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Practice Test 42 - AP Biology

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

  1. Early Earth's atmosphere likely lacked free oxygen and contained gases such as CH4\mathrm{CH_4}, NH3\mathrm{NH_3}, H2\mathrm{H_2}, and water vapor. In a lab simulation, researchers circulated these gases over boiling water and applied electrical sparks for one week. Chemical analysis detected multiple amino acids and other small organic molecules in the collection trap, while an identical setup without sparks produced far fewer organics. Which conclusion is best supported by these results about hypotheses for the origin of life on Earth?
  2. Immune cells exposed to lipid signal L show an increase in cytosolic Ca2+^{2+} within seconds. L is hydrophobic and crosses the plasma membrane. A receptor protein R is found primarily in the cytosol and binds L with high affinity. When R is mutated so it cannot bind L, Ca2+^{2+} does not increase after L addition, even though L still enters the cell. Which of the following is the most consistent prediction about the earliest step in this signaling pathway?
  3. Two distantly related taxa share an adaptive trait. Which evidence best supports common ancestry?
  4. A soluble protein folds so that nonpolar R groups are buried and polar/charged R groups are exposed to water. A mutation replaces a surface serine with phenylalanine, increasing local hydrophobicity without changing the primary structure length. The protein becomes less soluble and forms aggregates, reducing its normal binding function. Which feature best explains the aggregation and loss of function?
  5. A researcher compares chloroplasts under two light conditions with equal CO2_2: low light and high light. In both, the Calvin cycle uses ATP and NADPH to incorporate carbon in the stroma. Increasing light intensity increases the rate of photon absorption by photosystems, increasing electron excitation and electron flow through the thylakoid electron transport chain. Which outcome is most likely when shifting from low to high light, assuming CO2_2 remains nonlimiting?
  6. In muscle cells, myosin movement along actin requires repeated cycles of ATP binding and hydrolysis. In an experiment, ATP is replaced with a nonhydrolyzable ATP analog that can bind myosin but cannot be cleaved. The myosin heads bind actin but show little movement and remain attached longer than normal. Which explanation best accounts for the reduced movement when ATP cannot be hydrolyzed?
  7. Two cube-shaped cells have identical membrane composition and are in the same solution. Cell X has side length 2 units; Cell Y has side length 6 units. Nutrient entry rate is proportional to surface area, and nutrient use is proportional to volume. Which explanation best accounts for which cell more readily maintains nutrient concentration by diffusion?
  8. In a lab, a student compares two human cell types from the same individual: a skeletal muscle cell and a pancreatic beta cell. DNA sequencing shows the same alleles for several genes in both cells. However, RT-PCR detects abundant insulin mRNA in beta cells but not in muscle cells, while muscle cells show abundant myosin heavy-chain mRNA that is low in beta cells. Both cell types contain nuclei and are maintained under the same culture conditions for 24 hours. Which explanation best accounts for the different proteins produced by the two cell types?
  9. A population of frogs breeds in two ponds of equal size. In Pond 1, egg mass density is low, and 55% of tadpoles reach metamorphosis. In Pond 2, egg mass density is high, and only 18% reach metamorphosis. Measurements show similar water temperature and dissolved oxygen in both ponds. The main predator is a beetle larva that increases in number when tadpoles are abundant. Which outcome is most likely if tadpole density continues to increase in Pond 2?
  10. A population of 25 rabbits on an isolated peninsula has allele P frequency 0.48. Over 15 generations, allele P reaches fixation (1.00). Throughout the period, food availability, predator presence, and reproductive output show no consistent differences among genotypes, and no migration occurs. The population remains small each generation. Which factor most directly explains fixation of allele P?
  11. After a wildfire, ecologists compared plant communities in two nearby sites. Site X had 18 plant species with relatively similar abundances. Site Y had 6 plant species, and one grass species made up about 80% of individuals. Soil type and rainfall were similar at both sites. Which conclusion is best supported about biodiversity and community structure?
  12. Cells are exposed to 100 mM fructose outside and 10 mM inside. A saturable uptake rate is observed: increasing extracellular fructose from 100 mM to 200 mM produces little additional increase in uptake. Adding an inhibitor that binds a membrane protein decreases uptake immediately, while ATP levels remain constant. Which explanation best accounts for the saturable fructose uptake under these conditions?
  13. A fish population in a small lake was estimated each spring for 8 years. The population grew from 500 to 900 fish in the first two years, then increased more slowly to 1,050 fish by year 5. From years 5–8, estimates remained near 1,060–1,090 fish despite similar water temperature and no fishing. Surveys showed decreased dissolved oxygen near the bottom and reduced invertebrate prey density as fish density increased. Which growth pattern best describes this population over time?
  14. A researcher compares two solutions separated by an artificial membrane containing aquaporin proteins. Water moves across the membrane much faster than across an identical membrane lacking aquaporins. Which feature best explains the increased water movement?
  15. A symporter in the plasma membrane binds Na+^+ and an amino acid outside the cell and releases both into the cytosol. In the experiment, extracellular Na+^+ is high and cytosolic Na+^+ is low. The amino acid accumulates in the cytosol even when its cytosolic concentration exceeds the extracellular concentration. Transport stops when the Na+^+ gradient is eliminated but continues when ATP is not directly available to the symporter. Which process is most likely occurring?
  16. Plant cells with a cytosolic solute concentration of 0.25 M are placed in a 0.25 M solution of a nonpenetrating solute. Water can cross the membrane, and the solute cannot. After 15 minutes, cell size looks unchanged. Which outcome is most likely for net water movement?
  17. A lab grows identical cells under conditions where only passive diffusion supplies O2. The cells increase in diameter over several hours, while membrane composition remains unchanged. Later measurements show that internal O2 concentration decreases as diameter increases. Which explanation best accounts for the trend in internal O2 concentration?
  18. Which molecular evidence best resolves very ancient branches in a phylogeny?
  19. In a yeast cell, pheromone activates a GPCR that triggers a MAPK cascade. The terminal MAPK phosphorylates a GTPase-activating protein (GAP), increasing GAP activity toward the upstream G protein alpha subunit. Increased GAP activity accelerates GTP hydrolysis on the G protein, reducing signaling despite continued pheromone. A GAP mutant that cannot be phosphorylated shows prolonged MAPK activation. Which outcome best explains the feedback's effect?
  20. A phospholipid bilayer has a hydrophobic interior that favors diffusion of nonpolar molecules. Increasing polarity decreases permeability because polar solutes interact strongly with water and are poorly soluble in the membrane core. Compare methanol (CH3OH) and ethylene glycol (HO–CH2–CH2–OH). Both are small and uncharged, but ethylene glycol has two hydroxyl groups, making it more polar. No transport proteins are present. Which molecule would most likely diffuse across the bilayer faster?

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