State whether the following is true or false.
The plasma membrane is symmetrical, with the same molecules on both faces.
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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Simple diffusion across a membrane requires
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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Facilitated diffusion differs from simple diffusion in that it
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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State whether the following is true or false.
Facilitated diffusion requires ATP because it involves a membrane protein.
Answer: B
The protein provides a route, not a push, so the movement is still driven by the concentration gradient alone. Requiring a protein and requiring metabolic energy are quite separate matters, and only active transport needs both of them together.
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Osmosis is best defined as the movement of water
Answer: C
Water moves down its own potential gradient, which means towards the solution with more dissolved particles dissolved in it. Aquaporins greatly speed the process, though a little water also crosses the bilayer directly between the phospholipids.
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Aquaporins are membrane proteins that
Answer: D
They form narrow channels selective for water and exclude ions, which is why they speed osmosis without dissipating the cell's electrical gradients. Kidney tubule cells are especially rich in them, which is how urine is concentrated.
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An animal cell placed in a hypotonic solution will
Answer: A
Water enters because the surroundings are more dilute than the cytoplasm, and with no cell wall to resist the rising pressure the membrane may rupture. In the case of a red blood cell this is called haemolysis, and it releases haemoglobin into the surrounding fluid.
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A plant cell placed in a hypertonic solution becomes
Answer: B
Water leaves the vacuole and the protoplast shrinks away from the rigid wall. The wall itself does not prevent water movement at all, since it is freely permeable to water and small solutes alike. Only the plasma membrane controls what crosses.
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Assertion (A): A plant cell in pure water becomes turgid but does not burst.
Reason (R): The cell wall exerts an inward pressure that opposes further water entry.
Answer: C
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. Water enters by osmosis and the protoplast presses outward against the wall, which resists and develops turgor pressure. That pressure opposes further entry, so a point is reached where the net flow stops and the cell is turgid but intact. An animal cell has no wall to develop such a pressure, which is why it lyses instead.
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State whether the following is true or false.
A red blood cell placed in distilled water will shrink.
Answer: D
It swells and bursts, because distilled water is hypotonic to the cytoplasm and water moves inward. Shrinkage happens in the opposite case, when the surroundings are hypertonic, and the puckered appearance it produces is called crenation.
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