Practice Test 96 - AP Biology

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

The emergence of new infectious diseases, like avian influenza or COVID-19, often involves a pathogen 'jumping' from an animal host to humans. What evolutionary process is the most direct cause of a pathogen acquiring this ability?
Two plant cells are connected by plasmodesmata. A fluorescently labeled small sugar is injected into Cell 1 and is detected in Cell 2 within seconds. A larger fluorescently labeled protein injected into Cell 1 remains in Cell 1. When plasmodesmata are experimentally blocked, neither label appears in Cell 2. Which of the following best explains the communication mechanism between Cell 1 and Cell 2?
A freshwater snail population includes shell colors light and dark, controlled by alleles $L$ and $l$ . In a lake with dark sediment, fish consume light snails more often. Over ten generations, allele $l$ increases from 0.40 to 0.85, and light shells become rare. Which outcome best describes the evolutionary change occurring in this population?
A synthetic nucleic acid is built using nucleotides that include a phosphate group, a pentose sugar, and a nitrogenous base. During polymer formation, the nucleotides link into a strand with repeating sugar-phosphate units, while the bases extend from the backbone. The strand has polarity, with distinct 5′ and 3′ ends determined by the sugar's carbon numbering. The chemical linkage between adjacent nucleotides is the same throughout the backbone. Which feature best explains what creates the repeating backbone of a nucleic acid strand?
Two groups of genetically identical bean plants were watered with solutions differing only in salt concentration. Plants exposed to higher salinity developed shorter roots and accumulated more compatible solutes in root cells than plants in low salinity. DNA sequencing of a root-growth gene was identical in both groups. When high-salinity plants were moved to low-salinity water, newly formed roots grew longer than earlier roots. Which explanation best accounts for the root phenotype differences?
A population of lizards has allele frequencies $p=0.9$ and $q=0.1$ and is initially in Hardy-Weinberg equilibrium. A small group founds a new isolated population. Which outcome is most likely in the new population after a few generations?
In a population of 500 chickens, feather color is influenced by two alleles: $B$ and $b$ . Breeders mate only black-feathered birds each generation and do not breed white-feathered birds. After several generations, far fewer chicks hatch with white feathers. Which outcome is most likely regarding allele frequencies in the population?
A ligand (L) binds a G protein–coupled receptor (GPCR) in the plasma membrane, causing the Gα subunit to exchange GDP for GTP and dissociate. Gα-GTP activates adenylyl cyclase, increasing intracellular cAMP. cAMP binds protein kinase A (PKA), releasing active catalytic subunits that phosphorylate a cytosolic enzyme E, increasing E activity. In a cell-free assay, adding L increases E activity 8-fold. When a nonhydrolyzable GTP analog is provided, E activity remains high even after L is removed. When a phosphodiesterase that degrades cAMP is added during L exposure, E activity increases only 1.5-fold. Which change would most likely prevent amplification of the signal from receptor binding to enzyme E activation?
A frog population in wetlands was counted before and after a highway was built that reduced connectivity among ponds. One year before construction, the population was 900. In the first year after construction, births were 320 and deaths were 310; immigration was 20 and emigration was 110. Which explanation best accounts for the population change after construction?
A student models hydrothermal vent chemistry by mixing $\mathrm{CO_2}$ and $\mathrm{H_2}$ with iron-sulfide minerals at high temperature and pressure. After several hours, the student detects increased amounts of simple organic acids compared with a control lacking the minerals. The student argues that mineral surfaces could have contributed to prebiotic synthesis. Which conclusion is best supported by the evidence provided?
Cells respond to ligand Y with a rapid increase in cytosolic Ca $^{2+}$ . When a drug that chelates cytosolic Ca $^{2+}$ is applied, ligand binding to the receptor is unchanged, but the early Ca $^{2+}$ -dependent response is not observed. Which of the following best explains the role of Ca $^{2+}$ in early transduction in this case?
A plant cell is placed in a concentrated salt solution. The plasma membrane pulls away from the cell wall, and the cytoplasm shrinks inward. The cell wall remains rigid and retains its shape. Which feature best explains why the wall does not shrink with the cytoplasm?
In a culture dish, neuron A releases neurotransmitter X into a synaptic cleft. Muscle cell B contracts only when it expresses receptor R on its membrane. When an R-blocking drug is added, X is still released but B does not contract; a nearby cell lacking R shows no response in either condition. Which of the following best explains the specificity of this cell-to-cell communication?
A rabbit population includes alleles C and c. In year 1, $p(c)=0.30$ . A new predator arrives; over four years, genotype cc individuals contribute fewer offspring to the next generation than Cc or CC individuals. By year 5, $p(c)=0.12$ , and the population size remains large with no immigration observed. Which process is most likely responsible for the decrease in allele c frequency?
During aerobic respiration, the electron transport chain proteins are embedded in the inner mitochondrial membrane. As electrons pass through the chain, protons are pumped from the matrix into the intermembrane space, creating a proton gradient across the inner membrane. ATP synthase uses the movement of protons back into the matrix to phosphorylate ADP to ATP. If the inner membrane becomes freely permeable to protons, oxygen consumption may continue but ATP production drops sharply. Which conclusion is best supported about how compartmentalization affects cellular efficiency?
A researcher compares two beakers: one contains pure water, and the other contains water with dissolved sucrose. Sucrose has many hydroxyl (–OH) groups. Water is polar, with partial negative charge near oxygen and partial positive charge near hydrogen. Hydrogen bonds form when the partially positive hydrogen of one molecule is attracted to a partially negative oxygen on another molecule. In the sucrose solution, water molecules frequently form hydrogen bonds with sucrose's oxygen atoms as well as with other water molecules. This affects how sucrose disperses throughout the beaker. Which statement best explains why sucrose dissolves readily in water?
In chloroplasts, water is oxidized at a photosystem, releasing electrons that pass through an electron transport chain and ultimately reduce NADP $^+$ to NADPH. The electron transport chain also moves protons into the thylakoid lumen, creating a gradient that drives ATP synthase. A researcher adds a chemical that makes the thylakoid membrane highly permeable to protons without blocking electron transfer. Which outcome is most likely in illuminated chloroplasts after the chemical is added?
A researcher uses a fluorescent dye that labels the nuclear envelope. The envelope appears continuous with a network of membranes in the cytoplasm studded with ribosomes. Which feature best explains this observation?
A diploid organism has 2n = 4. In one meiotic cell, both homologs of chromosome 1 move to the same pole during anaphase I, while the homologs of chromosome 2 separate normally. Sister chromatids stay together through meiosis I. After meiosis II proceeds normally, four gametes are produced. Which chromosome composition is most likely among the gametes from this cell?
In a lizard population, some individuals have a heritable allele for longer hind limbs, while others have shorter limbs. After a hurricane increases open sandy habitat, longer-limbed lizards run faster on sand and produce more offspring than shorter-limbed lizards. Limb-length variation remains among hatchlings. Which outcome is most likely over time in this population?

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