Predict which of the following organisms will have the highest percentage of unsaturated phospholipids in their membranes ?

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Correct — option (1), "Antarctic fish". The question is really about one variable: what temperature are the organism's cell membranes actually at? A membrane must stay fluid to work. Its proteins have to move within it, transport has to occur across it, and vesicles have to bud and fuse. Cool a membrane and its phospholipid tails pack more tightly together until it stiffens into a gel and stops functioning. The cell's defence against that is to build its phospholipids from unsaturated fatty acids. The reason is geometric. A saturated fatty acid tail is a straight hydrocarbon chain, and straight chains stack neatly against one another, which is what allows a membrane to solidify. An unsaturated tail carries one or more carbon-carbon double bonds, and in the cis configuration found in membrane lipids each double bond puts a permanent kink in the chain. Kinked tails cannot pack closely; they hold their neighbours apart, and the membrane stays fluid at a temperature at which a membrane of straight tails would have set. Cooking oils make the same point at kitchen scale — animal fats, rich in saturated chains, are solid at room temperature, while vegetable oils, rich in unsaturated chains, are liquid. Organisms adjust the proportion accordingly, a response called homeoviscous adaptation: the colder the operating temperature of the membrane, the higher the proportion of unsaturated phospholipids needed to keep its viscosity in the working range. Now apply that to the four organisms. Two of them are endotherms — the human being and the polar bear — and an endotherm holds its core temperature near 37 °C whatever the weather outside. Their cell membranes never experience Antarctic or Saharan conditions, so neither is a candidate for the extreme. That disposes of options (3) and (4) together, and it disposes of them for the same reason, which is a strong hint that neither is intended. The remaining two are ectotherms, whose tissue temperature follows the environment. A desert snake's tissues run hot, so it needs comparatively saturated membranes to avoid becoming excessively fluid and leaky. An Antarctic fish lives in seawater at around −1.8 °C, the freezing point of sea water, and its tissues sit at that temperature continuously. Of the four organisms it is the only one whose membranes must remain functional at close to zero, so it is the one that requires the highest proportion of unsaturated phospholipids. Option (1). Note how the option set is built. Option (4), the polar bear, is the trap, and it is a good one: the word 'polar' points at cold, and a candidate matching habitat words to the answer will take it. But the polar bear's adaptation to cold is insulation — fur, a thick layer of blubber, and a high metabolic rate — precisely so that its cells do not have to operate cold. The Antarctic fish has no such option and must adapt at the level of the membrane itself. Reading past the habitat to the physiology is the whole of this question.

Why the other options are wrong — (b) Option (2) is the opposite extreme. A snake is an ectotherm, so its tissue temperature does track its environment — but that environment is a hot desert, and a membrane at high temperature is at risk of becoming too fluid and leaky rather than too rigid. The adaptive response is therefore a higher proportion of SATURATED phospholipids, whose straight tails pack more closely and stiffen the membrane. Of the four organisms this one would be expected to have the lowest proportion of unsaturated lipids, not the highest.  (c) Option (3) misses that a human is an endotherm. Whatever the outside temperature, core body temperature is regulated at about 37 °C, and the cell membranes of internal tissues experience that regulated temperature and nothing else. There is no cold stress for the membrane to adapt to, so no reason for an unusually high proportion of unsaturated phospholipids. A human sits in the middle of this list, not at either end, which is exactly why an examiner includes it.  (d) Option (4) is the intended trap, because 'polar' matches 'Antarctic' as a cold-habitat cue. But the polar bear is an endotherm and keeps its core near 37 °C; it survives the Arctic by insulating its cells from the cold, with dense fur, a thick subcutaneous fat layer and a high metabolic rate, rather than by rebuilding its membranes to work cold. Its cell membranes are at much the same temperature as a human's. The organism that must actually run its biochemistry near the freezing point of sea water is the Antarctic fish.

Concept: The plasma membrane is described by the fluid mosaic model of Singer and Nicolson (1972): a bilayer of phospholipids, hydrophilic heads outward and hydrophobic tails inward, behaving as a two-dimensional fluid in which proteins are embedded and can move laterally. Fluidity is not incidental to membrane function — it is required for it. Membrane proteins must diffuse to meet their partners, transporters must change shape, vesicles must bud off and fuse, and cells must divide and move. Four factors set the fluidity. The degree of unsaturation of the fatty acid tails is the most important: cis double bonds kink the chains, prevent close packing and keep the membrane fluid at lower temperatures. Chain length matters too, since shorter tails interact less and pack less tightly. Temperature itself is the external driver, with fluidity falling as temperature drops until the membrane undergoes a transition to a gel-like state. And in animal cells cholesterol acts as a fluidity buffer, restraining movement at higher temperatures while wedging between the phospholipid tails at lower temperatures and preventing the tight packing that would otherwise solidify the bilayer.

State PCS 2026 Q57 — Science & Technology PYQ with Answer

State PCS 2026 Q57 — Science & Technology PYQ with Answer