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Practice Test 46 - AP Biology

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

  1. A cell is placed in a hypertonic solution with higher solute concentration than its cytosol. Over several minutes, the cell shrinks as water exits. The rate of water loss increases when aquaporins are inserted into the membrane and is unaffected by ATP depletion. Net water movement is from cytosol to outside, down the water concentration gradient. Which mechanism best explains the water movement?
  2. A eukaryotic cell is treated with a drug that depolymerizes microtubules. Shortly after, chromosomes fail to separate during cell division, and vesicles accumulate near the Golgi instead of moving to the cell periphery. Which feature best explains both observations?
  3. In a population of frogs, an enzyme locus has alleles E and e. Researchers calculate allele frequencies as p(E)=0.40p(E)=0.40 and q(e)=0.60q(e)=0.60 from a sample of adults. Under Hardy-Weinberg equilibrium, expected genotype frequencies are 0.16 EE, 0.48 Ee, and 0.36 ee. The observed adult genotype frequencies are 0.16 EE, 0.32 Ee, and 0.52 ee. The population is large and shows random mating based on breeding-call observations, and migration is not detected. Which inference is best supported by the difference between expected and observed genotypes?
  4. An enzyme is assayed at constant enzyme concentration and saturating substrate concentration. When the temperature is increased from 20°C to 35°C, the initial rate increases. When temperature is further increased to 60°C, the initial rate drops sharply, and repeating the assay at 35°C with the same enzyme sample does not restore the original rate. pH is constant and substrate remains saturating. Which explanation best accounts for the persistent loss of activity after exposure to 60°C?
  5. Two fish populations live in separate lakes. In Lake 1, allele C frequency is 0.80; in Lake 2, allele C frequency is 0.20. A canal is opened, allowing fish to move freely between lakes and breed. After several generations, both lakes show allele C frequency near 0.50, with no consistent differences in survival among genotypes within either lake. Which process is most likely responsible for the convergence in allele frequencies?
  6. A cell maintains Ca2+ at 0.0001 mM in the cytosol and 1.0 mM outside. A membrane pump moves Ca2+ from cytosol to extracellular fluid. When ATP is depleted, Ca2+ export rapidly decreases, and cytosolic Ca2+ rises. The lipid bilayer is not permeable to Ca2+. Which mechanism best explains Ca2+ export under normal conditions?
  7. In epithelial cells, ligand A binds receptor R, activating kinase K1, which phosphorylates K2. Phosphorylated K2 activates a small GTPase G by promoting GDP-to-GTP exchange. Active G recruits enzyme Z to the membrane, where Z converts inactive precursor U to active U* by proteolytic cleavage. U* then activates ion channel C, increasing Cl−^- efflux within 2 minutes. A mutation prevents K2 phosphorylation; A no longer increases Cl−^- efflux. Which manipulation would most likely restore Cl−^- efflux in the mutant cells after A addition?
  8. An enzyme-catalyzed reaction is measured at constant substrate concentration (well above the enzyme's KmK_m) and constant temperature. Trial 1 contains enzyme E only and shows an initial rate of 120 nmol/min. Trial 2 contains the same amount of enzyme plus 10 µM inhibitor Y and shows an initial rate of 60 nmol/min. When the enzyme concentration is doubled in both trials (substrate still in excess), Trial 1 increases to 240 nmol/min, but Trial 2 increases only to 120 nmol/min. Which explanation best accounts for these results at the molecular level?
  9. During aerobic respiration, electrons removed from organic molecules reduce NAD+ to NADH during glycolysis and the citric acid cycle. NADH then transfers electrons to the electron transport chain in the inner mitochondrial membrane. As electrons move through the chain, energy is used to pump protons from the matrix to the intermembrane space, creating an electrochemical gradient. Oxygen is present and accepts electrons at the end of the chain, forming water. ATP synthase uses the proton gradient to phosphorylate ADP. Which statement best describes oxygen's role in ATP production?
  10. In a cell, a ligand activates a GPCR that increases IP3_3 and Ca2+^{2+}. Ca2+^{2+} activates an enzyme that phosphorylates the IP3_3 receptor on an inhibitory site, decreasing its probability of opening at a given IP3_3 concentration. Which outcome best illustrates how this feedback affects cytosolic Ca2+^{2+} dynamics during continuous ligand stimulation?
  11. In a multicellular organism, cells secrete signal P that binds receptors on the same cell type. When cells are cultured at very low density, they show little response to P. When cultured at higher density without changing receptor number per cell, the response increases. Which of the following best explains the density effect?
  12. A student places identical onion epidermal cells (internal solute concentration 0.28 M) into Solution Q containing 0.18 M solute. The solute does not cross the membrane during the observation period. After 10 minutes, the cytoplasm presses firmly against the cell wall. Which outcome is most likely for water movement in Solution Q?
  13. A lab measures melting temperatures of two equal-length DNA fragments: Fragment 1 is 70% GC, Fragment 2 is 30% GC. Which feature best explains the higher melting temperature of Fragment 1?
  14. In a bird population, beak depth is heritable and affects seed-cracking. During a drought spanning 5 generations, birds with deeper beaks produce more fledglings on average than birds with shallow beaks. Across these generations, the population mean beak depth increases, and the frequency of alleles associated with deeper beaks rises. Population size remains large, and migration is not detected. Which pattern best illustrates the selection acting on beak depth?
  15. A cytosolic enzyme includes several ionic interactions between acidic and basic R groups that help maintain tertiary structure. A student adds a reagent that reduces disulfide bonds, converting S—S\text{S—S} linkages to two —SH\text{—SH} groups without changing the amino acid sequence. After treatment, the enzyme's activity decreases even though temperature and pH are unchanged. Which feature best explains the decreased activity?
  16. A plant species has two populations that overlap geographically. Cross-pollination experiments show that pollen from population A lands on stigmas of population B and germinates, but pollen tubes usually fail to reach ovules; the reciprocal cross shows the same pattern. Within-population crosses produce seeds at high rates. Field observations show pollinators visit both populations, and flowering times overlap. Allele frequencies differ between populations at multiple loci. Which reproductive barrier most directly limits gene flow between the populations?
  17. A scientist compares two RNA molecules of similar length. RNA X contains only nucleotides with standard bases (A, U, G, C). RNA Y is identical except several uracils are chemically modified so they can no longer form hydrogen bonds with adenine. Under the same conditions, RNA Y shows reduced formation of stem regions and remains more single-stranded. The scientist concludes that the modification affects RNA secondary structure. Which statement best explains the observed difference at the molecular level?
  18. To evaluate the RNA world hypothesis, researchers compared two systems in water at similar temperature and pH. In system 1, short RNA strands were supplied with activated ribonucleotides; in system 2, short DNA strands were supplied with activated deoxyribonucleotides. After incubation, system 1 produced longer RNA products without added proteins, while system 2 showed little to no strand extension. Which claim is best supported by these results about early genetic systems?
  19. In smooth muscle cells, acetylcholine activates a GPCR that increases IP3_3 and releases Ca2+^{2+} from the ER. Ca2+^{2+} activates myosin light chain kinase (MLCK). Active MLCK phosphorylates and activates a Ca2+^{2+} pump in the plasma membrane, increasing Ca2+^{2+} efflux. Blocking MLCK-dependent pump phosphorylation increases the duration of cytosolic Ca2+^{2+} elevation after acetylcholine. Which change would most likely result from blocking this feedback?
  20. Two neighboring frog populations differ in allele K frequency: Population 1 has p(K)=0.20p(K)=0.20, and Population 2 has p(K)=0.80p(K)=0.80. After a wet season creates a temporary corridor, many frogs move between ponds. In the following breeding season, Population 1 has p(K)=0.35p(K)=0.35 and Population 2 has p(K)=0.68p(K)=0.68. Both populations remain large, and no genotype at this locus shows consistent differences in survival or reproduction. Which factor most directly explains the convergence in allele frequencies?

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