Plasma Membrane Structure and Transport Questions and Answers

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.

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.

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.

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.

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.

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.

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.

Answer: C

Lipids and many proteins drift sideways much as boats drift on water, while the sheet itself remains intact. The membrane is not a liquid, and calling it fluid describes the mobility of its parts rather than its state.

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: an animal cell in a hypertonic solution loses water and crenates, and a plant cell in a hypotonic solution becomes turgid. Statement 3 has the direction wrong - a plant cell in a hypertonic solution loses water and the protoplast shrinks away from the wall, which is plasmolysis. Bursting requires water entering, and the wall prevents that in any case.

Answer: A

Water crosses in both directions at equal rates, so there is no net movement and the cell keeps its volume unchanged. Intravenous fluids are made isotonic to blood for exactly this reason, since anything else would swell or shrink the patient's own cells.