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Practice Test 13 - AP Biology

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

  1. Cells are exposed to a fluorescent dye that is polar and cannot cross lipid bilayers unaided. Fluorescence inside cells increases only when a specific membrane channel is present and open. The increase occurs without detectable ATP consumption and slows as internal dye concentration rises. Which explanation best accounts for dye entry into the cells?
  2. A phospholipid bilayer has a hydrophobic core that restricts diffusion of polar and charged solutes. Two solutes are compared for permeability by simple diffusion with no protein assistance: urea (small, polar, uncharged) and K+ (small, positively charged). Both are similar in size, so charge is the key difference. Ions in aqueous solution are surrounded by a strong hydration shell due to electrostatic interactions. Entering the bilayer core would require disrupting these interactions, which is energetically unfavorable. Uncharged polar molecules may still cross slowly if they can transiently partition into the membrane.
  3. A student constructs two liposomes from identical phospholipids. Liposome X has no membrane proteins; Liposome Y includes a transmembrane protein that forms a continuous water-filled pore. When placed in a solution containing a small charged ion, the ion enters Liposome Y rapidly but shows minimal entry into Liposome X. The phospholipid bilayer interior is hydrophobic. Which feature best explains the difference in ion permeability between the two liposomes?
  4. A biologist places a thin strip of paper towel so that one end touches colored water in a beaker. Over time, the colored water rises through the towel fibers. Water molecules are polar, with partial charges that allow hydrogen bonding. Hydrogen bonds form between water molecules (cohesion) and can also form between water and polar groups on cellulose fibers in the towel (adhesion). As water molecules adhere to the fibers, additional water molecules are pulled along due to cohesion. This movement depends on intermolecular hydrogen bonding. Which statement best describes the molecular interactions causing the water to rise through the paper towel?
  5. A sensor protein at kinetochores recruits checkpoint proteins when microtubules are unattached, generating a diffusible inhibitor of APC/C. In a mutant, the sensor cannot recruit checkpoint proteins even when kinetochores are unattached. During mitosis, many chromosomes remain unattached, but APC/C is not inhibited. Which outcome is most likely in the mutant cells?
  6. A receptor is predicted to be a seven-transmembrane protein. Ligand binding leads to rapid activation of a cytosolic enzyme and increased cAMP. When the receptor's cytosolic loops are mutated, ligand binding remains normal but cAMP does not increase. Which of the following best explains the function of the receptor's cytosolic loops in early signaling?
  7. A ribosome translates an mRNA by matching codons with tRNA anticodons. A student writes an mRNA codon as 5′-AUG-3′ and proposes a tRNA anticodon written 5′-UAC-3′. In the ribosome, base pairing between codon and anticodon is antiparallel. The anticodon must align so that complementary bases pair correctly across the three positions. Which anticodon orientation is most likely to correctly pair with the codon 5′-AUG-3′ during translation?
  8. A lipid-soluble signaling molecule S is added to a suspension of animal cells. Within 2 minutes, S is detected inside cells, but no rapid change in cytosolic second messengers (cAMP or IP3_3) is observed. When the plasma membrane is removed experimentally, S still binds a specific intracellular protein. Which of the following best explains how S is received to initiate signaling?
  9. In a meiotic cell, homologous chromosomes pair during prophase I, forming tetrads. In one tetrad, a nonsister chromatid from the maternal homolog and a nonsister chromatid from the paternal homolog break and rejoin at corresponding loci, producing a visible chiasma. After this exchange, each homolog still separates from its partner in anaphase I, and sister chromatids separate in anaphase II. As a result, some chromatids now contain a mixture of maternal and paternal DNA segments along the same chromosome. Which process best explains this source of variation among gametes produced by the cell?
  10. A chimeric receptor has A's ligand-binding domain and B's kinase domain. A's ligand triggers what?
  11. An enzyme's active site forms when a single polypeptide folds so that distant amino acids in the primary structure become adjacent in the tertiary structure. Backbone hydrogen bonding stabilizes secondary structures, and interactions among R groups (ionic attractions, hydrogen bonds, and hydrophobic clustering) stabilize tertiary shape. In an experiment, the enzyme is placed in a solution that disrupts ionic interactions among side chains without breaking peptide bonds. The enzyme's amino acid sequence remains intact, but catalytic rate decreases sharply. Which statement best describes the molecular cause of the decreased activity?
  12. Identical strawberry plants were grown in a greenhouse with the same soil and water, but one group received short-day light cycles and the other received long-day light cycles. Short-day plants produced flowers earlier than long-day plants. Sequencing of a flowering-time gene showed identical DNA sequences in both groups. Which explanation best accounts for the earlier flowering under short days?
  13. A snail population includes individuals with thin and thick shells; shell thickness is heritable. A new crab predator is introduced and more easily crushes thin-shelled snails, reducing their survival to reproduction. Thick-shelled snails survive more often and produce more offspring. Which outcome is most likely after many generations with the crab present?
  14. A lake food chain is summarized by measured energy stored as new biomass per year: phytoplankton 9,000 kJ/m2^2/yr, zooplankton 900 kJ/m2^2/yr, small fish 90 kJ/m2^2/yr. Treat each value as energy available to the next trophic level. Which outcome is most likely if a new predator is added above small fish and has the same transfer efficiency as existing steps?
  15. Three grassland sites were sampled using identical quadrats. Site 1 contained 18 plant species with similar cover across species. Site 2 contained 18 plant species but 80% of cover was one grass species. Site 3 contained 9 plant species with similar cover across species. Which site has the highest biodiversity based on richness and evenness together?
  16. A grassland supports a population of grasshoppers. In Year 1, grasshopper density is low and the population increases rapidly. In Year 2, density is high and the population increases only slightly. In both years, a late-summer hailstorm occurs that kills approximately 30% of grasshoppers in the area, based on transect counts made before and after the storm. Which statement best classifies the hailstorm's effect on the grasshopper population?
  17. In a tissue culture, Cell A secretes peptide P, which diffuses a short distance. Only nearby Cell B shows a rapid ion flux after P is added. When Cell B is treated with a protease that removes extracellular portions of membrane proteins, P no longer triggers the ion flux, even though P is still present in the medium. Which of the following best explains why Cell B stopped responding to P?
  18. A phospholipid bilayer has a hydrophobic interior and hydrophilic surfaces. Consider five solutes at equal concentration: O2 (nonpolar), CO2 (nonpolar), H2O (small polar), glucose (large polar), and Na+ (small ion). None has a transport protein available. Molecules cross only by simple diffusion through the lipid portion of the membrane. Permeability depends on how well a solute dissolves in the hydrophobic core and how strongly it interacts with water. Charged solutes remain strongly hydrated and interact poorly with the nonpolar interior. Larger polar solutes form many hydrogen bonds with water, reducing partitioning into the membrane.
  19. In an experiment, ligand Q activates a membrane receptor that triggers production of second messenger M. M activates kinase A, which phosphorylates kinase B; kinase B phosphorylates enzyme E, increasing product formation. When kinase A is inhibited, neither kinase B nor E becomes phosphorylated after Q addition. When kinase B is inhibited, kinase A is still phosphorylated but E is not phosphorylated. Which conclusion is best supported by these results?
  20. A membrane-bound compartment contains many folded membrane structures and a high concentration of digestive enzymes. When the compartment membrane becomes leaky, the cytosol shows signs of damage. Which feature best explains why leakage is harmful?

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