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Practice Test 100 - AP Biology

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

  1. A scientist observes that a metabolic enzyme is active only when attached to the cytosolic face of the ER membrane, where its substrate is delivered by a nearby transporter. When the enzyme is experimentally released into the cytosol, total enzyme amount is unchanged but product formation decreases. Which feature best explains how localized environments affect enzyme efficiency in this case?
  2. In a cell, ligand activates an RTK that recruits and activates kinase A at the membrane. Kinase A phosphorylates and activates kinase B. Active kinase B phosphorylates kinase A on an inhibitory site, decreasing kinase A catalytic activity while ligand remains bound. A kinase A mutant lacking the inhibitory site shows higher kinase B activation over time. Which change would most likely result from this mutation?
  3. In a group of insects, derived traits were mapped relative to an outgroup. All taxa (V, W, X, Y) have trait 1. Only W, X, and Y have trait 2. Only X and Y have trait 3. Only Y has trait 4. Assume each trait evolved once and was not lost. Which pair of taxa shares the most recent common ancestor?
  4. A student compares ATP generation in two conditions using the same number of glucose molecules. In Condition 1, cells have normal oxygen and functional mitochondria; in Condition 2, oxygen is absent but glycolysis enzymes are intact. In both conditions, glycolysis produces ATP by substrate-level phosphorylation and generates NADH. In Condition 1, NADH delivers electrons to the electron transport chain, supporting proton pumping and ATP synthase activity. In Condition 2, electron transport cannot proceed because oxygen is unavailable as the terminal electron acceptor. Which prediction about ATP yield per glucose is most likely correct?
  5. A researcher inserts a eukaryotic gene into a plasmid and expresses it in bacteria. The gene contains three exons and two introns, and its promoter is recognized by bacterial RNA polymerase. RNA is isolated from the bacteria and is the same length as the full genomic region from exon 1 through exon 3, including both introns. No changes were made to the coding or intron sequences. Which explanation best accounts for the bacterial RNA length?
  6. Cells expressing a receptor tyrosine kinase (RTK) are exposed to Ligand B. Within 1 minute, the receptor becomes phosphorylated on its cytosolic tyrosines, and adaptor proteins bind to the receptor's cytosolic region. A mutant receptor binds Ligand B normally but cannot dimerize. In cells with the mutant receptor, receptor phosphorylation is not detected after ligand addition. Which of the following best explains the early transduction defect?
  7. A cytosolic enzyme is tested at constant temperature with two buffers: pH 7.2 and pH 7.8. At pH 7.2 the rate is 18 units; at pH 7.8 it is 10 units. Which explanation best accounts for the difference?
  8. A population of fish has alleles BB and bb at a locus. In year 1, a random sample shows allele frequencies p(B)=0.60p(B)=0.60 and q(b)=0.40q(b)=0.40. The population is large, mating is random, and no selection is detected. In year 2, after a storm connects the lake to a nearby river, the allele frequencies change to p(B)=0.52p(B)=0.52 and q(b)=0.48q(b)=0.48. No other conditions are known to change. Which evolutionary force is best supported as causing the allele-frequency change?
  9. To test whether repeated wet-dry cycles could promote polymer formation, scientists alternated dehydration and rehydration of a solution containing amino acids on a warm surface. After multiple cycles, analysis detected short peptide chains. A parallel sample kept continuously wet at the same temperature showed far fewer peptides. No biological catalysts were added. Which conclusion is best supported by these results about prebiotic chemistry on early Earth?
  10. A single population of insects begins using two host plants within the same forest. Mating occurs on the host plant where individuals feed. Over 100 generations, insects on host plant X and host plant Y show distinct allele frequencies, and mark-recapture studies indicate most matings occur within each host-associated group. There is no physical separation, and mating seasons overlap. Which isolating mechanism most directly reduces gene flow between the host-associated groups?
  11. A cell in G2 has completed DNA replication and contains duplicated chromosomes (two sister chromatids each). The cell enters mitosis, and chromosomes align at the metaphase plate with microtubules attached to all kinetochores. Just before anaphase, a chemical is added that prevents separase activity, so cohesin is not cleaved. The spindle remains intact and kinetochores stay attached. The cell does not show chromatid movement toward poles. Which stage is the cell most likely to remain in as a result of this treatment?
  12. A pure phospholipid bilayer is exposed to equal concentrations of Ca2+^{2+} (40 Da, charged) and CO2_2 (44 Da, nonpolar). No proteins are present. Which molecule would most likely show greater permeability across the membrane?
  13. An enzyme is tested at constant pH and substrate concentration but in two solutions differing only in ethanol concentration. With 0% ethanol, product forms at 40 μmol/min; with 20% ethanol, product forms at 8 μmol/min. Ethanol can disrupt hydrophobic interactions that help stabilize protein tertiary structure, potentially changing the active site's shape. The assay time is short, and enzyme amount is unchanged between trials. Which outcome is most likely in 20% ethanol compared with 0% ethanol?
  14. In a fish, body pattern shows incomplete dominance: allele S produces stripes and allele s produces no stripes. SS fish are striped, ss fish are unstriped, and Ss fish are lightly striped. A lightly striped fish is crossed with an unstriped fish (Ss × ss). Which outcome best illustrates the expected offspring phenotypes?
  15. A protein-free phospholipid bilayer is tested with two solutes: solute T (120 Da, polar uncharged) and solute U (120 Da, charged). Both have the same size. Which solute would most likely be more permeable across the bilayer?
  16. A researcher compares two isolated cells with the same total volume. Cell 1 is a single sphere. Cell 2 is divided into many smaller spherical compartments separated by membranes, increasing total membrane area while keeping total cytoplasmic volume the same. Both rely on diffusion across membranes for glucose uptake. Which explanation best accounts for the difference in glucose uptake capacity per unit cytoplasm?
  17. A mutation reduces the activity of a protein complex in the inner mitochondrial membrane that transfers electrons from NADH into the electron transport chain. Oxygen is present, and ATP synthase is functional. Glycolysis and the citric acid cycle still generate NADH and FADH2 from organic molecules. Because fewer electrons enter the chain from NADH, fewer protons are pumped across the inner membrane per unit time, reducing the proton gradient. No change occurs in the membrane's permeability to protons. Which outcome is most likely in mutant cells compared with normal cells?
  18. A biochemist examines two single-stranded nucleic acids of equal length. Molecule P has many adenine bases, while Molecule Q has many cytosine bases. The molecules are mixed separately with short complementary strands designed to bind by Watson-Crick pairing. Complementary binding depends on hydrogen bonds: A pairs with U (or T) using two hydrogen bonds, and C pairs with G using three hydrogen bonds. All strands remain single-stranded except where they hybridize. Which hybrid is expected to be more stable at the same temperature and salt concentration, and why?
  19. Ligand W is hydrophilic and added to cells. W triggers a response only when applied outside intact cells; injecting W directly into the cytosol does not trigger the response. Which of the following best explains this result?
  20. During DNA replication, DNA polymerase adds nucleotides to a growing DNA strand by forming phosphodiester bonds between the sugar of the incoming nucleotide and the phosphate of the existing strand. The reaction requires a free hydroxyl group on the 3' carbon of the terminal sugar. Which feature best explains why DNA elongation occurs only in the 5'→3' direction?

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