The biological membranes are composed of :

Question

UnlockIAS · Accepted Answer

Correct — A, "Lipids and proteins in varying combinations which are specific to each species, cell type and organelle". Every biological membrane is lipoproteinaceous: a continuous bilayer of phospholipids, with their polar heads facing the watery surroundings and their hydrophobic tails packed inward, into which proteins are inserted. NCERT's Class XI biology text puts numbers on it — the membrane of the human erythrocyte is approximately 52 per cent protein and 40 per cent lipid, the remainder being carbohydrate carried on glycolipids and glycoproteins on the outer face. Cholesterol sits between the phospholipid tails and tunes fluidity. Membrane proteins are classified by how easily they can be pulled out: peripheral proteins lie on the surface, integral proteins are partially or totally buried in the bilayer. This is the fluid mosaic model, proposed by S. J. Singer and G. L. Nicolson in 1972 and still the accepted picture — the quasi-fluid lipid lets proteins drift laterally through the sheet, and that fluidity is what makes cell growth, the formation of intercellular junctions, secretion, endocytosis and cell division possible. The clause the examiner is really rewarding is 'varying combinations'. Membrane composition is not a constant of nature; it tracks how much work a particular membrane has to do. The inner mitochondrial membrane, crowded with the electron-transport chain complexes and ATP synthase, runs at roughly 75 per cent protein. The myelin sheath wrapped round an axon, whose only job is electrical insulation, runs at about 18 per cent protein and is overwhelmingly lipid. Same basic architecture, opposite ratios, because one is a chemical factory and the other is a cable jacket. That is precisely the difference the option's phrase 'specific to each species, cell type and organelle' captures, and it is why the Commission upheld this key, noting that biological membranes are lipoproteinaceous in nature and composed of lipids and proteins in various combinations depending on cell type and species.

Why the other options are wrong — (b) This is the picture biology held before 1935 — Overton inferred in 1895 that membranes were lipid because lipid-soluble substances crossed them fastest, and Gorter and Grendel showed in 1925 that the lipid extracted from red cells spread to twice the cells' surface area, proving a bilayer. Lipid genuinely is the continuous phase and the permeability barrier. But a bare lipid sheet cannot pump sodium, cannot carry a hormone receptor and cannot act as an enzyme; the erythrocyte membrane is about 52 per cent protein by weight. This option is a half-memory of 'phospholipid bilayer'.  (c) No membrane anywhere in biology is protein-only. Even the most protein-rich membrane in the human body — the inner mitochondrial membrane at roughly 75 per cent protein — still needs its lipid bilayer, cardiolipin included, because only a hydrophobic lipid core can seal the compartment against water-soluble ions and hold the proton gradient that ATP synthase runs on. Proteins are water-soluble machines; they cannot by themselves form a barrier against water. Strip the lipid out and there is no compartment left to work in.  (d) 'None of the above' can only be right if all three preceding options are wrong, and option (a) is a textbook-accurate statement of the fluid mosaic model, so this collapses automatically. It is the refuge of a candidate who distrusts a long option on principle. That instinct is backwards in composition and definition questions: an option is often long precisely because it has been made accurate, with the qualifiers — here 'in varying combinations', 'specific to each species, cell type and organelle' — doing the discriminating work.

Concept: A biological membrane is not just the outer skin of the cell. The same architecture builds the plasma membrane, the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, the double membrane of the mitochondrion with its folded inner cristae, and the thylakoids of the chloroplast — which is exactly why eukaryotes have compartments and prokaryotes, lacking membrane-bound organelles, do not. The building block is the phospholipid, an amphipathic molecule with a water-loving phosphate head and two water-hating fatty-acid tails. Put such molecules in water and they self-assemble, tails inward and heads outward, into a bilayer roughly 7 to 8 nanometres thick; no energy or template is needed, the hydrophobic effect does it. That bilayer is an excellent barrier — ions and polar molecules cannot cross it — which is useful but also useless on its own, because a cell must import glucose, expel sodium, sense hormones and recognise its neighbours. All of that is done by proteins embedded in or attached to the lipid. Small non-polar molecules diffuse straight through; water moves by osmosis; polar molecules need a carrier protein to ferry them down the gradient; and moving a substance against its concentration gradient costs ATP, as the sodium-potassium pump does. Thus lipid supplies the seal and the fluidity, protein supplies the selectivity and the machinery, and carbohydrate on the outer face supplies identity — the ABO blood-group antigens are membrane glycolipids and glycoproteins. Composition varies because function varies.

State PCS 2025 Q142 — Science & Technology PYQ with Answer

State PCS 2025 Q142 — Science & Technology PYQ with Answer