Practice Test 52 - AP Biology

20 questions. Answer them in any order, then submit to see your score and every explanation.

In an experiment, mitochondria are given excess substrate and oxygen. When ADP is added, oxygen consumption rises sharply; when ADP is depleted, oxygen consumption falls. Which explanation best accounts for the dependence of oxygen use on ADP availability?
A plant cell is placed in a solution with the same total solute concentration as its cytosol, but the external solutes are impermeable while the cell contains many small permeable solutes. Over time, the permeable solutes diffuse out of the cell through existing membrane channels. Water can cross the membrane freely. Which change is most likely as the permeable solutes leave the cell?
A desert lizard is observed at noon (ground temperature $45^{\circ}\mathrm{C}$ ) and at dusk (ground temperature $28^{\circ}\mathrm{C}$ ). At noon, the lizard spends most time in shade and holds its body off the ground; at dusk, it forages in open areas and lies closer to the ground. Which response best explains the lizard's short-term regulation of body temperature?
A membrane contains a carrier protein that binds lactose on the outside and undergoes a conformational change to release it inside. As extracellular lactose increases, the transport rate increases but then reaches a maximum. Net movement occurs only when lactose is higher outside than inside, and ATP removal does not affect transport. Which mechanism best explains lactose transport?
A muscle cell contains many mitochondria positioned near contractile fibers. In an experiment, the inner mitochondrial membrane is selectively damaged, while the outer membrane remains mostly intact. Soon after, the cell's oxygen consumption decreases and ATP levels drop, even though glycolysis continues in the cytosol. The inner membrane normally has extensive folds called cristae. Which feature best explains why damage to the inner membrane reduces ATP production?
A receptor protein binds a hormone using a pocket formed by its tertiary structure, maintained by hydrogen bonds between polar R groups and ionic interactions between charged R groups. When the pH is lowered, several acidic side chains become protonated, changing their charge state. The receptor's primary structure is unchanged, but hormone binding decreases. Which statement best describes the molecular cause of decreased binding?
Two neighboring frog populations differ at allele $G$ of a neutral locus: in Population 1, $p(G)=0.20$ ; in Population 2, $p(G)=0.80$ . A new highway prevents movement between ponds for 30 generations. After isolation, Population 1 has $p(G)=0.19$ and Population 2 has $p(G)=0.81$ , with large population sizes and no genotype fitness differences detected. Which factor most directly explains why allele frequencies changed very little?
A scientist isolates mitochondria and supplies them with ADP + Pi and either NADH or succinate as the electron donor. NADH donates electrons to an early electron transport complex that pumps protons, while succinate donates electrons downstream, bypassing one proton-pumping step. Oxygen is present in both conditions, and ATP synthase is functional. The scientist measures ATP produced per molecule of electron donor oxidized. Which outcome is most likely?
After a wildfire, ecologists compared plant communities in two grassland sites one year later. Site 1 had 15 plant species with relatively even cover across species. Site 2 had 15 plant species, but one grass species accounted for 80% of total cover. Which conclusion is best supported about biodiversity at the community level?
A group of cells is analyzed for DNA content using fluorescence intensity. One subset shows a single peak at 2C, another subset shows a peak at 4C, and a third subset has intermediate values between 2C and 4C. Microscopy of the intermediate subset shows an intact nuclear envelope and uncondensed chromatin. Microscopy of the 4C subset shows an intact nuclear envelope and uncondensed chromatin as well. Which cell-cycle phase most likely corresponds to the intermediate DNA-content subset?
A population of freshwater snails is counted monthly in a small pond with no immigration or emigration. After a nutrient runoff event, algae increases sharply. Snail abundance rises from 120 to 210 over two months, then remains near 205–215 for the next four months while algae levels fall to a steady, moderate level. Predatory fish abundance does not change during the study. Which explanation best accounts for the snails leveling off after the initial increase?
A cell is placed in a solution where the extracellular glucose concentration is 10 mM and intracellular glucose is 2 mM. The plasma membrane is impermeable to glucose unless a specific carrier protein is present. When the carrier is expressed, glucose enters the cell rapidly; adding an ATP synthesis inhibitor does not change the rate. No vesicles form, and the carrier shows saturation at high extracellular glucose. Which mechanism best explains glucose entry under these conditions?
A student measures gas exchange from a photosynthesizing leaf in light. In the thylakoid, water is oxidized, releasing O $_2$ and electrons; electrons move to NADP $^+$ , and ATP is produced using a proton gradient. The Calvin cycle in the stroma uses ATP and NADPH to incorporate CO $_2$ into organic molecules. If the leaf is moved to darkness while CO $_2$ remains available, which process is most likely to stop first?
On a coral reef, cleaner wrasse remove ectoparasites from larger client fish. When cleaner wrasse are excluded from several reef patches using mesh that allows clients to enter, client fish show higher parasite loads than in nearby control patches. The abundance of client fish remains similar between patches during the study. Which interaction best explains the lower parasite loads in control patches?
A phospholipid bilayer has a hydrophobic interior. A solute's permeability depends on how well it can enter this nonpolar region. Small, nonpolar molecules cross readily; polar molecules cross slowly; large polar molecules cross very slowly; ions cross least. Compare two uncharged molecules: ribose (a 5-carbon sugar with multiple hydroxyl groups) and isopropanol (a 3-carbon alcohol with one hydroxyl group). No transport proteins are present. Which molecule would most likely be more permeable across the bilayer?
Two cell types have similar shapes and identical membrane permeability to water. Cell M has a volume of 1,000 µm³; Cell N has a volume of 8,000 µm³. Both are transferred from an isotonic solution to a hypotonic solution. The initial rate of swelling is greater in Cell M than in Cell N. Which explanation best accounts for the difference in swelling rate?
Two amino acids have identical carbon backbones but different side chains. Amino acid X has a side chain containing sulfur as a thiol (–SH). Amino acid Y has a side chain containing only C and H. In an oxidizing environment, two thiol groups from amino acid X can form a covalent disulfide bond (–S–S–) between different parts of the same polypeptide chain. This covalent linkage reduces flexibility and stabilizes a particular three-dimensional shape. Which feature best explains why amino acid X can stabilize protein structure more than amino acid Y?
A newly discovered unicellular eukaryote contains a small energy-producing organelle. The organelle is surrounded by two membranes, contains circular DNA, and has ribosomes that are similar in size to those of bacteria rather than those in the cytosol. During cell division, the organelle replicates by a process resembling binary fission. Which evidence best supports a mechanism for the origin of this organelle?
Immune Cell X displays a membrane-bound signaling molecule S. Immune Cell Y shows a response only when it physically contacts Cell X; when separated by a permeable membrane that allows small molecules to pass, Cell Y does not respond. Adding purified soluble S to the medium also fails to trigger Cell Y. Which of the following best explains how Cell X communicates with Cell Y?
In a population of wild rabbits, a virus spreads that causes high mortality. A gene has two alleles, R and r, associated with resistance; rabbits with genotype RR have the highest survival, Rr intermediate, rr lowest. Before the outbreak, R = 0.40. Two years later, R = 0.58, and the virus remains present. No migration is detected. Which statement best explains why allele frequencies changed in the population?

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