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Practice Test 75 - AP Biology

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

  1. When soil becomes dry, many plant stomata close within an hour and leaf water loss decreases. Which response best explains this short-term change?
  2. Immune cell I displays membrane-bound ligand L. Only cells physically contacting I show a response within minutes. When a membrane-impermeable enzyme cleaves L from I's surface, nearby cells no longer respond even though soluble fragments of L are detected in the medium. Which of the following best explains why contact is required?
  3. In a species with 2n=122n=12, a student counts chromosomes in a cell at metaphase II and observes 6 chromosomes aligned at the metaphase plate. Which conclusion is most consistent with this observation about the earlier meiotic division?
  4. During replication, each parental strand serves as a template for synthesis of a complementary strand, producing two double helices. A researcher provides a nucleotide pool lacking thymine (T) but containing adenine (A), cytosine (C), and guanine (G). Which outcome is most likely during DNA synthesis?
  5. In a population of 1,200 coastal mice, fur color is controlled by alleles B and b. Before a storm, the b allele frequency was 0.40. A hurricane randomly killed many individuals, leaving 60 survivors that founded the next generation. In the offspring produced by these survivors, the b allele frequency is 0.62. No evidence indicates differential survival by fur color, and no new mice immigrated during recovery. Which process is most likely responsible for the change in allele frequency between generations?
  6. Which part of a phylogeny is most affected by a misplaced outgroup?
  7. An experiment tracks labeled oxygen atoms (18^{18}O) introduced as molecular oxygen gas (O2_2) to a suspension of mitochondria provided NADH and ADP + Pi. During oxidative phosphorylation, electrons move through the electron transport chain and are ultimately transferred to oxygen, which becomes reduced. Water molecules accumulate in the mitochondrial matrix during the run. The researcher analyzes where the 18^{18}O label appears after respiration proceeds. Which result is most likely?
  8. A researcher compares cholesterol (a steroid with four fused rings and a small polar -OH group) with a typical phospholipid (two fatty acid tails and a phosphate-containing head). In a membrane, cholesterol positions its -OH near phospholipid head groups while its rings associate with fatty acid tails. Which feature best explains cholesterol's ability to insert into membranes in this orientation?
  9. A plant cell is exposed to a small, hydrophilic signaling peptide. A fluorescent tag shows the peptide remains outside the plasma membrane, yet cytosolic protein kinase activity increases within 20 seconds. When the receptor's extracellular domain is removed by protease treatment, the peptide no longer triggers kinase activation. Which of the following best explains how the signal is received?
  10. In epithelial cells, a ligand activates an RTK that triggers a kinase cascade culminating in kinase X activation at the plasma membrane. Kinase X phosphorylates an upstream adaptor protein, increasing the adaptor's affinity for the activated RTK and recruiting additional kinase cascade components. When the adaptor cannot be phosphorylated, kinase X activation reaches a lower peak after the same ligand dose. Which outcome best illustrates how feedback alters pathway behavior?
  11. A cell has a membrane carrier that binds amino acid Y. Extracellular Y is 30 mM and intracellular Y is 3 mM. When a nonhydrolyzable ATP analog is added, Y uptake continues unchanged; when a mutation reduces Y binding affinity, uptake rate decreases at the same gradient. Which explanation best accounts for these results?
  12. A grassland contains a three-level food chain: grasses → mice → owls. In one year, grasses add 8,000 kJ/m2^2 of biomass energy, mice add 400 kJ/m2^2, and owls add 20 kJ/m2^2. Assume mice eat grasses and owls eat mice. Which conclusion is best supported about energy flow in this chain?
  13. Students compare the amino acid sequence of cytochrome c from five organisms. Species A and B differ at 2 positions, A and C differ at 15 positions, A and D differ at 40 positions, and A and E differ at 41 positions. All sequences are from the same protein and aligned across the same length. Which inference is best supported about common ancestry?
  14. In a large plant population, allele T has frequency 0.60. After a wildfire, the population quickly regrows to a similar size from surviving seeds in the soil. Genetic sampling shows allele T frequency is 0.59 in the first postfire generation and 0.60 in the second postfire generation. No immigrants are observed, and no consistent differences in survival or reproduction among genotypes are detected. Which evolutionary mechanism best explains the minimal change in allele frequencies across generations?
  15. In a comparative study, two organelles are analyzed. Organelle X has a double membrane, circular DNA, and ribosomes similar to bacterial ribosomes; it replicates by fission. Organelle Y is bounded by a single membrane, lacks DNA, and is continuously replenished by vesicles budding from an internal membrane network connected to the nuclear envelope. Which inference best connects organelle origins to these observations?
  16. The plasma membrane's phospholipid bilayer contains hydrophilic heads and hydrophobic tails, creating a selective barrier. Some embedded proteins form channels that allow specific ions to cross. A researcher blocks ion channels with a chemical that binds to the channel's pore but does not disrupt phospholipids. After treatment, ion movement across the membrane decreases sharply, while movement of small nonpolar molecules is unchanged. Which feature best explains why blocking channels affects ions more than nonpolar molecules?
  17. A ligand binds to a receptor and triggers rapid production of diacylglycerol (DAG) in the plasma membrane. When cells are treated with a drug that prevents cleavage of PIP2_2, DAG does not increase, but ligand binding remains normal. A fluorescent probe shows DAG normally accumulates at the inner leaflet of the plasma membrane. Which of the following best explains the earliest molecular source of DAG in this pathway?
  18. Two artificial membranes are made from phospholipids. Membrane 1 is composed mostly of saturated fatty acid tails; Membrane 2 is composed mostly of unsaturated tails with cis double bonds. Both membranes lack transport proteins. A small, nonpolar gas diffuses through Membrane 2 faster than through Membrane 1 at the same temperature. Phospholipid heads face outward toward water, and tails face inward. Which membrane feature best explains the higher permeability of Membrane 2?
  19. An enzyme's activity is measured at 20°C in buffers of different pH. The reaction rate is highest at pH 6.5 and decreases sharply at pH 4.5. The substrate concentration is saturating in all trials. Protonation state changes can alter hydrogen bonding and ionic interactions that position catalytic residues in the active site. Which condition would most likely restore the higher reaction rate after a trial at pH 4.5?
  20. Compared with a similar-size ion, O2 crosses the lipid bilayer faster because O2 can

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