Practice Test 4 - AP Biology

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

An RNA molecule folds into a stem-loop when complementary regions within the same strand base-pair. Which feature best explains how a single RNA strand can form double-stranded segments?
A fluorescent ligand binds to receptors on the surface of living cells. At low ligand concentration, binding is detected and increases with concentration; at high concentration, binding reaches a maximum and no longer increases. The intracellular response also plateaus at high ligand concentration. Which of the following best explains the plateau in the context of signal reception?
A eukaryotic lineage contains an organelle that performs aerobic respiration. The organelle has a double membrane, and its inner membrane includes many proteins related to bacterial electron transport proteins. Antibiotics that inhibit bacterial ribosomes reduce protein synthesis inside the organelle but do not affect cytosolic protein synthesis. The organelle contains a small circular genome. Which evidence best supports the origin of this organelle by endosymbiosis?
In pea plants, purple flowers (P) are completely dominant to white flowers (p). A student crosses two purple-flowered plants and is told both parents are heterozygous (Pp). Each parent produces gametes carrying either P or p with equal probability. Assume fertilization is random and the gene is autosomal. Which proportion of offspring is expected to have white flowers?
In an isolated mitochondrion preparation supplied with abundant NADH and ADP + Pi, researchers add cyanide, which blocks electron transfer to oxygen at the end of the electron transport chain. Protons had been pumped into the intermembrane space as electrons moved through complexes, creating a gradient used by ATP synthase. After cyanide addition, electron flow through the chain stops and NADH can no longer be oxidized to NAD+. Which outcome is most likely to occur immediately in this preparation?
In a rabbit population, fur thickness varies and is heritable. A new parasite attaches more successfully to rabbits with very thick fur, reducing their reproductive success compared with rabbits with thinner fur. Both fur types continue to appear among newborns. Which explanation best accounts for how allele frequencies will change over generations?
A snail population shows genetic variation in shell thickness. A crab predator is introduced and can crush thin shells more easily than thick shells. After several years, surveys show an increased proportion of thick-shelled snails, and breeding experiments indicate shell thickness is heritable. Which statement best explains the role of variation in the population's evolution?
In a diploid cell (2n=4), homologous chromosome pair 1 carries alleles A and a, and pair 2 carries alleles B and b. During metaphase I, the two homologous pairs align independently at the metaphase plate, so either the maternal or paternal homolog of each pair can face the same pole. No crossing over occurs in this cell. After meiosis I and II, four haploid gametes form. Considering only the arrangement of whole chromosomes into gametes, the cell can produce gametes with different combinations of the A/a chromosome and the B/b chromosome. Which meiotic process best explains the production of gametes AB, Ab, aB, and ab from this one cell?
A word-described phylogeny for four plant species is: (A,B) form a clade; that clade is sister to C; the clade containing A, B, and C is sister to D. No timing information is given. Based on this branching pattern, which statement is best supported?
Researchers measure early signaling in cells exposed to ligand M. M binds a membrane receptor, and within 10 seconds IP $_3$ levels rise sharply. When phospholipase C (PLC) is inhibited, M still binds the receptor but IP $_3$ does not increase. Which of the following best explains the role of PLC in early transduction here?
A biologist monitored two fenced grassland plots of equal size. Plot 1 began with 20 rabbits, and Plot 2 began with 80 rabbits. Over three months, rainfall and temperature were similar in both plots. By the end, Plot 2 had more individuals with bite wounds and a lower proportion of pregnant females, while Plot 1 showed few wounds and higher pregnancy rates. No new predators entered the fences. Which factor is most likely density dependent in regulating the rabbit populations?
A mitotic checkpoint protein (M) binds APC/C and inhibits it when M is phosphorylated. A phosphatase removes the phosphate from M after kinetochores attach, releasing APC/C. In mutant cells, M cannot be dephosphorylated, but kinetochore attachment occurs normally. Which outcome is most likely in the mutant cells?
A membrane protein has a transmembrane region enriched in amino acids with nonpolar side chains composed mostly of C and H, and an external region enriched in polar side chains containing O and N. The phospholipid bilayer interior is largely hydrocarbon, while the surrounding cytosol is water-based. Nonpolar side chains interact favorably with the bilayer's hydrocarbon tails via dispersion forces, whereas polar side chains can form hydrogen bonds with water. Which statement best explains why the C- and H-rich segment spans the membrane?
A stream is sampled upstream and downstream of a wastewater outfall. Upstream, macroinvertebrates include 18 taxa with fairly similar abundances. Downstream, the sample includes 7 taxa, and 85% of individuals are one worm taxon; total individuals collected are equal at both sites. Water temperature is similar. Which conclusion is best supported about the community-level consequences downstream?
A spherical cell doubles in radius; its membrane then folds, doubling surface area. Its SA:V is now...
In a coastal grass population, individuals vary in leaf wax thickness. Wax thickness is heritable and determined largely by alleles at one gene. After several unusually dry growing seasons, plants with thicker wax lose less water and produce more seeds than plants with thinner wax. No new individuals immigrate during this period. Which outcome is most likely in the population after several generations of these dry seasons?
In a temperate pond, larval dragonflies consume mosquito larvae. When dragonflies are experimentally removed from several enclosures, mosquito larvae density increases and grazing on algae by mosquito larvae increases, causing algae biomass to decrease compared with control enclosures. No other species are added or removed, and abiotic conditions are similar across enclosures. Which interaction best explains the initial decrease in mosquito larvae in control enclosures?
In a cell-free system, activated receptor fragments phosphorylate adaptor protein Ad on tyrosines. Ad recruits enzyme PI3K, which converts membrane lipid PIP $_2$ to PIP $_3$ . PIP $_3$ recruits kinase Akt to the membrane, where Akt becomes phosphorylated and then phosphorylates cytosolic target X. When a lipid phosphatase that converts PIP $_3$ back to PIP $_2$ is added, Akt phosphorylation decreases and X phosphorylation drops. Which change would most likely counteract the effect of the lipid phosphatase?
A lipid-soluble signaling molecule L is added to two cell types. Cell Type 1 responds even when a membrane-impermeable antagonist blocks all external ligand-binding sites on surface receptors. Cell Type 2 does not respond unless a specific membrane transporter is present. In both cell types, L remains chemically unchanged in the medium. Which of the following best explains the difference in communication mechanism?
In plant cells, the central vacuole can accumulate high concentrations of ions and organic acids compared with the cytosol. In an experiment, disrupting the tonoplast (vacuolar membrane) causes the cytosolic ion concentration to rise and several cytosolic enzymes to lose activity, even though the enzymes remain intact. Which feature best explains how compartmentalization contributes to cellular control in this situation?

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