Unit 2: Cell Structure and Function. Practice questions with scoring guidelines.

Bacterium S has channel proteins in its plasma membrane that open only when the membrane is stretched; when open, each channel lets ions and other small solutes through. Cells of the normal strain and of a mutant strain that lacks these channels, all with intact cell walls, were grown in a medium with a high solute concentration and then moved suddenly into distilled water or into fresh growth medium. In some cells, ATP synthesis was blocked for 10 minutes before the move. The table shows the percentage of cells surviving 10 minutes after the move.

Which explanation of how the normal strain survives the move into distilled water is best supported by the data?

The figure shows a mitochondrion in section, with the measured area of each of its two membranes. The mitochondrion is placed in a solution with a very low solute concentration, so water enters the matrix, which swells and becomes rounded. A membrane tears if it is stretched by more than a few percent. For a sphere of radius r, surface area = 4πr² and volume = (4/3)πr³. Which outcome is most likely by the time the matrix has swollen to a volume of 5 µm³?

A student cuts equal-sized cylinders from a fresh potato, blots and weighs each one, then submerges each cylinder in a sucrose solution of known molarity for 60 minutes. The cylinders are removed, blotted, and reweighed, and the mean percent change in mass is calculated. The results are shown in the table.

Predict what would happen if a fresh cylinder were left in distilled water (0.0 M) for a much longer period of time. Which outcome is most likely?

The Ca²⁺ concentration in the cytosol of an animal cell is about 0.0001 mM, and the fluid around the cell contains about 2 mM Ca²⁺. Ca²⁺ enters the cytosol continually through open channel proteins in the plasma membrane, while Ca²⁺ pumps in the plasma membrane move it back out. When the cells were treated with a drug that closes the channels, the cytosolic Ca²⁺ concentration stayed at about 0.0001 mM, but the cells' total rate of ATP use fell by 4 percent. Which statement best explains these results?

A species of bacterium releases toxin T, a protein that kills animal cells by making large holes in their plasma membranes. Researchers made two kinds of artificial vesicle, each filled with a dye: one whose membrane contained phospholipids only, and one whose membrane contained the same phospholipids together with cholesterol, in about the proportion found in animal plasma membranes. Each kind of vesicle was kept for 10 minutes in a solution with or without toxin T, and the percent of its dye that leaked out was measured in eight replicate samples. The results are shown in Table 1.

The bacterium that makes toxin T is enclosed by a plasma membrane but is not harmed by the toxin. Which explanation is best supported by the data?

Two species of single-celled alga, P and Q, each have one chloroplast. In both species, the proteins that capture light energy and release O₂ are embedded in the thylakoid membranes inside the chloroplast. Researchers measured the volume of the chloroplast, the total area of its thylakoid membranes, and the rate at which it released O₂ in bright light. Table 1 shows the means for 30 cells of each species. A student studying the table writes: "Per µm³ of chloroplast, Q releases O₂ four times as fast as P. So each light-capturing protein in Q must work four times as fast as each one in P."

Which statement best identifies the flaw in the student's reasoning?

The figure shows part of a leaf cell. In the chloroplast, O₂ is released into the space inside the thylakoids. Some of this O₂ diffuses to the matrix of the mitochondrion in the same cell. How many membranes, each a single phospholipid bilayer, must an O₂ molecule cross on this path, and how does it cross them?

The figure is a model of a cube-shaped cell from the lining of an earthworm's gut. Its top face, which faces the food in the gut, carries 2,000 finger-like projections of the plasma membrane (microvilli), each filled with cytoplasm. Model each projection as a cylinder of radius r and height h whose curved side adds 2πrh of membrane area and πr²h of volume to the cell; its flat tip only replaces the patch of top face it stands on. By about how many times do the projections raise the cell's surface area-to-volume ratio, compared with the same cube without them?

A biologist compares a bacterial cell with a plant cell using electron microscopy. Which structural difference best reflects the greater compartmentalization of the eukaryotic cell?

Researchers tried to remove the cell walls of a plant, a fungus and a bacterium without killing the cells, in a medium with the same solute concentration as their cytoplasm. Each of three different polysaccharide-digesting enzymes removed the walls of just one of the three kinds of cell, a different kind for each enzyme, leaving the cells alive and bounded by their plasma membranes. An enzyme that digests proteins removed none of the walls, and an enzyme that digests phospholipids killed all the cells but left their walls in place. Which conclusion is best supported?

Figure 1 is a model of carrier protein T in the plasma membrane of a cell. Solute S is a polar molecule that does not cross the phospholipid bilayer at a measurable rate. T switches back and forth between shape 1, in which its binding site for S faces the outside of the cell, and shape 2, in which the site faces the cytosol. T switches at random, whether or not S is bound, and uses no ATP. The site binds S equally well in either shape, and a bound molecule of S is released on whichever side the site faces after a switch. S is 5 mM outside the cell and 1 mM in the cytosol.

Which statement best explains why there is net movement of S into the cell through T?

Cylinders of beetroot tissue of equal size were rinsed and then placed in separate tubes of water held at different temperatures for 20 minutes. Beetroot cells store a red pigment inside their vacuoles; when the cell membranes are damaged, the pigment leaks out into the surrounding water. The absorbance of the surrounding water (a measure of how much pigment leaked) was recorded. Table 1 shows the results.

A student predicts that adding a lipid-dissolving solvent such as ethanol to a beetroot cylinder held at 20 °C would increase pigment leakage. Which reasoning best supports this prediction?

Researchers investigated how a drug that blocks aquaporin channels affects the movement of water into cells. Cells were exposed to one of four treatments, and the rate of osmotic water uptake was measured in eight replicate trials for each treatment. Table 1 shows the mean rate of water uptake ± standard error (SE). All cells were placed in the same hypotonic solution.

Which statement best describes the effect of the aquaporin blocker on the rate of water uptake, based on Table 1?

Red blood cells were placed in a series of sodium chloride (NaCl) solutions of different concentrations for 10 minutes. The relative volume of the cells, compared with their normal volume in blood plasma (taken as 100%), was then measured. Red blood cells lack a cell wall. Table 1 shows the results.

Which NaCl concentration is closest to being isotonic to the cytoplasm of the red blood cells?

Researchers investigated whether the uptake of large particles by cultured macrophage-like cells requires the cytoskeletal protein actin. Cells drawn from a single culture were divided between two treatments and incubated for 30 minutes at 37 °C with fluorescent latex beads supplied at the same concentration to both treatments. One treatment received buffer alone; the other received cytochalasin D, a compound that prevents actin filaments from assembling. Each cell was then examined under a fluorescence microscope and scored as either 'beads internalized' or 'no beads internalized', and 200 cells were scored in each treatment.

Which of the following states the null hypothesis for this investigation?

Researchers made two fluorescent probe molecules, F1 and F2. Each has two long nonpolar hydrocarbon tails joined to a small head group. The head of F1 carries a full negative charge; the head of F2 is polar but uncharged. Each probe was added to the solution around cultured cells, where within a minute it entered the outer layer of phospholipids of the plasma membrane. At intervals, compound Q was added to samples of the cells. Q cannot cross membranes, and it dims the glow of any probe head that it touches. No probe left the plasma membrane during the experiment. Table 1 shows the percent of each probe's glow that Q dimmed.

Which of the following best explains the results?