B. Amino acids contain both amino and carboxyl functional groups.

C. Serine is an aromatic amino acid.

D. Amino acids contain no nitrogen.

Correct Answer: B. Amino acids contain both amino and carboxyl functional groups.

Explanation: Standard amino acids contain an amino group and a carboxyl group attached to the alpha carbon, along with a hydrogen atom and a variable side chain. Serine is a polar amino acid, not an aromatic amino acid.

Question 176. Concanavalin A

Exam: NEET 2026 (Cancelled Paper)

Concanavalin A is classified as a:

A. Lectin

B. Steroid

C. Polysaccharide

D. Nucleic acid

Correct Answer: A. Lectin

Explanation: Concanavalin A is a carbohydrate-binding protein obtained from jack beans. Lectins recognise and bind specific carbohydrate structures and are used in biological research.

Question 177. Antibodies

Exam: NEET 2026 (Re-NEET)

The primary biological function of antibodies is to:

A. Store glucose

B. Fight infection by recognising specific antigens

C. Catalyse all metabolic reactions

D. Form the cellulose cell wall

Correct Answer: B. Fight infection by recognising specific antigens

Explanation: Antibodies are immunoglobulin proteins produced by B lymphocytes and plasma cells. They bind specific antigens and help the immune system neutralise pathogens or mark them for destruction.

Question 178. Which statement about denaturation of proteins is correct?

Exam: Odisha NEET 2019

A. Denaturation always breaks peptide bonds.

B. Denaturation destroys the primary structure of a protein.

C. Denaturation can disrupt secondary and tertiary structures while leaving the primary structure intact.

D. Denaturation converts every protein into an active enzyme.

Correct Answer: C. Denaturation can disrupt secondary and tertiary structures while leaving the primary structure intact.

Explanation: Denaturation alters the native three-dimensional structure of a protein, commonly affecting its secondary, tertiary and sometimes quaternary structure. The peptide bonds of the primary structure generally remain intact.

Question 179. Non-essential amino acid

Exam: NEET 2019

Which of the following is a non-essential amino acid in humans?

A. Lysine

B. Valine

C. Leucine

D. Alanine

Correct Answer: D. Alanine

Explanation: Alanine can normally be synthesised by the human body. Lysine, valine and leucine are essential amino acids and must be obtained from the diet.

Question 180. Amylose and amylopectin

Exam: NEET 2018

Which statement correctly distinguishes amylose from amylopectin?

A. Amylose is highly branched, while amylopectin is unbranched.

B. Amylose is mainly unbranched, while amylopectin is branched.

C. Both are made of amino acids.

D. Amylose contains peptide bonds, while amylopectin contains phosphodiester bonds.

Correct Answer: B. Amylose is mainly unbranched, while amylopectin is branched.

Explanation: Both are components of starch and consist of glucose units. Amylose is mainly composed of alpha-1,4 glycosidic linkages. Amylopectin contains alpha-1,4 linkages in its chains and alpha-1,6 linkages at branch points.

Question 181. Water-soluble vitamins

Exam: AIPMT 2011 Prelims

Which of the following is not a fat-soluble vitamin?

A. Vitamin A

B. Vitamin D

C. Vitamin E

D. Vitamin B complex

Correct Answer: D. Vitamin B complex

Explanation: Vitamins A, D, E and K are fat-soluble. The B-complex vitamins and vitamin C are water-soluble.

Question 182. Denaturation of proteins and DNA

Exam: AIPMT 2011 Mains

Consider the following statements:

1. Denaturation of proteins causes loss of secondary and tertiary structures.
2. Denaturation of DNA can convert its double-stranded structure into single strands.
3. Denaturation necessarily destroys the primary structure of a protein.

Which statements are correct?

A. 1 and 2 only

B. 1 and 3 only

C. 2 and 3 only

D. 1, 2 and 3

Correct Answer: A. 1 and 2 only

Explanation: Protein denaturation usually disrupts higher-order structures without breaking the peptide bonds of the primary structure. DNA denaturation separates the two strands by disrupting base-pairing interactions, without normally breaking the covalent sugar-phosphate backbone.

Question 183. Structural formula and biological function

Exam: AIPMT 2011 Prelims

Which of the following pairs is correctly identified along with its related function?

A. Uracil — a component of DNA

B. Triglyceride — a major energy-storage molecule

C. Adenine — a complete nucleotide

D. Lecithin — a carbohydrate reserve in plants

Correct Answer: B. Triglyceride — a major energy-storage molecule

Explanation: Triglycerides are important energy-storage lipids. Uracil is normally found in RNA rather than DNA. Adenine is a nitrogenous base, not a complete nucleotide. Lecithin is a phospholipid associated with cell membranes.

Question 184. Enzyme reaction diagram

Exam: AIPMT 2010 Mains

In an enzyme-reaction energy diagram, which statement correctly describes the activation energy?

A. Activation energy is always higher in the presence of an enzyme.

B. An enzyme lowers the activation energy required for the reaction.

C. An enzyme changes the overall energy difference between reactants and products.

D. An enzyme changes the equilibrium constant.

Correct Answer: B. An enzyme lowers the activation energy required for the reaction.

Explanation: Enzymes accelerate reactions by providing a pathway with a lower activation-energy barrier. They do not change the reaction’s equilibrium constant or the overall free-energy difference between reactants and products. This is a concept-based reformulation of the AIPMT 2010 diagram question, not a verbatim reproduction of its labelled figure.

Question 185. Non-protein component of an enzyme

Exam: NEET/AIPMT

Which of the following is a non-protein component that may be required for enzyme activity?

A. Cofactor

B. Polypeptide chain

C. Substrate protein

D. Peptide bond

Correct Answer: A. Cofactor

Explanation: A cofactor is a non-protein component needed by some enzymes. It may be a metal ion, such as zinc, or an organic molecule such as a coenzyme. The protein portion is the apoenzyme; the complete active enzyme with its required component is the holoenzyme.

Question 186. ATP is

Exam: AIPMT 2000

ATP is classified as a:

A. Nucleotide

B. Nucleoside

C. Nucleic acid

D. Vitamin

Correct Answer: A. Nucleotide

Explanation: ATP (adenosine triphosphate) contains adenine, ribose and three phosphate groups. Because it contains a nitrogenous base, a pentose sugar and phosphate groups, it is a nucleotide. It serves as an important energy-transfer molecule in cells.

Question 187. Carbohydrate-containing conjugated protein

Exam: AIPMT 2000

Which of the following has a carbohydrate component attached to its protein portion?

A. Glycoprotein

B. Chromoprotein

C. Lipoprotein

D. Nucleoprotein

Correct Answer: A. Glycoprotein

Explanation: Glycoproteins contain carbohydrate groups covalently attached to proteins. They play important roles in cell recognition, immunity and cell-surface interactions.

Question 188. Enzymes in viruses

Exam: AIPMT 2000

Which option was identified in the original question as an organismal group in which enzymes are not found?

A. Fungi

B. Algae

C. Viruses

D. Cyanobacteria

Expected textbook answer: C. Viruses

Explanation: Viruses lack independent cellular metabolism and depend on host cells for replication. However, the wording is scientifically oversimplified: some viruses carry enzymes, and viral replication may involve enzymes supplied by either the virus or its host.

Question 189. Feedback inhibition

Exam: AIPMT 2000

Which factor is responsible for inhibition of an enzymatic process during feedback regulation?

A. Substrate

B. Enzyme

C. End product

D. Temperature

Correct Answer: C. End product

Explanation: In feedback inhibition, the final product of a metabolic pathway inhibits an enzyme involved in an earlier step. This prevents unnecessary production and conserves cellular resources.

Question 190. Essential amino acid

Exam: AIPMT 2000

Which of the following is an essential amino acid in humans?

A. Phenylalanine

B. Glycine

C. Aspartic acid

D. Serine

Correct Answer: A. Phenylalanine

Explanation: Phenylalanine must be obtained from the diet because the human body cannot synthesise enough to meet its needs. Glycine, aspartic acid and serine are generally classified as non-essential amino acids.

Question 191. Most abundant organic compound on Earth

Exam: AIPMT 2001

Which of the following is commonly described as the most abundant organic compound on Earth?

A. Protein

B. Cellulose

C. Lipids

D. Steroids

Correct Answer: B. Cellulose

Explanation: Cellulose is a structural polysaccharide found in plant cell walls and is commonly described in biology textbooks as the most abundant organic compound in the biosphere.

Question 192. Composition of enzymes

Exam: AIPMT 2002

Enzymes are composed of which of the following?

A. Edible proteins

B. Proteins with specific structures

C. Nitrogen-containing carbohydrates

D. Carbohydrates

Correct Answer: B. Proteins with specific structures

Explanation: Most enzymes are proteins with specific three-dimensional structures that determine their catalytic activity. Some RNA molecules, called ribozymes, also act as biological catalysts.

Question 193. Incorrect statement about amino acids and proteins

Exam: AIPMT 2002

Which of the following statements is incorrect?

A. The alpha carbon of an alpha-amino acid is asymmetric except in glycine.

B. Proteins in the human body are generally composed of L-amino acids.

C. The human body can synthesise all the amino acids it needs.

D. At physiological pH, amino and carboxyl groups can exist in ionised forms.

Correct Answer: C. The human body can synthesise all the amino acids it needs.

Explanation: Humans cannot synthesise all essential amino acids in sufficient amounts. These must be obtained from the diet. Glycine is achiral because its alpha carbon has two hydrogen atoms attached to it.

Question 194. Phospholipids

Exam: AIPMT 2003

Phospholipids are esters of glycerol with:

A. Only amino acids

B. Fatty acids and phosphoric acid

C. Only glucose

D. Nucleotides and nitrogenous bases

Correct Answer: B. Fatty acids and phosphoric acid

Explanation: Phospholipids generally contain glycerol, fatty acid chains, a phosphate group and a polar head group. They are important structural components of biological membranes.

Question 195. Vitamin E

Exam: AIPMT 2001

Which of the following is correctly matched?

A. Vitamin E — Tocopherol

B. Vitamin D — Riboflavin

C. Vitamin B1 — Calciferol

D. Vitamin A — Thiamine

Correct Answer: A. Vitamin E — Tocopherol

Explanation: Vitamin E is a group of compounds known as tocopherols and tocotrienols. Vitamin D includes calciferols, vitamin B1 is thiamine, and vitamin B2 is riboflavin.

Question 196. Enzyme that hydrolyses triglycerides

Exam: AIPMT 2004

Which enzyme hydrolyses triglycerides into fatty acids and glycerol?

A. Lipase

B. Zymase

C. Pepsin

D. Maltase

Correct Answer: A. Lipase

Explanation: Lipases catalyse the hydrolysis of ester bonds in fats. This digestion releases fatty acids and glycerol or related partial glycerides, depending on the enzyme and reaction conditions.

Question 197. Hormone involved in glycogen formation

Exam: AIPMT 2004

Which hormone facilitates the conversion of glucose into glycogen?

A. Bile acids

B. Adrenaline

C. Insulin

D. Cortisone

Correct Answer: C. Insulin

Explanation: Insulin promotes glucose uptake and glycogen synthesis, especially in liver and skeletal muscle. Adrenaline generally promotes glycogen breakdown in response to immediate energy demands.

Question 198. Anomers of glucose

Exam: AIPMT 2000

Alpha-D-glucose and beta-D-glucose are:

A. Epimers

B. Anomers

C. Functional isomers

D. Chain isomers

Correct Answer: B. Anomers

Explanation: Alpha-D-glucose and beta-D-glucose differ in configuration at the anomeric carbon (C1). Such stereoisomers are called anomers.

Question 199. Fehling’s solution test

Exam: AIPMT 2001

Which compound gives a positive Fehling’s solution test?

A. Sucrose

B. Glucose

C. Fats

D. Protein

Correct Answer: B. Glucose

Explanation: Glucose is a reducing sugar and gives a positive Fehling’s test under appropriate conditions, producing a brick-red precipitate of copper(I) oxide. Sucrose is a non-reducing sugar.

Question 200. Correct statement about carbohydrates

Exam: AIPMT 2001

Which of the following statements is correct?

A. Starch is a polymer of alpha-glucose.

B. Amylose is a component of cellulose.

C. Proteins are composed of only one type of amino acid.

D. The cyclic structure of fructose contains four carbon atoms and one oxygen atom.

Correct Answer: A. Starch is a polymer of alpha-glucose.

Explanation: Starch is composed of alpha-glucose units and includes amylose and amylopectin. Amylose is a component of starch, not cellulose. Proteins generally contain different amino acids, and common cyclic forms of fructose contain five- or six-membered rings.

Question 201. Acid-soluble pool

Exam: NEET Biology practice question

Which of the following is generally found in the acid-soluble pool of living tissues?

A. Protein

B. Nucleic acid

C. Amino acid

D. Polysaccharide

Correct Answer: C. Amino acid

Explanation: Amino acids are small biomolecules generally found in the acid-soluble pool. Proteins, nucleic acids and polysaccharides are macromolecules typically found in the acid-insoluble fraction.

Question 202. Lipids in the acid-insoluble fraction

Exam: NEET Biology practice question

Why are lipids commonly found in the acid-insoluble fraction despite having relatively low molecular masses?

A. They are made of nucleotides.

B. They form aggregates and membrane structures that do not behave like freely soluble small molecules.

C. They are always covalently attached to DNA.

D. They are polymers of amino acids.

Correct Answer: B. They form aggregates and membrane structures that do not behave like freely soluble small molecules.

Explanation: Lipids are not true polymers like proteins or nucleic acids. Their hydrophobic nature causes them to form aggregates or membrane structures, so they can appear in the acid-insoluble fraction during tissue analysis.

Question 203. Nucleoside versus nucleotide

Exam: NEET Biology practice question

Which statement correctly distinguishes a nucleoside from a nucleotide?

A. A nucleoside contains a base, sugar and phosphate group.

B. A nucleotide contains only a nitrogenous base.

C. A nucleoside contains a nitrogenous base and sugar; a nucleotide also contains phosphate.

D. A nucleotide contains amino acids instead of a sugar.

Correct Answer: C. A nucleoside contains a nitrogenous base and sugar; a nucleotide also contains phosphate.

Explanation: Adenosine is a nucleoside consisting of adenine and ribose. AMP (adenosine monophosphate) is a nucleotide containing adenine, ribose and one phosphate group.

Question 204. Effect of temperature on enzyme activity

Exam: NEET Biology practice question

Why does the activity of many enzymes decrease sharply above their optimum temperature?

A. The enzyme always gains additional active sites.

B. The enzyme may lose its functional three-dimensional structure.

C. The substrate becomes a nucleotide.

D. The enzyme changes into a carbohydrate.

Correct Answer: B. The enzyme may lose its functional three-dimensional structure.

Explanation: High temperatures can disrupt interactions that maintain an enzyme’s structure, causing denaturation and reducing catalytic activity. The exact temperature at which this happens varies among enzymes.

Question 205. Enzyme classification

Exam: NEET Biology practice question

An enzyme that joins two molecules by forming a new chemical bond belongs to which traditional enzyme class?

A. Ligase

B. Hydrolase

C. Isomerase

D. Lyase

Correct Answer: A. Ligase

Explanation: Ligases join two molecules by forming a new bond, commonly coupled to the use of energy from ATP or another nucleoside triphosphate. Hydrolases break bonds using water, isomerases rearrange atoms within molecules, and lyases remove or add groups by mechanisms other than hydrolysis or oxidation.