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Practice Test 10 - AP Biology

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

  1. In a large coastal snail population, shell color is controlled by a single gene with two alleles: snails with genotype BBBB or BbBb have dark shells, and snails with genotype bbbb have light shells. Before a storm, allele BB frequency is 0.60 and allele bb is 0.40. The storm deposits dark sediment, after which birds more easily spot and eat light-shelled snails, so dark-shelled snails are more likely to survive and reproduce. No snails migrate into or out of the population. Five generations after the storm, allele BB frequency is 0.78. Which statement best explains how variation affected this change in allele frequency across generations?
  2. An enzyme-catalyzed reaction is run at saturating substrate concentration (well above the level that produces a rate plateau). In Trial 1, enzyme concentration is 1×. In Trial 2, enzyme concentration is doubled while all other conditions are unchanged and no inhibitor is present. The initial rate in Trial 2 is approximately twice that of Trial 1. Which explanation best accounts for this result at the molecular level?
  3. A cell's cytosol contains 1 mM Ca2+, while the extracellular fluid contains 2 mM Ca2+. A specific Ca2+ transporter moves Ca2+ from the cytosol to the extracellular space, and transport stops when ATP is absent. Ca2+ continues to be exported even though cytosolic Ca2+ is lower than extracellular Ca2+. Which mechanism best explains Ca2+ export?
  4. In a freshwater lake, a fungal pathogen infects a dominant zooplankton species. During weeks with high fungal infection, zooplankton grazing pressure decreases and phytoplankton biomass increases, while fish abundance remains stable. In weeks with low infection, zooplankton grazing increases and phytoplankton biomass decreases. Which interaction best explains the reduced zooplankton grazing during high-infection weeks?
  5. A membrane protein forms a pore that allows K+^+ to cross the membrane. In an experiment, extracellular K+^+ is 2 mM and cytosolic K+^+ is 120 mM. When the pore is opened, net K+^+ movement is from the cytosol to the outside until concentrations approach equilibrium. The movement continues when ATP synthesis is inhibited but stops when the pore is blocked. Which mechanism best explains K+^+ movement through the pore?
  6. In an experiment, a membrane vesicle contains 100 mM sucrose and is placed in 100 mM sucrose solution. The membrane is impermeable to sucrose but permeable to water. Then 50 mM NaCl is added only to the external solution; Na+ and Cl− cannot cross the membrane. The vesicle's volume decreases over several minutes, and ATP is absent. Which mechanism best explains the volume change?
  7. In a eukaryotic cell line, gene Y transcription requires an activator that binds an enhancer 2 kb upstream. The activator recruits a histone acetyltransferase (HAT), increasing acetylation of nearby histone tails and correlating with higher gene Y mRNA. A drug that inhibits the HAT is added; the activator still binds DNA, but histone acetylation near the enhancer decreases. Which change would most likely restore high transcription of gene Y in the presence of the drug?
  8. Which of the following best explains, from an evolutionary perspective, why the second antibiotic treatment was ineffective?
  9. A researcher monitors NADH and NAD+ levels in a cell suspension. When oxygen is removed, NADH levels rise and NAD+ levels fall within minutes, even though glucose is still available. ATP production decreases compared with oxygen-present conditions. No inhibitors are added. The observation is limited to changes in electron carrier oxidation state and ATP output under different terminal electron acceptor availability. This scenario focuses on how availability of a terminal electron acceptor affects electron flow, NADH oxidation, and the cell's capacity to generate ATP via oxidative phosphorylation.
  10. Two groups of genetically identical fruit fly larvae are raised on diets that differ only in protein content. Adults from the high-protein diet are larger and have higher expression of insulin-like signaling genes in fat body cells; DNA sequencing shows no differences in those genes. If high-protein larvae are switched to low protein early in development, adult size decreases. Which explanation best accounts for the diet-dependent differences in adult body size?
  11. A scientist examines two cell sizes of the same species. Both have the same membrane thickness and the same permeability to CO2. Under identical conditions, larger cells show higher internal CO2 levels than smaller cells. Which explanation best accounts for the higher CO2 levels in larger cells?
  12. A mitotic cell reaches metaphase with all chromosomes aligned at the metaphase plate. However, one chromosome is attached to microtubules from only one spindle pole (monotelic attachment), while all others are bi-oriented. The spindle checkpoint detects the improper attachment and prevents the onset of anaphase. Which outcome is most likely if the improper attachment persists?
  13. Two marine food chains begin with the same phytoplankton production of 10,000 kJ/m2^2/yr. Chain 1: phytoplankton → krill → penguins. Chain 2: phytoplankton → krill → small fish → seals. Assume ~10% of energy stored at one level becomes new biomass at the next level. Which outcome is most likely when comparing top-level biomass production between the two chains?
  14. In a coastal grass population, blade waxiness varies continuously. After a decade of frequent salt spray, plants with higher waxiness produce more seeds per plant than less waxy plants. Over four generations, the mean waxiness increases from 4.1 to 6.3 (arbitrary units), while the variance stays similar. Which pattern best illustrates the type of natural selection acting on waxiness in this population?
  15. A lab measures oxygen consumption and ATP synthesis in mitochondria supplied with ADP + Pi and a steady source of NADH. When ADP becomes depleted, oxygen consumption drops sharply even though oxygen is still present. Electron transport proteins remain intact, and no inhibitors are added. The inner membrane remains impermeable to protons except through ATP synthase. Because ATP synthase lacks its substrate, protons accumulate outside the matrix, strengthening the gradient and opposing further proton pumping by the electron transport chain. Which explanation best accounts for the decreased oxygen consumption when ADP is depleted?
  16. In a receptor signaling pathway, ligand L activates receptor R, which activates kinase K. K phosphorylates inhibitor I, causing I to release enzyme E. Free E then catalyzes conversion of substrate to product. In cells expressing a mutant I that cannot be phosphorylated, L binding to R is normal but product formation does not increase. Which step is most directly disrupted by the mutation?
  17. A researcher isolates two membrane fractions from the same eukaryotic cell. Fraction 1 contains proteins embedded in a lipid bilayer and is enriched in cholesterol. Fraction 2 contains proteins embedded in a lipid bilayer but has little cholesterol. At low temperatures, proteins in fraction 1 remain more laterally mobile than proteins in fraction 2. Both fractions have similar phospholipid types and protein densities. Which feature best explains the difference in lateral mobility at low temperature?
  18. Five species (A–E) were analyzed for a DNA marker. Pairwise differences (number of nucleotide substitutions) are: A–B:2, A–C:7, A–D:8, A–E:9; B–C:7, B–D:8, B–E:9; C–D:3, C–E:4; D–E:2. Assume fewer substitutions indicate a more recent common ancestor. Which inference is best supported by the data?
  19. Scientists tested whether protocells could grow by incorporating additional lipids. Fatty-acid vesicles were placed in solution, and extra fatty acids were added gradually. Over time, vesicle diameters increased and some vesicles divided into smaller vesicles when gently agitated. A control with no added fatty acids showed little size change. Which claim is best supported by these observations about protocells?
  20. A photosynthetic protist contains an organelle where light reactions occur. The organelle has two surrounding membranes, and inside it are membrane stacks resembling thylakoids. The organelle contains circular DNA with sequences most similar to cyanobacterial genes, and its ribosomes are inhibited by chloramphenicol. During cell division, the organelle divides by binary fission, independent of mitosis. Which explanation best accounts for the origin of this organelle?

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