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Practice Test 45 - AP Biology

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

  1. A plasma membrane is a phospholipid bilayer in which hydrophilic heads face the aqueous environments and hydrophobic tails form a core that limits passage of ions and most polar solutes. Some molecules cross by simple diffusion through the bilayer, while others require embedded transport proteins. A researcher replaces many membrane phospholipids containing unsaturated fatty acid tails with phospholipids containing saturated tails, leaving the number of transport proteins unchanged. After the replacement, fewer small nonpolar molecules enter the cell per unit time. Which change would most likely account for the decreased diffusion of small nonpolar molecules?
  2. A cell expresses a membrane antiporter that exchanges Ca2+ out of the cell while Na+ moves into the cell. Intracellular Na+ is 12 mM and extracellular Na+ is 140 mM; intracellular Ca2+ is 0.0001 mM and extracellular Ca2+ is 2 mM. When extracellular Na+ is reduced to 5 mM, Ca2+ export by the antiporter decreases sharply without changing ATP levels. Which mechanism best explains Ca2+ export under normal extracellular Na+?
  3. A student observes a cell with an intact nuclear envelope and diffuse chromatin. The cell's DNA content is measured as 4C, indicating DNA replication has occurred. No spindle apparatus is present, and chromosomes are not condensed. The student concludes the cell is in G2. If the cell proceeds normally into M phase, which structural change is most likely to occur first?
  4. An enzyme is assayed at constant enzyme and substrate concentrations. In a second trial, a molecule structurally similar to the substrate is added; the initial reaction rate decreases. When substrate concentration is increased tenfold in the presence of the molecule, the initial rate returns close to the original trial. The enzyme remains stable and pH and temperature are unchanged. Which prediction is most consistent with the inhibitor's mechanism?
  5. A population of cells is exposed to the same external toxin that enters by diffusion across the plasma membrane. Toxin entry rate depends on membrane surface area, and toxin accumulation depends on cytoplasmic volume. Larger cells show a lower toxin concentration increase per minute than smaller cells, despite identical membrane permeability. Which explanation best accounts for the lower concentration increase in larger cells?
  6. A diploid cell with 2n=102n=10 completes DNA replication before meiosis. In metaphase I, homologous pairs align as five tetrads. The cell then proceeds through meiosis I and meiosis II with normal segregation. Which outcome is most likely for the number of chromosomes in each of the four final cells produced at the end of meiosis II?
  7. A student measures initial rates for an enzyme at a fixed enzyme concentration while varying substrate concentration. When substrate is increased from 0.2 mM to 0.4 mM, the rate approximately doubles. When substrate is increased from 5 mM to 10 mM, the rate changes very little. The enzyme remains stable and no inhibitor is present. Which inference is most consistent with these observations?
  8. Genetically identical human skin cells in culture are exposed to UV light for a short period, while a control culture is not exposed. UV-exposed cells show increased transcription of DNA repair genes and higher levels of repair proteins, but sequencing shows the repair genes' coding regions are unchanged. After 24 hours without UV, repair gene mRNA returns near control levels. Which explanation best accounts for the change in repair protein levels?
  9. A drug blocks a specific membrane carrier. After treatment, a polar nutrient no longer enters cells, even though extracellular nutrient concentration remains higher than intracellular. ATP levels in the cell are unchanged. When the drug is removed, nutrient entry resumes and stops at equilibrium. Which mechanism best explains nutrient entry when the carrier is not blocked?
  10. A cell with 2n=22n=2 is observed during meiosis. At metaphase I, the two homologous chromosomes are paired as a tetrad. At anaphase I, the homologs separate to opposite poles. At metaphase II in one of the resulting cells, the single chromosome aligns at the metaphase plate. If meiosis II proceeds normally, which description best matches the chromosome content of each final gamete?
  11. A plant leaf cell contains chloroplasts with stacked thylakoid membranes (grana). When thylakoid membranes are experimentally disrupted, the cell's ability to convert light energy into chemical energy drops sharply. Which feature best explains this decrease?
  12. In a plant guard cell, hormone A binds receptor GPCR-like protein R at the plasma membrane. R activates G protein, which activates phospholipase C (PLC). PLC cleaves PIP2_2 into DAG and IP3_3. IP3_3 binds an ER channel, releasing Ca2+\mathrm{Ca^{2+}} into the cytosol. Cytosolic Ca2+\mathrm{Ca^{2+}} activates kinase K, which phosphorylates an anion channel, increasing anion efflux and reducing turgor pressure. A cytosolic phosphatase removes phosphate from the anion channel. Which manipulation would most likely block the response downstream of IP3_3 production but upstream of anion efflux?
  13. A lab replaces thymine with uracil in a nucleic acid polymer. Which statement best identifies the polymer based on this structural feature?
  14. In a lab, a student shines a bright light on one side of a container holding 20 pill bugs. After 5 minutes, 16 pill bugs are on the shaded side. When the light is turned off, pill bugs redistribute evenly within 5 minutes. Which outcome is most likely explained by a short-term response mechanism to the stimulus?
  15. A phospholipid bilayer membrane is tested with different solutes. The membrane interior is hydrophobic, so permeability increases as solutes become smaller and less polar. Ions are especially impermeable because their charge is stabilized by water and unfavorable in the membrane core. Compare chloride ion (Cl−) and chlorine gas (Cl2). Both contain chlorine atoms, but one is charged and the other is nonpolar. Which solute would most likely diffuse across the bilayer more rapidly?
  16. A eukaryotic cell line is treated with a chemical that inhibits poly(A) polymerase in the nucleus. Shortly afterward, newly synthesized RNA from several genes is still detected, and intron removal proceeds normally. However, the 3′ ends of these RNAs lack long stretches of adenosines that are present in untreated cells. No changes are observed in the DNA sequences of the genes. Which outcome best accounts for the observed difference in the RNA molecules?
  17. A student observes that urea enters cells faster when a specific membrane protein is present. Urea concentration is higher outside than inside. Uptake does not require ATP, but it stops when the protein is inhibited. At very high external urea, uptake rate approaches a maximum. Which mechanism best explains urea entry under these conditions?
  18. In an in vitro assay, enzyme E catalyzes S → P. Reaction rate is measured with [E] held constant at 1.0 µM while [S] increases from 0.1 to 50 mM. The rate rises quickly at low [S] but approaches a maximum of ~120 µM P/min and does not increase further above 10 mM S. Temperature and pH are constant, and no inhibitor is present. Which explanation best accounts for the rate plateau at high substrate concentration?
  19. A cell is placed in a solution containing urea at a higher concentration outside than inside. Net urea entry is slow unless a urea-specific transporter is present; with the transporter, entry is faster and reaches a maximum rate at high urea concentrations. Transport stops when inside and outside urea concentrations are equal and does not require ATP. Which mechanism best explains urea movement when the transporter is present?
  20. A student records the breathing rate of a resting mouse before and after placing it in a chamber with 10% oxygen (normal air is about 21% oxygen). Within 2 minutes, the mouse's breathing rate increases from 140 breaths/min to 210 breaths/min, and the mouse becomes more active. When normal air is restored, the breathing rate returns near baseline within 5 minutes. Which outcome is most likely caused by a short-term response mechanism to low oxygen?

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