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Practice Test 34 - AP Biology

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

  1. A population of meadow voles lives in two fenced fields of equal area with similar vegetation. Field X begins with 20 voles; Field Y begins with 120 voles. Over the next month, both fields experience the same rainfall and temperature. Researchers observe that the average number of offspring per female is lower in Field Y, and more voles in Field Y show bite wounds and spend more time hiding. No new predators enter either field. Which outcome is most likely caused by a density-dependent factor in Field Y?
  2. A breeder starts with a mixed population of pigeons and repeatedly breeds only individuals with the largest beaks. Beak size is influenced by alleles G (larger) and g (smaller). After many generations, nearly all pigeons have large beaks, and genetic testing finds allele g is rare. Which statement best describes a likely evolutionary consequence of this breeding program?
  3. In pancreatic cells, glucose causes membrane depolarization that opens voltage-gated Ca2+^{2+} channels. Ca2+^{2+} activates a cytosolic enzyme that produces second messenger SM from precursor P. SM binds and activates kinase K, which phosphorylates target R within 45 s. A drug that chelates cytosolic Ca2+^{2+} prevents SM production and R phosphorylation. Which manipulation would most likely restore R phosphorylation while Ca2+^{2+} remains chelated?
  4. Two groups of genetically identical bean seedlings were grown with the same light and water, but one group received nitrogen-rich fertilizer and the other received nitrogen-poor fertilizer. After 3 weeks, nitrogen-rich plants had higher leaf protein content and deeper green leaves. DNA sequences of a leaf protein gene were identical in both groups. Which explanation best accounts for the observed differences?
  5. In an experiment, cells are exposed to ligand L and the fraction of phosphorylated substrate S is measured. With L alone, S phosphorylation reaches 80% at 2 minutes. With L plus a competitive receptor antagonist, S phosphorylation reaches only 20%. With L plus a kinase inhibitor downstream of the receptor, S phosphorylation remains at 0%. Which change would most likely increase S phosphorylation in the presence of the receptor antagonist?
  6. A structural protein contains repeated β-pleated sheets stabilized by hydrogen bonds between backbone atoms. A mutation replaces several glycine residues with bulkier tryptophan residues within a region that must pack tightly. The protein becomes less flexible and its ability to form stable fibers decreases. Which feature best explains the reduced fiber formation?
  7. A diploid cell (2n = 4) completes DNA replication before meiosis. In metaphase I, two tetrads align at the metaphase plate, and spindle attachment allows either homolog of each pair to face either pole. Homologs then separate in anaphase I, and sister chromatids separate in anaphase II. Which outcome is most likely due only to the metaphase I alignment described?
  8. In cultured bacteria, gene X encodes a sugar transporter. A repressor protein binds a specific operator sequence overlapping the promoter and blocks RNA polymerase binding. When sugar S is added, S binds the repressor and reduces its affinity for the operator. After S addition, levels of gene X mRNA increase within minutes. Which change would most likely prevent the increase in transcription after adding sugar S?
  9. A fish is transferred to water with higher salinity; within hours, it drinks more and excretes concentrated urine. Which mechanism best explains this response?
  10. A eukaryotic cell contains many peroxisomes, each bounded by a single membrane and containing enzymes that transfer hydrogen from substrates to oxygen, forming hydrogen peroxide that is then broken down. Which outcome is most likely if peroxisomes are absent from this cell?
  11. A marine snail is placed in a tank where the water becomes gradually more acidic over 30 minutes. As pH drops from 8.1 to 7.6, the snail reduces movement and remains attached to the tank wall. When pH is returned to 8.1, the snail resumes crawling within 10 minutes. The snail's shell shows no visible change during the hour-long observation. Which response best explains the snail's reduced movement during lower pH?
  12. In a pancreatic beta cell, glucose metabolism increases ATP, closing ATP-sensitive K+^+ channels and depolarizing the membrane. Depolarization opens voltage-gated Ca2+^{2+} channels, raising cytosolic Ca2+^{2+}. Ca2+^{2+} activates a Ca2+^{2+}-dependent K+^+ channel that repolarizes the membrane, reducing Ca2+^{2+} channel opening. With sustained high glucose, Ca2+^{2+} levels show brief spikes rather than a continuously increasing plateau. Which outcome best explains how feedback shapes the Ca2+^{2+} response?
  13. Two droplets are placed on a waxy leaf surface: one droplet is water and the other is ethanol. The water droplet remains more rounded, while the ethanol droplet spreads more. Water molecules are polar and form extensive hydrogen bonds with each other because partially positive hydrogens are attracted to partially negative oxygens. On a waxy surface, there are few polar groups available for hydrogen bonding with water. As a result, water–water cohesion can dominate over water–surface adhesion. Which feature best explains why the water droplet stays more rounded on the waxy leaf?
  14. In a eukaryotic nucleus, transcription factor TF binds a promoter-proximal element of gene Z and increases transcription when TF is phosphorylated. A signaling kinase phosphorylates TF in response to hormone H. In cells treated with H, TF is present but gene Z mRNA does not increase. Sequencing shows TF is unchanged, but the kinase has a loss-of-function mutation. Which change would most likely increase transcription of gene Z in response to H?
  15. Two spherical cells have radii of 3 µm and 9 µm. Both have the same number of glucose transporters per µm² of membrane and the same internal glucose utilization rate per unit volume. When placed in the same glucose solution, the larger cell shows a lower steady-state internal glucose concentration. Which explanation best accounts for this observation?
  16. A peptide ligand L binds receptor tyrosine kinase (RTK) on the plasma membrane, causing receptor dimerization and autophosphorylation of cytosolic tyrosines. An adaptor protein binds these phosphotyrosines and recruits Ras, which becomes GTP-bound. Active Ras triggers a kinase cascade: kinase 1 phosphorylates kinase 2, which phosphorylates kinase 3. Kinase 3 phosphorylates cytosolic target T, changing its activity within 2 minutes. When a drug prevents RTK dimerization, Ras remains GDP-bound and T is not phosphorylated. Which change would most likely allow T phosphorylation even when RTK dimerization is blocked?
  17. A cell membrane is a phospholipid bilayer with a hydrophobic interior and embedded proteins. Researchers compare transport of two solutes: oxygen (small and nonpolar) and sucrose (large and polar). The membrane contains no sucrose-specific transport proteins. Both solutes are present at higher concentration outside the cell than inside. Which feature best explains why oxygen enters the cell more readily than sucrose?
  18. A protist contains an energy-processing organelle with two membranes. Sequencing shows several genes inside the organelle are most similar to genes in modern alphaproteobacteria, while the host nuclear genome lacks these gene versions. The organelle's membranes contain transport proteins that import many nucleus-encoded proteins. Which evidence best supports endosymbiosis as the origin of this organelle?
  19. In an epithelial cell, EGF binds EGFR, causing receptor dimerization and activation of its kinase domain. EGFR phosphorylates a membrane-associated adaptor that recruits Ras-GEF to activate Ras-GTP. Active Ras promotes activation of a downstream kinase (ERK). ERK phosphorylates Ras-GEF, decreasing Ras-GEF's ability to activate Ras. When EGF remains present, Ras-GTP levels peak and then decline. Which outcome best demonstrates the effect of the feedback loop on pathway behavior?
  20. A botanist sampled two desert washes after the same rainfall event. Wash A contained 7 annual plant species, each with 12–18 individuals. Wash B contained 7 annual plant species, but one species accounted for 90% of individuals. Total individuals were similar between washes. Which conclusion is best supported about biodiversity differences?

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