Practice Test 63 - AP Biology

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

In pancreatic cells, glucose increases ATP, closing K $_{ATP}$ channels and depolarizing the membrane. Depolarization opens voltage-gated Ca $^{2+}$ channels, increasing cytosolic Ca $^{2+}$ . Ca $^{2+}$ activates a phosphatase that dephosphorylates the Ca $^{2+}$ channel, lowering its open probability during depolarization. Blocking the phosphatase increases Ca $^{2+}$ influx during sustained depolarization. Which outcome best illustrates the feedback's role in regulating signaling?
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?
Two groups of genetically identical mice were fed for 8 weeks: Diet 1 contained adequate methyl-donor nutrients (folate and choline), and Diet 2 lacked these nutrients. Mice on Diet 2 produced lower levels of a liver enzyme protein, although the enzyme-coding DNA sequence was unchanged. Which explanation best accounts for the reduced enzyme protein level in Diet 2 mice?
A laboratory maintains a population of fruit flies. Each generation, technicians allow only flies with the darkest body color to mate. Body color is associated with alleles D (darker) and d (lighter). After 15 generations, the population shows less variation in body color than at the start. Which explanation best accounts for the reduced variation?
A researcher assays an enzyme at two pH values using identical enzyme concentration, substrate concentration, and temperature. At pH 6.5, the initial rate is 15 μM/min; at pH 7.5, it is 16 μM/min. The enzyme contains several acidic and basic side chains, and its tertiary structure is stabilized by multiple noncovalent interactions. Small pH changes can shift protonation states, but not all shifts cause major changes to active-site geometry. Based on the data, which inference is most supported about changing pH from 6.5 to 7.5?
Human skin cells from the same individual are cultured with either normal oxygen (21% O₂) or low oxygen (1% O₂). Under low oxygen, cells show increased VEGF mRNA and secrete more VEGF protein, but DNA sequencing reveals no changes. Which explanation best accounts for the increased VEGF production in low oxygen?
In a small island population of birds, a locus has alleles $D$ and $d$ . In generation 1, allele frequencies are $p(D)=0.50$ and $q(d)=0.50$ . The birds mate randomly, and no migration, mutation, or selection is detected. A hurricane then reduces the population to 12 surviving individuals, and in generation 2 the allele frequencies are $p(D)=0.67$ and $q(d)=0.33$ . Which conclusion is best supported about the cause of the allele-frequency change?
A gardener forgets to water two identical potted plants. After several hours in dry soil, both plants have drooping leaves. When one plant is watered, its leaves become more upright within 30 minutes, while the unwatered plant remains drooped. The air temperature and light level stay constant. Which response best explains the rapid change in leaf position after watering?
An enzyme-catalyzed reaction is run at 30°C with saturating substrate in two buffers: pH 7.4 and pH 5.4. The rate at pH 7.4 is 15 nmol/min, and the rate at pH 5.4 is 6 nmol/min. pH can change the ionization of amino acid side chains involved in substrate binding and catalysis. Which outcome is most likely at pH 5.4?
In an experiment, a pure phospholipid bilayer is exposed to equal concentrations of K $^+$ (39 Da, charged) and argon gas (40 Da, nonpolar). Their masses are similar. Which molecule would most likely cross the membrane faster by simple diffusion?
A globular protein's primary structure determines how it folds into secondary (α-helices/β-sheets) and tertiary structures through R-group interactions. Hydrophobic R groups tend to cluster away from water, while polar or charged R groups often face the aqueous environment, helping stabilize the folded shape. A researcher substitutes several surface-exposed polar amino acids with nonpolar amino acids without changing chain length. In water, the altered protein aggregates and loses its normal binding specificity to a ligand. Which statement best describes the molecular cause of the lost function?
Based on this information, what is the most likely mode of inheritance for this disorder?
A scientist compares two RNA samples transcribed from the same eukaryotic gene. In sample X, sequences matching introns are present. In sample Y, intron sequences are absent, and exon sequences are joined precisely at canonical splice junctions (GU at the 5′ intron end and AG at the 3′ intron end). The gene's DNA contains these GU/AG motifs at intron boundaries. Which explanation best accounts for the precise exon joining observed in sample Y?
A phospholipid bilayer forms a hydrophobic barrier to most polar molecules. Some embedded proteins function as aquaporins, providing a hydrophilic channel for water. Two cell membranes have the same phospholipid composition, but Membrane 1 contains many aquaporins and Membrane 2 contains few. When placed in the same hypotonic solution, cells with Membrane 1 change volume more rapidly than cells with Membrane 2. Which feature best explains the faster volume change in cells with Membrane 1?
Clonal strawberry plants are grown at either 15°C or 25°C. Plants at 15°C produce darker red fruit and show higher expression of anthocyanin biosynthesis genes in fruit skin cells; sequencing shows identical DNA sequences for those genes in both groups. When plants are moved from 25°C to 15°C before fruit ripening, new fruit become darker. Which explanation best accounts for the temperature-dependent fruit color differences?
In an animal cell, gene E is regulated by a transcription factor that binds the promoter only when it is dimerized. A ligand binds the transcription factor and promotes dimer formation. Without ligand, chromatin at the promoter is accessible but gene E transcription is low. After ligand addition, gene E primary transcript levels increase. Which change would most likely block ligand-induced transcription of gene E?
Two animal cells are compared: Cell 1 has a plasma membrane with a higher proportion of unsaturated phospholipid tails than Cell 2. At the same temperature, Cell 1's membrane is observed to be more fluid. Which feature best explains the increased fluidity in Cell 1?
A population of freshwater snails has a shell-band gene with alleles S and s. In a sample of 500 snails, the genotype counts are 245 SS, 210 Ss, and 45 ss. From these counts, allele frequencies are $p(S)=0.70$ and $q(s)=0.30$ . If the population were in Hardy-Weinberg equilibrium, expected genotype frequencies would be $p^2=0.49$ , $2pq=0.42$ , and $q^2=0.09$ . The observed genotype frequencies are 0.49 SS, 0.42 Ss, and 0.09 ss. No migration into or out of the pond is detected during the sampling interval. Which conclusion is best supported by the data?
Enzyme R catalyzes S → P. At fixed enzyme concentration, the initial rate is measured at different substrate concentrations. Without inhibitor, the rate reaches 100 µM/min at high [S]. With inhibitor J present, the rate reaches only 50 µM/min even at very high [S], and increasing [S] does not restore the original maximum. pH and temperature remain constant. Which explanation best accounts for J's effect on the reaction?
Based on the information, how does the distribution of lactase persistence support the idea that evolution is a continuous process that responds to environmental pressures?

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