Plasma Membrane Structure and Transport Questions and Answers

Q: 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.

Correct answer: FalseBilayer 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.

21.

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.

A.True only at large scales
B.False only at small scales
C.True
D.False

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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22.

In the bilayer, the hydrophobic fatty acid tails are oriented

A.inward, facing each other
B.outward, facing the water
C.randomly in all directions
D.parallel to the membrane surface

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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23.

Cholesterol in an animal cell membrane acts to

A.provide the energy needed for active transport
B.moderate fluidity across a range of temperatures
C.carry glucose molecules across the whole bilayer
D.attach the membrane firmly to the cell wall

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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24.

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.

A.A is true but R is false
B.A is false but R is true
C.Both A and R are true, and R is the correct explanation of A
D.Both A and R are true, but R is not the correct explanation of A

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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25.

State whether the following is true or false.
Increasing the proportion of unsaturated fatty acids makes a membrane less fluid.

A.True only at large scales
B.False only at small scales
C.True
D.False

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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26.

Integral membrane proteins differ from peripheral ones in that integral proteins

A.are embedded within the lipid bilayer
B.lie loosely on the membrane surface
C.are always found on the outer face only
D.consist entirely of carbohydrate chains

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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27.

The glycocalyx on the outer surface of an animal cell is composed of

A.phospholipids arranged in a second bilayer
B.carbohydrate chains attached to lipids and proteins
C.a layer of cellulose fibres laid down outside
D.cholesterol molecules facing the extracellular fluid

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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28.

State whether the following is true or false.
The plasma membrane is symmetrical, with the same molecules on both faces.

A.False only at small scales
B.True
C.False
D.True only at large scales

Answer: C

The two leaflets differ markedly in their lipid composition, and carbohydrate chains are found only on the outer face. This asymmetry is established during synthesis and actively maintained by the cell using enzymes that flip lipids between the leaflets.

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29.

Simple diffusion across a membrane requires

A.energy from ATP but no protein
B.a transport protein but no energy
C.both a protein and energy from ATP
D.no energy and no transport protein

Answer: D

Molecules move down their own concentration gradient, and the gradient itself supplies the direction. Only small non-polar substances such as oxygen and carbon dioxide cross readily this way, since the hydrophobic core blocks anything charged or strongly polar.

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30.

Facilitated diffusion differs from simple diffusion in that it

A.uses a channel or carrier protein
B.moves substances against the gradient
C.consumes ATP at every step
D.occurs only in prokaryotic cells

Answer: A

The protein provides a path for substances the lipid core would otherwise block, such as ions and glucose. It remains a passive process, since movement is still down the concentration gradient and no metabolic energy is spent by the cell.

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