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Practice Test 30 - AP Biology

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

  1. A researcher compares a red blood cell precursor and a pancreatic beta cell from the same individual. Both have identical nuclear DNA. The precursor contains abundant mRNA for hemoglobin, while the beta cell contains abundant mRNA for insulin; the other transcript is nearly absent in each cell type. The researcher finds no difference in ribosome number between the cells. Which explanation best accounts for the observed cell-type-specific protein production?
  2. Two cell types have different numbers of the same glucose transporter in their plasma membranes. Both are placed in 10 mM extracellular glucose with 1 mM intracellular glucose. Cell type 1 shows a faster initial glucose uptake rate than cell type 2, and neither shows a measurable change in ATP concentration during uptake. Which explanation best accounts for the difference in initial uptake rates?
  3. A neuron maintains a much higher concentration of Na+^+ outside the plasma membrane than inside the cytosol. The lipid bilayer restricts ion diffusion, and membrane proteins move Na+^+ to sustain the difference. When the membrane is damaged and becomes freely permeable to Na+^+, the concentration difference rapidly collapses. Which feature best explains how compartmentalization across the plasma membrane supports cellular function?
  4. In an experiment, identical plasmids carrying a GFP reporter under the control of a neuron-specific promoter are introduced into cultured neurons and cultured kidney epithelial cells. Both cell types take up the plasmid DNA, confirmed by PCR. GFP mRNA and fluorescence are detected only in neurons. Which explanation best accounts for GFP expression occurring only in neurons?
  5. A lab measures uptake of a fluorescent dye that enters cells only by diffusion through the plasma membrane. Two populations of cells have the same membrane composition and are equally exposed to the dye. Population 1 has average cell radius 6 μm6\,\mu m; population 2 has average cell radius 18 μm18\,\mu m. After the same exposure time, population 2 shows lower dye concentration inside cells. Which explanation best accounts for the lower intracellular dye concentration in population 2?
  6. In a large population of snails, allele frequencies are p=0.8p=0.8 and q=0.2q=0.2. Individuals preferentially mate with the same phenotype. No selection, mutation, or migration occurs. Which change is most likely after several generations?
  7. A phospholipid has a glycerol backbone with two fatty acid tails and a phosphate head group. In one variant, the head group includes an additional charged group, increasing its overall polarity; the tails are unchanged. Increased head-group polarity strengthens interactions with water molecules at the membrane surface, while the hydrophobic tails still cluster away from water. When bilayers made from the two variants are compared in water, both remain intact. Which statement best predicts a molecular-level difference caused by the more polar head group?
  8. Enzyme Y catalyzes S → P. At constant [E] and saturating [S], adding inhibitor L decreases the initial rate by 50%. After L is removed by dialysis, the enzyme solution is retested under the same conditions and the initial rate returns to the original value. No changes in pH or temperature occur. Which explanation best accounts for L's reversible effect on enzyme activity?
  9. A protein enzyme is a polymer of amino acids linked by peptide bonds (primary structure). Its polypeptide chain folds into secondary structures stabilized by hydrogen bonds between backbone groups, then into a tertiary structure stabilized by interactions among R groups (hydrophobic clustering, ionic attractions, hydrogen bonds, and disulfide bridges between cysteines). The enzyme's active site depends on precise R-group positioning. In a mutant, a cysteine in the interior is replaced with serine; all other amino acids remain unchanged. The mutant enzyme shows greatly reduced catalytic rate at the same temperature and pH. Which feature best explains the decreased activity?
  10. In plant cells, the central vacuole can accumulate ions and small solutes, creating a lower water potential inside the vacuole than in the cytosol. Water moves into the vacuole by osmosis, increasing turgor pressure against the cell wall. If the tonoplast (vacuolar membrane) becomes permeable to solutes, solute concentrations equalize between vacuole and cytosol and turgor pressure decreases. Which feature best explains how compartmentalization affects this cellular property?
  11. A student places a cell with cytosol at 0.30 M nonpenetrating solute into a solution at 0.60 M nonpenetrating solute. The membrane is permeable to water but not solute, and the external solution stays at 0.60 M. The cell starts at normal volume and is observed for 2 minutes. Which outcome is most likely regarding net water movement?
  12. In an animal cell line, ligand L triggers a rapid increase in intracellular IP3_3. Researchers find that blocking phospholipase C (PLC) prevents the IP3_3 increase, even though L still binds its receptor. A separate treatment with a membrane-permeable IP3_3 analog produces the intracellular response without ligand L. The receptor is a transmembrane protein with an extracellular ligand-binding site and a cytosolic region that interacts with signaling proteins. Which of the following is the most likely early transduction event after ligand binding?
  13. A eukaryotic gene normally produces a protein with an N-terminal signal peptide that targets it to the endoplasmic reticulum (ER). A point mutation changes the start codon in the coding strand from 5'-ATG-3' (mRNA 5'-AUG-3') to 5'-ACG-3' (mRNA 5'-ACG-3'). All downstream codons remain unchanged, including a second AUG at codon 25. Transcription produces normal amounts of mRNA. Which outcome is most likely from this mutation?
  14. In fruit flies, red eyes (R) are completely dominant to sepia eyes (r). Two red-eyed flies are crossed, and both are known to be heterozygous (Rr × Rr). Each fly produces gametes carrying one allele, and fertilization is random. Assume a single gene controls eye color with complete dominance. Which genotype ratio is expected among the offspring?
  15. A cell is experimentally induced to form many gap junctions with neighboring animal cells. Soon after, ions and small molecules are observed moving rapidly between the cells. Which feature best explains this increased intercellular movement?
  16. A lipid droplet is composed mainly of triglycerides, each with three fatty acid chains attached to glycerol. A cell membrane is composed mainly of phospholipids, each with two fatty acid tails and a phosphate-containing polar head. When exposed to an aqueous cytosol, the droplet remains spherical while the membrane forms a stable sheet. Which statement best explains the different shapes based on lipid structure?
  17. Two populations of unicellular eukaryotes are grown in identical media. Population S has average cell diameter 8 μm8\,\mu m; population L has average cell diameter 16 μm16\,\mu m. Oxygen enters cells by simple diffusion across the plasma membrane, and the external oxygen concentration is kept constant. Both populations have similar internal oxygen consumption per unit cytoplasmic volume. Which explanation best accounts for why cells in population L are more likely to develop low oxygen levels in the cytoplasm?
  18. A coastal survey compared two tide pools of equal area. Pool 1 contained 60 barnacles, 60 mussels, 60 limpets, and 60 algae patches. Pool 2 contained 220 barnacles, 10 mussels, 5 limpets, and 5 algae patches. Both pools had four species present, and total individuals were similar. Which conclusion is best supported about biodiversity across the two pools?
  19. Two desert shrubland sites were surveyed for lizard communities. Site M had 5 species and 500 total individuals, with one species making up 92% of individuals. Site N had 5 species and 120 total individuals, with each species 15–25% of individuals. Which statement is best supported about community biodiversity?
  20. A eukaryotic gene is transcribed by RNA polymerase II to produce pre-mRNA. In an experiment, cells are treated with a drug that specifically inhibits the enzyme responsible for adding the 5′ cap, without affecting RNA polymerase activity or splicing enzymes. After treatment, the amount of newly made RNA containing exon sequences is unchanged, but the RNA lacks the modified guanine cap at its 5′ end. Which outcome best describes the direct effect of the drug on RNA processing?

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