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Practice Test 18 - AP Biology

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

  1. Initial reaction rates are measured at multiple substrate concentrations for an enzyme with and without Inhibitor Z. With Z present, the maximum rate observed at very high substrate concentration is lower than without Z, and increasing substrate does not restore the original maximum. Enzyme concentration, pH, and temperature are constant. Which explanation best accounts for Inhibitor Z's effect?
  2. A mammalian gene Z is normally transcribed at low levels. The promoter contains a CpG island. After treatment with a DNA methyltransferase activator, methylation at the CpG island increases, and gene Z mRNA decreases. Histone deacetylase (HDAC) activity is unchanged. Which change would most likely increase transcription of gene Z despite high CpG methylation at its promoter?
  3. A muscle cell is observed to contain an extensive smooth endoplasmic reticulum (SR) network closely associated with bundles of contractile proteins in the cytoplasm. When the SR membrane is experimentally made leaky to ions, the cell shows prolonged contraction and delayed relaxation even when ATP is available. Which feature best explains how the intact SR normally supports rapid contraction cycles?
  4. A researcher compares two membrane preparations. Membrane A contains a higher proportion of unsaturated phospholipid fatty acid tails than Membrane B, while both have similar cholesterol content. At the same temperature, Membrane A shows greater lateral movement of lipids and embedded proteins. Which feature best explains the increased membrane fluidity in Membrane A?
  5. A lipid bilayer with a hydrophobic interior is exposed to two solutes without any transport proteins: methane (CH4; very small, nonpolar) and methanol (CH3OH; small, polar due to a hydroxyl group). Both are similar in size, so polarity is the main difference. Nonpolar molecules have weak interactions with water and more readily dissolve in the membrane's nonpolar core. Polar molecules form hydrogen bonds with water, increasing the energetic cost of entering the hydrophobic interior. Therefore, the solute with lower polarity is expected to diffuse across faster.
  6. In a lab assay, cells are exposed to ligand Q. Binding of Q to a membrane receptor is confirmed, and within 20 seconds GTP-bound G protein increases. When a nonhydrolyzable GDP analog is introduced, Q still binds the receptor but GTP-bound G protein does not increase. Which of the following best explains the inhibited early transduction event?
  7. During translation, a ribosome reads an mRNA segment 5′-AUG GCU UAA-3′. A tRNA with an anticodon complementary to GCU delivers alanine. The AUG codon is recognized by an initiator tRNA carrying methionine, and UAA is a stop codon that is recognized by a release factor rather than a tRNA. The ribosome moves along the mRNA one codon at a time and forms peptide bonds between adjacent amino acids as matching tRNAs bind. Which outcome is most likely for the synthesized polypeptide?
  8. Two populations of cells are given equal amounts of glucose. Population X has normal mitochondria; Population Y has mitochondria lacking a functional electron transport chain, but glycolysis enzymes are normal. In both populations, glycolysis produces pyruvate and NADH in the cytosol. In Population X, NADH-derived electrons enter the electron transport chain, supporting proton pumping and ATP synthase activity in the mitochondria. In Population Y, mitochondrial electron transport does not occur, so NADH is not efficiently oxidized by the chain. Which outcome is most likely in Population Y compared with Population X?
  9. In a dark room, a human's pupils enlarge within seconds, increasing light entry. Which mechanism best explains this response to low light?
  10. In a lab, a molecule is identified as a chain of repeating subunits formed when the carboxyl group of one subunit reacts with the amino group of another, releasing water. The resulting covalent linkage repeats to form a long polymer with variable side chains (R groups) projecting from the backbone. Such polymers can fold based on interactions among R groups, affecting molecular shape and chemical properties. Which statement best describes the macromolecule category and polymerization process for this molecule?
  11. A cell-surface receptor is suspected to be a ligand-gated ion channel. When ligand S2 is added, membrane potential changes within 2 seconds. A different treatment blocks all G proteins but does not affect the immediate membrane potential change after S2 addition. Which of the following best supports the conclusion that the receptor is a ligand-gated ion channel?
  12. D-glucose uptake saturates without ATP; L-glucose uptake is linear with concentration. This shows that
  13. A student examines meiosis in a species where one homologous chromosome pair contains alleles F and f. Prior to meiosis, DNA replicates so each homolog has two sister chromatids. During prophase I, homologs synapse, and a crossover occurs between nonsister chromatids at a locus between two markers on the chromosome. After meiosis II, two of the four gametes contain chromatids that include one marker region from the maternal homolog and the other marker region from the paternal homolog. Which meiotic process best explains the formation of these recombinant chromatids?
  14. A cell is treated with a drug that prevents formation of the contractile ring, so cytokinesis cannot occur. The cell progresses normally through G1, S, and G2, entering mitosis with duplicated chromosomes. The spindle checkpoint is satisfied, sister chromatids separate at anaphase, and two chromosome sets reach opposite poles. Nuclear envelopes reform around each set during telophase, but the cytoplasm does not divide. Which result is most likely immediately after telophase in the treated cell?
  15. Cells are placed in a solution where the extracellular solute concentration is higher than the cytosol. Over time, cell volume decreases. Blocking aquaporins slows the rate of volume change but does not prevent it. No ATP is required for the volume change, and the direction reverses when cells are moved to a more dilute solution. Which mechanism best explains the change in cell volume?
  16. A diploid organism has 2n=82n=8. During meiosis I in one spermatocyte, homologous chromosomes separate normally except one homologous pair fails to separate at anaphase I, with both homologs moving to the same pole. Meiosis II proceeds normally, separating sister chromatids. Which set of gametes is most likely produced from this cell?
  17. A steroid-like signaling molecule S is membrane-impermeable and binds a cell-surface receptor that activates a heterotrimeric GsG_s protein. GsG_s stimulates adenylyl cyclase, producing cAMP. cAMP activates a cAMP-gated ion channel in the plasma membrane, allowing Cl−^- efflux that changes membrane potential. A regulator protein binds adenylyl cyclase and decreases its catalytic rate, reducing cAMP production without affecting receptor binding. In cells overexpressing the regulator protein, S causes little change in membrane potential. Which result is best supported if the regulator protein is inhibited while S is present?
  18. The plasma membrane is composed of a phospholipid bilayer with hydrophobic tails that form a nonpolar interior. Some phospholipids have unsaturated fatty acid tails that introduce kinks, reducing tight packing. A researcher compares two artificial membranes: Membrane X has mostly saturated tails, and Membrane Y has more unsaturated tails. Both lack transport proteins. When exposed to the same small nonpolar solute, Membrane Y shows faster diffusion of the solute. Which feature best explains the higher permeability of Membrane Y?
  19. A cell's cytosol contains 0.28 M nonpenetrating solute. It is moved to a solution with 0.12 M nonpenetrating solute, and the membrane is permeable only to water. Soon after transfer, the cell's volume increases. Which outcome is most likely?
  20. In chloroplasts, the thylakoid membrane encloses a lumen. During light reactions, H+ accumulates in the thylakoid lumen relative to the stroma. When a chemical uncoupler allows H+ to cross the thylakoid membrane freely, ATP synthesis in the chloroplast decreases even though light absorption continues. Which feature best explains how compartmentalization affects ATP synthesis in chloroplasts?

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