Practice Test 70 - AP Biology

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

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1. A cell is placed in an isotonic solution, and a nonpenetrating solute is added to the outside, increasing extracellular solute concentration. The cell shrinks as water leaves. No ATP is required, and changing the membrane's protein composition does not eliminate the volume change, though it can alter the rate. Which mechanism best explains the water movement responsible for shrinkage?
2. During a heat wave, a group of rabbits is observed in a field at midday. Compared with cool mornings, rabbits spend more time in shaded areas and have visibly faster breathing with open mouths. A thermometer shows shaded burrow entrances are $8\,^{\circ}\mathrm{C}$ cooler than open ground. When air temperature drops in the evening, the rabbits' breathing slows and they resume foraging in open areas. Which response best explains the rabbits' behavior and physiology in the heat?
3. A signaling pathway in yeast begins when ligand L binds a membrane receptor and activates a G protein. The G protein activates a MAPK cascade: MAPKKK phosphorylates MAPKK, which phosphorylates MAPK. Active MAPK phosphorylates cytosolic enzyme V, increasing its activity. A scaffold protein holds the three kinases in a complex; deleting the scaffold reduces the amount of phosphorylated MAPK after L addition, even though each kinase is present at normal levels. Which mechanism best explains the scaffold's effect on MAPK activation?
4. A receptor R is normally found on the plasma membrane and binds ligand L outside the cell. A mutation deletes R's transmembrane domain, and the mutant R is secreted into the extracellular fluid. Cells with only mutant R show no response to L, even though L binds mutant R in solution. Which of the following best explains the loss of signaling?
5. A severe winter storm kills most canopy trees in a temperate forest stand. During the following spring, understory plant cover increases and soil temperature rises. Which consequence is most likely for ecosystem processes that year?
6. A fish population in a river has two alleles at a locus affecting tolerance to low oxygen: H and h. In upstream pools with frequent low-oxygen events, H = 0.62; in downstream fast-flowing water, H = 0.29. Over time, a dam reduces flow downstream, and low-oxygen events become common there. Five years later, downstream H increases to 0.51. Which statement best explains the role of variation in the downstream population's change?
7. A mainland bird population has allele A at frequency 0.20. Each year, several hundred birds from a nearby island population (allele A frequency 0.70) join the mainland breeding population. Over 10 years, allele A on the mainland increases steadily to 0.45, while survival and reproductive output do not differ among genotypes on the mainland. Mutation rates are assumed typical and constant. Which factor most directly explains the increase in allele A frequency on the mainland?
8. In a plant, leaf shape shows incomplete dominance. Allele L produces long leaves and allele l produces round leaves. LL plants have long leaves, ll plants have round leaves, and Ll plants have oval leaves. A botanist crosses a homozygous long plant with an oval plant (LL × Ll). Which phenotype ratio among offspring is expected?
9. Cells are treated with a chemical that prevents sister chromatid cohesion from forming during S phase. DNA replication occurs, so each chromosome is duplicated, but sister chromatids do not remain paired. The cells enter mitosis and attach chromatids to spindle microtubules. The spindle checkpoint is satisfied when each chromatid is attached to opposite poles. Which outcome is most likely at anaphase onset?
10. A student compares two phospholipids: Lipid X has two saturated fatty acid tails (no C=C double bonds), and Lipid Y has two unsaturated tails with cis C=C bonds that introduce kinks. Both lipids have the same phosphate-containing polar head group. Bilayers are made separately from pure X or pure Y at the same temperature. The bilayer made from Y shows greater lateral movement of lipids within the membrane. Which statement best describes the molecular basis for this difference?
11. A researcher compares two double-stranded DNA fragments of equal length. Fragment 1 contains 70% G≡C base pairs, while Fragment 2 contains 70% A=T base pairs. Both are placed in the same buffered solution and heated gradually. Which feature best explains why Fragment 1 requires a higher temperature to separate the strands?
12. In a lab, pill bugs are placed in a choice chamber with one side dry and one side moist. Within 5 minutes, 18 of 20 pill bugs are on the moist side, and several are observed moving rapidly when they enter the dry side. When the chamber sides are switched, most pill bugs move to the newly moist side within 5 minutes. No changes in body structure are observed during the trial. Which response best explains the pill bugs' distribution across the chamber?
13. Cells express a steroid hormone's response protein but not its receptor. What is likely after adding hormone?
14. During transcription, RNA nucleotides align with a DNA template strand. Which feature best explains why RNA can base-pair with DNA during this process?
15. A researcher inhibits ATP synthase in mitochondria while providing oxygen and a constant supply of NADH. Electron transport chain components can still accept and donate electrons, but protons pumped into the intermembrane space cannot return through ATP synthase. As protons accumulate, the electrochemical gradient becomes very steep, which increasingly opposes additional proton pumping by the electron transport chain. Over time, electron flow through the chain slows because it is coupled to proton pumping. Which outcome is most likely after ATP synthase inhibition?
16. Two mRNAs differ at a single codon within an otherwise identical coding sequence. At position 5, mRNA X contains a codon that is recognized by a specific tRNA carrying amino acid M, while mRNA Y contains a different codon recognized by a different tRNA carrying amino acid N. Neither codon is a stop codon, and initiation occurs at the same start codon for both mRNAs. All translation components are present. Which outcome is most likely when comparing the polypeptides produced?
17. A peptide hormone contains several nonpolar amino acids that pack inward during folding, while polar and charged side chains face the aqueous cytosol. The hormone's primary structure (amino acid sequence) determines where hydrogen bonds form along the backbone, producing local secondary structures (α-helices and β-sheets). These interactions contribute to a specific tertiary structure that creates a binding surface complementary in shape and charge to its receptor. In a variant, one interior leucine is replaced with aspartate. The overall sequence length is unchanged, but the hormone shows reduced receptor binding. Which feature best explains the reduced binding?
18. A cell in metaphase has 4C DNA content and all chromosomes aligned. The spindle checkpoint is functioning normally. A researcher observes that one sister chromatid pair is attached to microtubules from opposite poles, but the kinetochores are not under tension because both microtubule bundles are unusually short and slack. The cell remains in metaphase longer than typical. Which outcome is most likely once proper tension is established at all kinetochores?
19. A carbohydrate sample is composed of many repeating six-carbon sugars. Each linkage forms when a hydroxyl group on one sugar reacts with a hydroxyl group on another sugar, releasing water and creating a covalent bond between the sugars. Because many sugars can be joined, the molecule can become very large and has multiple hydroxyl groups that can form hydrogen bonds with water. Which statement best describes the bond formation that produces this macromolecule?
20. In immune cells, a cytokine receptor activates JAK kinase, which phosphorylates the receptor to recruit STAT. Separately, receptor activation stimulates a membrane-associated phosphatase (PTP) by phosphorylation. Active PTP removes phosphates from the receptor's docking tyrosines, lowering STAT recruitment even while cytokine remains bound. A selective PTP inhibitor increases both the peak and duration of receptor phosphorylation after cytokine addition. Which outcome best illustrates the feedback's role in this signaling pathway?

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