BPSC 71st CCE Prelims General Studies · Question 142 of 150

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.

• (b)Lipids only — 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)Proteins only — 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 — '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.

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.

Look at the shape of the options before you look at the biology. Three of them are absolutes — 'lipids only', 'proteins only', 'none of the above' — and one is a long, carefully qualified sentence. In a question about the composition of a real biological structure, the qualified statement almost always wins, because nothing in cell biology is pure. Then reason from function, which settles it without any memorised percentage. A membrane has to do two opposite things at once: seal the cell against everything water-soluble, and let selected ions and nutrients through at controlled rates. A lipid bilayer alone achieves the first and makes the second impossible; proteins alone achieve the second and cannot achieve the first, since a protein layer is not hydrophobic enough to keep water out. Both components are therefore necessary, which kills (b) and (c) together, and killing them kills (d) with them. What remains is to be sure the qualifier in (a) is true rather than just impressive, and one comparison settles that: the myelin sheath is about 18 per cent protein while the inner mitochondrial membrane is about 75 per cent protein. That single contrast is the discriminating fact — proof that the protein-to-lipid ratio really is specific to the cell type and the organelle. The trap here is the half-remembered line 'the plasma membrane is a phospholipid bilayer', which names only lipid and pushes a hurried candidate towards (b). The bilayer is the framework of the building, not the building.

• NCERT Class XI biology states that the membrane of the human erythrocyte is approximately 52 per cent protein and 40 per cent lipid, and that the ratio of protein to lipid varies considerably between different cell types — the textbook basis of the phrase 'varying combinations' in the correct option.
• The fluid mosaic model was proposed by Singer and Nicolson in 1972: the quasi-fluid nature of the lipid allows lateral movement of proteins within the bilayer, and this fluidity underpins cell growth, formation of intercellular junctions, secretion, endocytosis and cell division.
• Membrane proteins are classified by ease of extraction — peripheral proteins lie on the surface of the membrane, while integral proteins are partially or totally buried within it; the major lipids are phospholipids arranged in a bilayer with polar heads outward and hydrophobic tails inward, with cholesterol also present.
• Protein content tracks workload: the inner mitochondrial membrane is roughly 75 per cent protein because it carries the electron-transport chain and ATP synthase, whereas the myelin sheath is only about 18 per cent protein because insulation needs lipid, not machinery.
• The model was built in stages — Overton deduced the lipid nature of membranes in 1895 from permeability, Gorter and Grendel demonstrated the bilayer in 1925 using lipid extracted from red blood cells, Danielli and Davson proposed a protein-lipid-protein sandwich in 1935, Robertson's electron micrographs gave the trilaminar 'unit membrane' in 1959, and Singer and Nicolson replaced it in 1972.

• Choosing 'Lipids only' from the half-remembered phrase 'the plasma membrane is a phospholipid bilayer' — the bilayer is the framework, and roughly half the erythrocyte membrane's weight is protein
• Assuming every membrane has the same composition — the protein share runs from about 18 per cent in myelin to about 75 per cent in the inner mitochondrial membrane
• Reaching for 'None of the above' because the correct option looks suspiciously long — in composition questions the length is usually the accuracy, not the padding

BPSC asks membranes as a flat definitional recall in which the correct option is the long, hedged one and the wrong ones are one-word absolutes — the same shape it uses for other 'what is X made of' items, so read the qualifiers rather than skipping them. UPSC almost never asks the composition directly; it approaches the membrane through what it does, matching mineral uptake to the plasma membrane in a cell-organelle list in 1996 and testing the direction of solvent movement across a semipermeable membrane in osmosis and reverse osmosis in 2005. Learn the model for BPSC and the transport mechanics for UPSC.

• practice — not a real PYQ

  The widely accepted fluid mosaic model of the plasma membrane was proposed in 1972 by

  • (a)Gorter and Grendel
  • (b)Danielli and Davson
  • (c)Singer and Nicolson
  • (d)J. D. Robertson

  Answer(c) Singer and Nicolson — they proposed the fluid mosaic model in 1972. Gorter and Grendel demonstrated the lipid bilayer in 1925, Danielli and Davson gave the protein-lipid-protein sandwich in 1935, and Robertson's electron micrographs produced the trilaminar 'unit membrane' in 1959.

• practice — not a real PYQ

  The movement of Na+ and K+ ions across the plasma membrane against their concentration gradients by the sodium-potassium pump is an example of

  • (a)Simple diffusion
  • (b)Osmosis
  • (c)Facilitated diffusion
  • (d)Active transport

  Answer(d) Active transport — movement against the concentration gradient requires energy from ATP. Simple diffusion and facilitated diffusion both run down the gradient and cost no energy, and osmosis is specifically the diffusion of water across a selectively permeable membrane.