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Practice Test 73 - AP Biology

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

  1. In polarized epithelial cells, some membrane proteins are found only on the apical surface and others only on the basolateral surface. Tight junctions restrict lateral diffusion of proteins within the plasma membrane, keeping the two regions distinct. When tight junctions are disrupted, proteins mix between regions and transport across the epithelium becomes less directional. Which feature best explains how compartmentalization affects function in these cells?
  2. In a certain bird species, feather pattern is sex-linked on the Z chromosome. Males are ZZ and females are ZW. Allele ZBZ^B produces barred feathers and allele ZbZ^b produces nonbarred feathers; barred is dominant. A nonbarred female (ZbZ^b W) is crossed with a barred male that is heterozygous (ZBZ^B ZbZ^b). Which outcome best predicts the phenotypes of female offspring?
  3. Two spherical cells are in the same solution containing a small uncharged molecule. Both have the same membrane permeability and no active transport for this molecule. Cell J has radius 2 µm; Cell K has radius 8 µm. Both consume the molecule at the same rate per unit volume. The larger cell maintains a lower internal concentration at steady state. Which explanation best accounts for the lower steady-state concentration in the larger cell?
  4. A cell-cycle checkpoint at metaphase monitors kinetochore attachment to spindle microtubules. When even one kinetochore is unattached, a signaling complex inhibits the anaphase-promoting complex/cyclosome (APC/C). When all kinetochores are attached, inhibition stops, APC/C becomes active, and APC/C tags securin for degradation; separase then cleaves cohesin, allowing sister chromatids to separate. In an experiment, cells are treated with a drug that keeps the inhibitory kinetochore signal active even after all kinetochores are attached. Which change would most likely affect chromosome segregation in treated cells?
  5. A soluble enzyme is placed in a solution with very low pH, increasing the concentration of H+H^+. Many amino acid side chains can gain or lose protons depending on pH, changing their charges. The enzyme's peptide bonds remain intact, but activity decreases and the protein becomes less soluble. The active site depends on specific ionic attractions and hydrogen bonds among side chains that stabilize tertiary structure. Which feature best explains the decreased activity at low pH?
  6. A receptor couples to a G protein and normally increases cAMP after ligand binding. In a mutant cell line, ligand binding occurs normally, but cAMP does not increase. Biochemical assays show the G protein alpha subunit cannot exchange GDP for GTP. Adenylyl cyclase protein levels are unchanged. Which of the following best explains why cAMP fails to increase in the mutant cells?
  7. A scientist examines two cell types. Cell X has many mitochondria and an extensive inner membrane surface area; Cell Y has fewer mitochondria with smoother inner membranes. Cell X produces ATP at a higher rate. Which feature best explains this difference?
  8. A severe wildfire burns through a conifer forest, removing most canopy cover across a watershed. In the following rainy season, stream monitoring shows increased sediment in the water and reduced light penetration. Surveys find that periphyton (attached algae) biomass decreases, and aquatic insect larvae that graze on periphyton also decrease. Fish that feed on those insect larvae decline in abundance in the same stream reaches. No chemical pollutants are detected. Which explanation best accounts for the fish decline after the wildfire?
  9. In liver cells, epinephrine binds a GPCR that activates Gs, increasing adenylyl cyclase activity and raising cAMP. cAMP activates protein kinase A (PKA). Active PKA phosphorylates the GPCR's cytosolic tail, increasing recruitment of a regulatory protein that reduces coupling between the receptor and Gs. In experiments with constant epinephrine, cAMP rises rapidly and then declines to a lower steady level despite continued ligand presence. Which change would most likely reduce the decline in cAMP by disrupting the feedback mechanism?
  10. A single cell is observed over time. At the start, it is in late G1 with one chromatid per chromosome and a DNA content of 2C. It then completes S phase and enters G2 with a DNA content of 4C. During mitosis, chromosomes condense and align at the metaphase plate, and then sister chromatids separate and move to opposite poles. Cytokinesis completes, producing two daughter cells. Immediately after cytokinesis, each daughter cell has uncondensed chromosomes and an intact nuclear envelope. Which DNA content is most likely present in each daughter cell immediately after cytokinesis?
  11. In a temperate forest, energy stored as new biomass per year is estimated for a simple chain: tree leaves 15,000 kJ/m2^2/yr, caterpillars 1,500 kJ/m2^2/yr, songbirds 150 kJ/m2^2/yr. Assume each level's value is the energy available to the next level. Which statement is best supported by these data?
  12. A researcher performs Western blotting to detect a specific membrane protein in two cell types. Proteins from each sample are denatured with SDS, separated by polyacrylamide gel electrophoresis, and transferred to a membrane. The membrane is incubated with a primary antibody specific to the target protein, then with a labeled secondary antibody. A band appears at ~55 kDa in cell type 1 but not in cell type 2. Which conclusion best accounts for the observed banding pattern?
  13. A researcher adds 2,000 J of heat to 100 g of water and, separately, 2,000 J of heat to 100 g of a liquid whose molecules are similar in size to water molecules but cannot form hydrogen bonds. Both liquids start at 20°C. The specific heat of water is 4.18 J/(g·°C), and the specific heat of the other liquid is 2.0 J/(g·°C). No phase change occurs, and heat loss to the surroundings is negligible. Which statement best predicts and explains the outcome?
  14. Three island populations (G, H, J) and a mainland population (K) were compared for a mitochondrial gene. Differences from K are: G differs by 5 substitutions, H differs by 5 substitutions, and J differs by 1 substitution. Between island populations: G–H differs by 2, G–J differs by 6, and H–J differs by 6. Assume fewer differences indicate a more recent common ancestor. Which inference is best supported?
  15. A ribosome translates an mRNA segment 5′-AUG CCC AAA GGG UAA-3′ in the shown reading frame. A mutation deletes the second nucleotide of the second codon, changing the sequence to 5′-AUG CCA AAG GGU AA-3′. Translation initiates at AUG and then continues by reading codons in the new frame until a stop codon is reached. No additional nucleotides are added, and the ribosome does not realign to the original frame. Which outcome is most likely for the mutant translation product?
  16. Six taxa (A–F) share ancestral trait 0. Derived traits appear as follows (each evolved once, never lost): trait X occurs in B, C, D, E, F; trait Y occurs in C, D, E, F; trait Z occurs in E and F only. No other derived traits are considered. The most recent split separates E from F within the Z-bearing group. Based on these shared derived traits and the stated split, which statement about relatedness is best supported?
  17. A researcher compares two RNA molecules of the same length. RNA 1 contains many intramolecular complementary segments that can base-pair, forming multiple stems and loops. RNA 2 has the same nucleotide composition but the bases are arranged so that few complementary segments occur, leaving most bases unpaired. Hydrogen bonds between complementary bases stabilize stems, while unpaired regions remain flexible. Both RNAs have identical phosphodiester backbones and are in the same solution conditions. Which statement best describes how sequence arrangement affects the molecules' three-dimensional behavior?
  18. An enzyme is assayed at constant temperature in two solutions: pH 6.5 and pH 9.5. The rate is 28 units at pH 6.5 and 1 unit at pH 9.5. Which explanation best accounts for the near-zero activity at pH 9.5?
  19. In isolated thylakoid membranes supplied with water and ADP + Pi, researchers add a compound that makes the thylakoid membrane highly permeable to H+. Light still excites chlorophyll and electrons move through an electron transport chain to a terminal acceptor, but the H+ concentration difference across the membrane rapidly collapses. The rate of NADPH formation changes little, and oxygen evolution continues. Which outcome is most likely when the membranes are illuminated compared with untreated controls?
  20. Clonal human skin cells were cultured with either high UV exposure or low UV exposure for 48 hours. High-UV cultures produced more melanin and higher levels of tyrosinase mRNA, while sequencing of the tyrosinase gene showed no sequence change. Which explanation best accounts for the increased melanin production in high-UV cultures?

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