MPSC 2026 Prelims Paper-I · Question 57 of 100

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.

• (2)Desert snake — 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.
• (3)Human being — 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.
• (4)Polar bear — 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.

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.

The adjustment of membrane lipid composition to keep viscosity within a working range is called homeoviscous adaptation, and it is widespread. Bacteria shifted to a lower growth temperature alter the fatty acid composition of their membranes within a generation. Plants over-wintering in cold climates increase the unsaturation of their membrane lipids as part of cold acclimatisation, which is one reason a plant hardened gradually survives a frost that would kill the same plant moved abruptly outdoors. Fish of cold waters carry markedly more polyunsaturated fatty acids in their tissues than tropical fish, which is also the reason cold-water marine fish are the dietary source of omega-3 fatty acids. The Antarctic fishes go further still: the notothenioids, which dominate the Southern Ocean, additionally produce antifreeze glycoproteins that bind to nascent ice crystals in the body fluids and stop them growing, allowing the fish to live in water below the normal freezing point of their blood. Membrane unsaturation keeps their cells working; the antifreeze proteins keep the fish from freezing solid. The two are separate adaptations to the same problem.

• Unsaturated fatty acids carry cis double bonds that kink the hydrocarbon tail, preventing close packing of phospholipids and keeping the membrane fluid at low temperature. Saturated tails are straight, pack tightly and stiffen the membrane.
• Homeoviscous adaptation is the adjustment of membrane lipid composition to maintain working fluidity: colder operating temperatures call for a higher proportion of unsaturated phospholipids, warmer ones for more saturated phospholipids.
• Ectotherms such as fish and reptiles have tissue temperatures that follow the environment, so their membranes must adapt to it. Endotherms such as humans and polar bears hold a core temperature near 37 °C and insulate their cells from external extremes instead.
• Antarctic waters sit near −1.8 °C, the freezing point of sea water. Antarctic notothenioid fish combine highly unsaturated membrane lipids with antifreeze glycoproteins that bind to ice crystals and prevent their growth in the body fluids.
• The fluid mosaic model of the membrane was proposed by Singer and Nicolson in 1972. In animal cells cholesterol acts as a fluidity buffer — restricting movement at higher temperatures and preventing tight packing at lower ones.

• Matching a habitat word to the answer. 'Polar bear' sounds like the cold-adapted option but the bear is an endotherm whose cells sit near 37 °C; the cold is kept outside by fur and blubber, not met at the membrane.
• Forgetting that ectotherms are the only organisms whose membranes actually experience environmental extremes. In any question of this shape, sort the list into endotherms and ectotherms first.
• Reversing the geometry. Unsaturated tails are kinked and keep membranes fluid in the cold; saturated tails are straight and stiffen membranes, which is what a hot-habitat ectotherm needs.
• Assuming cholesterol simply increases or decreases fluidity. It buffers in both directions — restraining movement at high temperature and preventing solidification at low temperature.

This is an applied-reasoning question rather than a recall question, and MPSC has been using more of them in general science. The pattern is a principle from the syllabus — here membrane fluidity and its dependence on fatty acid saturation — applied to four organisms or situations, with one option built as a surface-cue trap. Related questions in the same area ask directly what makes a membrane fluid, what role cholesterol plays, why fats from animals are solid while vegetable oils are liquid at room temperature, or which model describes membrane structure and who proposed it. The technique that works across all of them is to name the underlying variable before looking at the options — here, the temperature the membrane actually operates at — and then to test each option against that variable rather than against the words in it.

No directly related past PYQ was found.

• practice — not a real PYQ

  Why does a high proportion of unsaturated fatty acids keep a cell membrane fluid at low temperature ?

  • (a)Because unsaturated fatty acids have shorter hydrocarbon chains
  • (b)Because cis double bonds kink the tails and prevent close packing of the phospholipids
  • (c)Because unsaturated fatty acids are hydrophilic and dissolve in water
  • (d)Because unsaturated fatty acids generate heat within the membrane

  Answer(b) Because cis double bonds kink the tails and prevent close packing of the phospholipids. A straight saturated tail stacks neatly against its neighbours, which is what allows a bilayer to stiffen into a gel when cooled; a kinked tail holds its neighbours apart and keeps the membrane fluid. Chain length does affect fluidity as well, but it is not what distinguishes unsaturated from saturated fatty acids, and the tails remain hydrophobic in both cases.

• practice — not a real PYQ

  The fluid mosaic model of the cell membrane was proposed in 1972 by :

  • (a)Watson and Crick
  • (b)Singer and Nicolson
  • (c)Robertson and Danielli
  • (d)Schleiden and Schwann

  Answer(b) Singer and Nicolson. Their model describes the membrane as a phospholipid bilayer behaving as a two-dimensional fluid, with proteins embedded in it like a mosaic and free to move laterally — replacing earlier static sandwich models of membrane structure. Watson and Crick described the structure of DNA, and Schleiden and Schwann formulated the cell theory.