Plasma Membrane Structure and Transport Questions and Answers

Q: Membrane proteins that act as receptors typically

Correct answer: bind a signal molecule on the outer faceA receptor is a transmembrane protein whose extracellular domain binds its ligand and whose intracellular domain then changes shape or activity, passing the message inward without the signal molecule itself ever entering the cell. That is what makes the cell responsive to its surroundings, and it is why the response depends on the receiving cell rather than on the signal.

Q: Exocytosis is used by a cell chiefly to

Correct answer: release secretory products to the outsideIn exocytosis a vesicle fuses with the plasma membrane and discharges its contents outside - hormones, digestive enzymes, neurotransmitters, or the components of the extracellular matrix. It also delivers new membrane and new membrane proteins to the cell surface, which is the point the next item turns on.

Q: State whether the following is true or false.A cell that secretes heavily by exocytosis must also carry out endocytosis to balance its membrane area.

Correct answer: TrueEach vesicle that fuses adds its own membrane to the cell surface, so a cell secreting heavily would expand indefinitely if nothing removed it. Endocytosis retrieves membrane at a matching rate, and the two run as a continuous cycle. A synaptic terminal is the clearest case: vesicle membrane is recovered and refilled within seconds so that transmission can be sustained.

41.

Membrane proteins that act as receptors typically

A.bind a signal molecule on the outer face
B.carry ions across against their gradient
C.join adjacent cells mechanically together
D.catalyse reactions within the lipid core

Answer: A

Correct answer: bind a signal molecule on the outer face

A receptor is a transmembrane protein whose extracellular domain binds its ligand and whose intracellular domain then changes shape or activity, passing the message inward without the signal molecule itself ever entering the cell. That is what makes the cell responsive to its surroundings, and it is why the response depends on the receiving cell rather than on the signal.

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42.

Consider the following statements about the fluid mosaic model.
1. Proteins can move laterally within the bilayer.
2. The membrane's two leaflets differ in composition.
3. Phospholipids flip readily between leaflets unaided.

A.All of them
B.1 and 2 only
C.3 only
D.1 only

Answer: B

The first two are correct. The third is wrong: flipping a polar head through the hydrophobic core is extremely slow without assistance, and enzymes called flippases carry it out where it is needed, which is how the asymmetry between the two leaflets is maintained.

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43.

State whether the following is true or false.
Membrane proteins are fixed rigidly in position within the bilayer.

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

Answer: C

Many drift laterally, as the classic experiment fusing mouse and human cells demonstrated when their surface proteins intermingled within an hour. Some are anchored by the cytoskeleton, but that is a restriction rather than the rule.

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44.

A cell wall differs from a plasma membrane in being

A.selectively permeable to chosen solutes
B.made largely of phospholipid
C.present in every kind of cell
D.freely permeable to most small molecules

Answer: D

The wall is a mechanical support that lets water and small solutes pass without discrimination. All the selection is done by the plasma membrane lying immediately inside it, so the wall shapes and supports the cell without controlling its contents.

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45.

Exocytosis is used by a cell chiefly to

A.release secretory products to the outside
B.take up large particles from outside the cell
C.move ions against their concentration gradient
D.generate ATP for the cell's metabolic work

Answer: A

Correct answer: release secretory products to the outside

In exocytosis a vesicle fuses with the plasma membrane and discharges its contents outside - hormones, digestive enzymes, neurotransmitters, or the components of the extracellular matrix. It also delivers new membrane and new membrane proteins to the cell surface, which is the point the next item turns on.

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46.

A vesicle of surface area 0.5 square micrometres fuses with the plasma membrane. If 40 such vesicles fuse, how much membrane area is added?

A.0.5 square micrometres
B.20 square micrometres
C.2 square micrometres
D.200 square micrometres

Answer: B

Forty vesicles at half a square micrometre each contribute twenty square micrometres. A secretory cell must retrieve membrane by endocytosis at a matching rate, or its surface would grow steadily larger while its volume stayed the same.

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47.

State whether the following is true or false.
A cell that secretes heavily by exocytosis must also carry out endocytosis to balance its membrane area.

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

Answer: C

Correct answer: True

Each vesicle that fuses adds its own membrane to the cell surface, so a cell secreting heavily would expand indefinitely if nothing removed it. Endocytosis retrieves membrane at a matching rate, and the two run as a continuous cycle. A synaptic terminal is the clearest case: vesicle membrane is recovered and refilled within seconds so that transmission can be sustained.

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48.

Tight junctions between epithelial cells function to

A.allow ions to pass directly from one cell to the next
B.anchor the cells firmly to the basement membrane below
C.carry nerve impulses between two adjacent cells
D.seal the gap so substances cannot leak between cells

Answer: D

Sealing the space forces everything crossing an epithelium to pass through the cells rather than between them, which lets the tissue control what it admits. Gap junctions do the quite different job of connecting cytoplasms.

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49.

Gap junctions differ from tight junctions in that gap junctions

A.let small molecules pass between adjacent cells
B.prevent any leakage through the intercellular space
C.attach the cell to the extracellular matrix outside
D.are found only in plant tissues and not in animals

Answer: A

Correct answer: let small molecules pass between adjacent cells

A gap junction is a channel joining two cytoplasms, so ions and molecules up to about a kilodalton pass directly from one cell to the next - which is how cardiac and smooth muscle are electrically coupled. A tight junction does the opposite: it seals the space *between* cells so that nothing leaks past them, which is what makes an epithelium a barrier.

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50.

State whether the following is true or false.
Plasmodesmata in plants are functionally comparable to gap junctions in animals.

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

Answer: B

Both create direct cytoplasmic continuity between neighbouring cells, allowing small molecules to pass without crossing a membrane. Their structures differ entirely, since plasmodesmata must traverse a cell wall as well.

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