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Practice Test 68 - AP Biology

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

  1. Chitin and cellulose are both linear polysaccharides with β-1,4 glycosidic bonds. In chitin, each monomer is N-acetylglucosamine, which includes an acetamide group (-NHCOCH3_3) replacing one hydroxyl group found in glucose. The acetamide groups can participate in additional hydrogen bonding between adjacent chains compared with cellulose. Which statement best predicts a consequence of chitin's monomer structure for the material properties of the polymer?
  2. A cell is observed under a microscope after being placed in a solution with a toxin that inhibits Na+/K+ pumps in the plasma membrane. Over time, the cell swells. Which feature best explains why swelling occurs when the pump is inhibited?
  3. In a meadow, an invasive plant forms dense patches that reduce light at ground level. In plots where the invasive plant is removed, several native grasses and forbs increase in cover. In plots where the invasive remains, native seedling survival is low, and the invasive plant's cover stays high. Soil moisture and nutrient levels are similar across treatments. Which interaction best explains the invasive plant's effect on native plant species?
  4. A protein enzyme shows maximal activity at 40°C. When the temperature is raised to 50°C, the measured rate decreases even though substrate remains abundant. Weak interactions (hydrogen bonds, ionic interactions, hydrophobic interactions) help maintain the enzyme's tertiary structure and active site geometry. Which outcome is most likely at 50°C compared with 40°C?
  5. A bacterial population contains two alleles at a locus affecting antibiotic resistance: R and r. When a low dose of an antibiotic is present each day, cells with genotype RR produce 3 times as many descendant cells as rr, and Rr produce 2 times as many as rr. After 200 generations, the frequency of allele R rises from 0.10 to 0.92 in the same culture, with large population size and no migration. Which pattern best illustrates the selection acting on this locus?
  6. In a dog-breeding program, only dogs with black coats are allowed to produce offspring for multiple generations. Coat color is determined mostly by one gene with alleles B (black) and b (brown), with black dominant. At the start, many dogs are brown. After several generations, brown dogs are uncommon, and most litters contain only black puppies. No migration occurs. Which outcome is most likely regarding genetic variation at the coat-color locus?
  7. In a replication bubble, two forks move in opposite directions from an origin. Each parental strand serves as a template, and new DNA is synthesized 5′→3′ by adding nucleotides to a 3′ end. Short fragments form on the lagging strand and are later joined into a continuous strand. A student claims that both new strands at a single fork can be synthesized continuously if polymerase simply moves 3′→5′ on one template. Which explanation best accounts for why this claim is incorrect?
  8. A transporter moves Cl−^- from the cytosol to the extracellular space. Cytosolic Cl−^- is 5 mM and extracellular Cl−^- is 120 mM, yet Cl−^- continues to be exported. Export stops when ATP is depleted. The transporter is not affected by disrupting Na+^+ gradients. Which mechanism best explains Cl−^- export?
  9. A protein contains several proline residues within an α-helix region. Proline's rigid ring structure restricts rotation of the polypeptide backbone and can introduce kinks that change secondary structure. A new mutation inserts an additional proline into the middle of a helix that helps position residues forming the active site. The amino acid sequence is otherwise unchanged. Enzyme activity decreases markedly. Which feature best explains the decrease in activity?
  10. A signaling molecule Q is produced by gland G and circulates throughout the body. Target cells in tissue R respond, but cells in tissue S do not. Both tissues receive equal blood flow, and Q concentration is the same in capillaries supplying R and S. Which of the following best explains why only tissue R responds?
  11. A gene encodes a membrane protein with a signal peptide at the N-terminus. A mutation changes the start codon from ATG to ACG in the coding strand, with the rest of the coding sequence unchanged. Assume no alternative start codon is used. Which outcome is most likely in the cell?
  12. In a large population of frogs, allele frequencies at a locus are p(F)=0.80p(F)=0.80 and q(f)=0.20q(f)=0.20. The population is isolated and mates randomly. A parasite reduces reproductive success of ffff individuals relative to FFFF and FfFf, while no mutation is detected. Which outcome is most consistent with this violation of Hardy-Weinberg conditions over multiple generations?
  13. A signaling ligand G binds to a receptor on the plasma membrane and quickly increases cytosolic cGMP. Cells pretreated with a drug that locks heterotrimeric G proteins in the GDP-bound state show no cGMP increase after ligand addition, even though ligand binding is unchanged. When a membrane-permeable cGMP analog is added, the intracellular response occurs despite the drug. Which of the following best explains the early transduction step normally required for cGMP production?
  14. In smooth muscle cells, ligand L binds receptor tyrosine kinase (RTK) and causes receptor dimerization and autophosphorylation on cytosolic tyrosines. An adaptor protein binds the phosphotyrosines and recruits a Ras-GEF, which converts Ras-GDP to Ras-GTP. Ras-GTP activates a kinase cascade: Raf phosphorylates MEK, and MEK phosphorylates ERK. Activated ERK phosphorylates a cytosolic enzyme, increasing its catalytic rate within minutes. A mutant cell line expresses Ras that can bind GDP but cannot exchange GDP for GTP; all other proteins are normal and L binding to RTK is unchanged. Which outcome is most likely when mutant cells are treated with ligand L?
  15. In an experiment, intact chloroplasts are illuminated while the pH of the stroma is artificially lowered to match the pH of the thylakoid lumen, reducing the H+ gradient across the thylakoid membrane. Electron flow from water to NADP+ continues, and oxygen is produced. However, the reduced gradient decreases the driving force for ATP synthase. CO2 concentration is kept constant and high. Which outcome is most likely for the ratio of NADPH to ATP produced during illumination compared with controls?
  16. An enzyme-catalyzed reaction S → P is measured at saturating substrate (100 mM S). At 0.5 µM enzyme, the initial rate is 30 µM/min; at 1.0 µM enzyme, the initial rate is 60 µM/min; at 2.0 µM enzyme, the initial rate is 120 µM/min. All other conditions are constant. Which explanation best accounts for the effect of changing enzyme concentration on reaction rate?
  17. In a developing embryo, scientists isolate two differentiated cell types that share the same genome. In Cell Type 1, a specific enhancer region is heavily methylated and the nearby gene's mRNA is low. In Cell Type 2, the same enhancer is unmethylated and the gene's mRNA is high. Which explanation best accounts for the difference in mRNA levels?
  18. In cultured liver cells, adding peptide hormone P causes a rapid increase in cytosolic Ca2+^{2+} within 10 seconds. P cannot cross the plasma membrane. When cells are treated with a drug that blocks GTP binding to G proteins, adding P no longer increases cytosolic Ca2+^{2+}, but P still binds to a specific protein on the cell surface. Which of the following best explains how the signal is initiated in untreated cells?
  19. In a lab, a student places a drop of water on clean glass and observes that it spreads into a thin film rather than forming a tall bead. The surface of clean glass carries many polar hydroxyl (–OH) groups. Which feature best explains why the water drop spreads on glass?
  20. A membrane contains a cotransporter that moves Na+^+ and an amino acid into the cell in the same direction. Amino acid uptake occurs even when its intracellular concentration is higher than extracellular. Uptake stops if the Na+^+ gradient is eliminated, even though ATP is still present. ATP is not used by the cotransporter itself, but another membrane protein uses ATP to maintain low intracellular Na+^+. Which process best explains amino acid uptake?

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