The fluid mosaic model of membrane structure was proposed by
Answer: A
Their 1972 model described proteins floating in and through a fluid lipid bilayer rather than coating it as a continuous sheet. Danielli and Davson had earlier proposed the protein sandwich model that this replaced, and Gorter and Grendel first showed the lipid was a bilayer.
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The basic structural framework of the plasma membrane is
Answer: B
Phospholipids assemble spontaneously into a bilayer in water, and proteins are embedded within that framework rather than forming it. Cholesterol modulates the bilayer's properties but does not build it, and plant cells substitute other sterols entirely.
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A phospholipid molecule is described as amphipathic because it has
Answer: C
Correct answer: a polar head and non-polar tails
Amphipathic means having both a water-loving and a water-avoiding part in the same molecule. The phosphate head is polar and hydrates readily; the two fatty acid tails are hydrocarbon and do not. That single structural fact is what makes the bilayer form and what gives the membrane its properties.
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State whether the following is true or false.
A phospholipid bilayer forms in water because the process requires an input of energy from the cell.
Answer: D
Correct answer: False
Bilayer formation is spontaneous and releases free energy rather than consuming it. Exposing hydrocarbon tails to water forces the surrounding water molecules into an ordered cage, which is entropically costly; sequestering the tails away from water frees that structure and the entropy of the system rises. The hydrophobic effect therefore drives assembly, which is also why a punctured membrane reseals by itself.
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In the bilayer, the hydrophobic fatty acid tails are oriented
Answer: A
The tails point into the interior of the sheet and the phosphate heads face the watery environments on either side. This is why the membrane core acts as a barrier to charged and polar substances, which must use a protein if they are to cross at all.
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Cholesterol in an animal cell membrane acts to
Answer: B
Correct answer: moderate fluidity across a range of temperatures
Cholesterol is a buffer against temperature in both directions. At warm temperatures its rigid ring system restrains the movement of the fatty acid tails and reduces fluidity; at cold temperatures it wedges between them and prevents the close packing that would otherwise gel the membrane. The result is that the membrane stays workable over a far wider range than it would alone.
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Assertion (A): Cholesterol both increases and decreases membrane fluidity depending on conditions.
Reason (R): It restricts phospholipid movement when warm and prevents close packing when cold.
Answer: C
Both statements are true and the reason resolves the apparent contradiction: the same molecule has opposite effects at the two temperature extremes, which is exactly what buffering means. The membrane therefore stays workable over a range no pure bilayer could manage.
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State whether the following is true or false.
Increasing the proportion of unsaturated fatty acids makes a membrane less fluid.
Answer: D
The kinks introduced by double bonds stop the tails packing closely, so fluidity rises rather than falls rather than dropping. Organisms living in the cold accordingly build membranes with a higher proportion of unsaturated lipid, keeping them workable at low temperature.
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Integral membrane proteins differ from peripheral ones in that integral proteins
Answer: A
Spanning or penetrating the bilayer requires a hydrophobic region that matches the membrane core, and removing such a protein needs a detergent. Peripheral proteins can be washed off by a change of salt concentration alone.
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The glycocalyx on the outer surface of an animal cell is composed of
Answer: B
These sugar chains are involved in recognition, adhesion and protection, and they distinguish one cell type from another during development and immune surveillance. The ABO blood group antigens are among the best known examples of such markers.
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