CDS (II) 2022 General Knowledge · Question 70 of 120

Correct — A, Bilayer of phospholipid molecule in which proteins and cholesterol are embedded. NCERT's account of the plasma membrane is exactly this: the membrane is mainly composed of lipids and proteins, 'the major lipids are phospholipids that are arranged in a bilayer', the lipids sit with 'the polar head towards the outer sides and the hydrophobic tails towards the inner part', and 'in addition to phospholipids, membrane also contains cholesterol'. Later work added the proteins, classified as integral where they are partly or wholly buried in the bilayer and peripheral where they lie on the surface. Singer and Nicolson put the pieces together in 1972 as the fluid mosaic model — a two-dimensional fluid of lipid in which protein molecules float, which is what allows a membrane to be flexible, to fuse, to bud and to let its components move sideways.

• (b)Bilayer of proteins molecule in which lipid and cholesterol are embedded — This inverts the structure, making protein the bilayer and lipid the inclusion. It is not an invented distractor: the sandwich model proposed in the 1930s did put protein layers on the outside of a lipid core, and it was displaced once electron microscopy and freeze-fracture work showed the lipid bilayer with proteins embedded in it.
• (c)Bilayer of neutral lipid in which proteins and cholesterol are embedded — Neutral lipids such as triglycerides have no polar head. They cannot form a stable bilayer in water, because the whole architecture depends on amphipathic molecules turning their water-loving heads outward and their water-fearing tails inward. Membrane lipids are phospholipids, which are polar.
• (d)Bilayer of neutral lipid lacking cholesterol and proteins — Fails twice over. It repeats the neutral lipid error and then strips out both cholesterol and protein, leaving a structure that could neither hold itself together in water nor do any of the transport, signalling and recognition work a membrane performs.

A phospholipid has a phosphate-bearing head that mixes with water and two fatty acid tails that do not. Put enough of them in water and they arrange themselves without help into a sheet two molecules thick, heads out, tails in. Cholesterol slots between the tails and steadies the sheet, keeping it fluid when cold and firm when warm. Proteins provide the function: channels, pumps, receptors, enzymes and identity markers. Carbohydrate chains attached to lipids and proteins face outwards and are how cells recognise one another.

The distractors here are built by changing one word at a time — bilayer of protein instead of lipid, neutral lipid instead of phospholipid, cholesterol removed — so the question rewards reading each option to the end rather than recognising the first familiar phrase. The word doing the real work is 'phospholipid'. Only a molecule with a water-loving head and water-fearing tails will self-assemble into a bilayer, and stating that is a complete justification of option (a) on its own. One printed note in passing: the stem reads 'a feature of Membrane' with a capital and no article, and option (b) prints 'proteins molecule', both of them slips in the paper's own typesetting rather than in the science being tested.

• The plasma membrane is mainly lipid and protein; the major lipids are phospholipids arranged in a bilayer.
• Lipids sit with polar heads outward and hydrophobic tails inward; cholesterol is also present in the membrane.
• Membrane proteins are integral, partly or wholly buried in the bilayer, or peripheral, lying on its surface.
• In the human red blood cell the membrane is roughly 52 per cent protein and 40 per cent lipid.
• The accepted structure is the fluid mosaic model of Singer and Nicolson, proposed in 1972.

• Accepting a protein bilayer because it appears in older textbooks; that model was superseded.
• Missing the difference between a neutral lipid and a phospholipid — only the second has the polar head that makes a bilayer possible.
• Forgetting cholesterol, which the question deliberately makes part of the correct description.

As a describe-the-structure item with one word altered in each wrong option, or as a which-statement-is-correct item on membrane composition.

• practice — not a real PYQ

  The fluid mosaic model of the plasma membrane was proposed by

  • (a)Robert Hooke
  • (b)Singer and Nicolson
  • (c)Schleiden and Schwann
  • (d)Watson and Crick

  Answer(b) Singer and Nicolson — their 1972 model describes proteins floating in a fluid lipid bilayer.

• practice — not a real PYQ

  Phospholipids form a bilayer in water because each molecule has

  • (a)two polar heads
  • (b)a polar head and non-polar tails
  • (c)only non-polar groups
  • (d)a rigid ring structure

  Answer(b) a polar head and non-polar tails — the heads face the water and the tails turn inward, away from it.