BIOLOGY NEET PYQ : CELL: THE UNIT OF LIFE

Explanation: Prokaryotic ribosomes have 50S and 30S subunits, and many prokaryotes possess plasmids. Mesosomes described in older accounts are generally regarded as preparation artefacts, and prokaryotes do not possess membrane-bound peroxisomes.

Explanation: Prokaryotic cytoplasmic ribosomes are 70S, consisting of 50S and 30S subunits. Eukaryotic cytoplasmic ribosomes are 80S, consisting of 60S and 40S subunits. The S values are sedimentation coefficients.

Explanation: Matthias Schleiden studied plants, while Theodor Schwann studied animals. Their work established that plants and animals are composed of cells. Virchow later emphasised that new cells arise from pre-existing cells.

Explanation: S. J. Singer and G. L. Nicolson proposed the fluid mosaic model in 1972. It describes the membrane as a fluid lipid bilayer containing embedded proteins.

Explanation: Pili and fimbriae primarily assist attachment to surfaces or other cells. Some specialised pili participate in DNA transfer, and certain pili can contribute to surface-associated movement. Bacterial flagella are the principal locomotory structures in many motile bacteria.

Explanation: Cellulose is a major structural component of plant cell walls, while lignin strengthens many specialised plant cell walls. Chitin is characteristic of fungal cell walls. Peptidoglycan is characteristic of most bacterial cell walls.

Explanation: Mitochondria and chloroplasts possess their own DNA and ribosomes and can divide within cells. However, many of their proteins are encoded by nuclear genes and supplied by the cell.

Explanation: The typical motile eukaryotic ciliary or flagellar axoneme contains nine peripheral microtubule doublets and two central microtubules. The basal body generally has nine peripheral triplets.

Explanation: These structures coordinate synthesis, modification, packaging, transport and intracellular digestion. Mitochondria, chloroplasts and peroxisomes are not included in the classical NCERT endomembrane system.

Explanation: Smooth ER lacks attached ribosomes and participates in lipid synthesis. Rough ER bears ribosomes and is involved in synthesising proteins destined for secretion, membranes and certain organelles.

Explanation: The nucleolus is a region within the nucleus where most ribosomal RNA is produced and ribosomal subunits are assembled.

Explanation: Mycoplasmas are wall-less prokaryotic organisms. They do not possess a membrane-bound nucleus or typical eukaryotic membrane-bound organelles.

Explanation: Plasmids are extra-chromosomal DNA molecules, commonly circular and double-stranded in bacteria. They possess replication origins that allow them to replicate independently of the main chromosome. Some plasmids carry genes associated with antibiotic resistance.

Explanation: Most bacterial cell walls contain peptidoglycan. Plant cell walls are mainly cellulose-based, fungal cell walls contain chitin, and the plant middle lamella is rich in calcium pectate.

Explanation: Cisternae are the flattened membrane-bound sacs of the Golgi apparatus. Cristae are folds of the mitochondrial inner membrane, while thylakoids are membrane compartments in chloroplasts.

Explanation: Robert Brown described the nucleus in plant cells in 1831. Robert Hooke had earlier observed cork compartments and introduced the term cell.

Explanation: Amyloplasts are starch-storing leucoplasts. Elaioplasts store oils and fats, while chromoplasts contain pigments that contribute to the colour of plant parts.

Explanation: Viruses are acellular infectious agents, not cellular organelles. They lack the complete cellular machinery required for independent metabolism and reproduction. Golgi apparatus, lysosomes and mitochondria are cellular organelles.

Explanation: The plasma membrane protects the cell and regulates material transport. Cell division is a coordinated process involving cellular structures and regulatory machinery; it is not a primary function of the plasma membrane.

Explanation: The mitochondrial inner membrane surrounds the matrix. The matrix contains enzymes involved in the Krebs cycle, mitochondrial DNA and ribosomes. The inner membrane contains components of the electron transport chain and ATP synthase.