Practice Test 82 - AP Biology

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

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1. Two cell types express the same membrane receptor for ligand Q. After Q addition, Cell type 1 shows a rapid increase in cGMP, while Cell type 2 shows no change in cGMP. Radiolabeled Q binds equally to both cell types. A biochemical assay shows that only Cell type 1 contains a membrane-associated guanylyl cyclase that can be activated by the receptor. Which of the following best explains the difference in early intracellular response?
2. A biologist monitors a yeast population in a closed flask with abundant sugar and stable temperature. Cell counts (millions/mL) are: 2 at 0 h, 4 at 2 h, 8 at 4 h, 16 at 6 h, and 32 at 8 h. Waste products remain low during this interval, and no cells are removed from the flask. Which explanation best accounts for the growth pattern observed from 0 to 8 hours?
3. A freshwater enzyme is assayed at 25°C in two solutions: 0.01 M salt and 0.50 M salt. The enzyme produces 10 units of product/min in 0.01 M salt but 4 units/min in 0.50 M salt. Increased salt can shield charges on amino acid side chains and change electrostatic interactions that help stabilize tertiary structure. Which explanation best accounts for the decreased activity in 0.50 M salt?
4. A single large population of 500 mice lives on an isolated island, with no migration on or off the island. A gene with codominant alleles $A$ and $a$ determines coat color, so $AA$, $Aa$, and $aa$ mice each have a distinct coat color. Genotype counts are 200 $AA$, 50 $Aa$, and 250 $aa$. Field studies show that survival and number of offspring do not differ among the three genotypes, and that mice almost always mate with partners of their own coat color. Which Hardy-Weinberg condition is most clearly not met, and why?
5. A simple cladogram is described in words: The first split separates species P from the lineage leading to Q, R, and S. The next split separates Q from the lineage leading to R and S. The final split separates R and S from each other. No branch lengths are provided. Based on this branching order alone, which species share the most recent common ancestor with each other?
6. A cell uses a membrane protein to transport solute X from low concentration outside to high concentration inside. When cellular ATP levels are experimentally depleted, transport of X stops even though the concentration gradient remains. In a separate trial, adding ATP restores transport of X without changing the gradient. ATP hydrolysis is exergonic, and moving X against its gradient is endergonic. The transporter has cytosolic sites that bind ATP and become transiently phosphorylated during the transport cycle. Which explanation best accounts for why ATP is required for X accumulation inside the cell?
7. A bacterial enzyme is encoded by a gene whose coding strand begins 5'-ATG AAA GGC CCT GAA ACC TGG...-3'. A mutation deletes nucleotide 4, the first A after the ATG start codon, so the mutant coding strand begins 5'-ATG AAG GCC CTG AAA CCT GG...-3'. Translation initiates at the AUG start codon, which is unaffected by the mutation. No other mutations occur. Which outcome is most likely for the protein produced from this mutated gene?
8. In a lab, red blood cells are placed into a solution with lower solute concentration than the cytosol. Over 10 minutes, the cells swell, and the membrane remains intact. No ATP inhibitors change the rate of swelling. Water crosses the plasma membrane through aquaporin proteins, and the net movement is from the solution into the cells, down water's concentration gradient. Which mechanism best explains the movement of water into the cells?
9. A coastal snail population includes shells with either thick or thin walls. A gene with two alleles influences shell thickness; heterozygotes show intermediate thickness. Before an increase in crab predators, allele frequencies are T = 0.55 and t = 0.45. After 10 years, T = 0.72 and t = 0.28, and crab abundance is high each year. No evidence of migration or mutation is found during the period. Which statement best explains how variation contributed to the change in allele frequencies?
10. A student compares two cells that are identical except for size. Both cells rely on simple diffusion across the plasma membrane for uptake of a nonpolar molecule, and both use the molecule at the same rate per unit volume. When the cell diameter is doubled, the time required for the internal concentration to approach the external concentration increases. Which explanation best accounts for the increased time with larger cell size?
11. Starch in plants includes amylose, a polysaccharide of glucose connected by α-1,4 glycosidic bonds. This linkage geometry promotes a helical conformation rather than a straight chain. In contrast, β-1,4 linkages favor extended chains that align side-by-side. Which feature best explains why amylose tends to form compact coils compared with β-linked glucose polymers?
12. In humans, red-green color blindness is caused by an X-linked recessive allele c; the normal-vision allele C is dominant. A woman with normal vision whose father was color-blind mates with a man with normal vision ($X^CY$). The woman's genotype is X^CX^c. Which outcome best predicts the probability that a son will be color-blind?
13. A plant cell with cytosol at 0.30 M nonpenetrating solute is transferred from an isotonic solution to a new solution at 0.10 M nonpenetrating solute. The plasma membrane is permeable to water but not solute, and the external concentration remains constant. Focus on osmotic water movement and resulting changes in cell volume and pressure against the cell wall. Which outcome is most likely after several minutes?
14. Two phospholipids have identical head groups and tail lengths, but one has a cis double bond in one tail while the other has no double bonds. The cis double bond introduces a bend that reduces how closely neighboring tails can align. In a bilayer made mostly of one type, the average spacing between tails differs. Which consequence is most likely in the bilayer enriched with the cis-unsaturated phospholipid?
15. A neuron releases neurotransmitter N into a synaptic cleft. Muscle cell M depolarizes only when N is present and returns to resting potential when N is removed. When a competitive antagonist that resembles N is added, N is still released normally, but M does not depolarize. Which outcome is most likely at the muscle cell membrane?
16. A student compares DNA and RNA molecules isolated from the same cell. The DNA sample is mostly double-stranded and contains thymine, while the RNA sample is mostly single-stranded and contains uracil. The student notes that RNA nucleotides have a ribose sugar with a 2′ hydroxyl group, whereas DNA nucleotides have deoxyribose lacking the 2′ hydroxyl. This structural difference affects the chemical stability of the sugar-phosphate backbone in aqueous conditions. Which statement best describes how the sugar structure relates to nucleic acid stability?
17. A deer population in a reserve was estimated each winter. After hunting was banned, the population rose from 300 to 600 over three years. In the next two years it rose to 720 and then 760. Over the same period, vegetation surveys showed increased browsing damage and reduced sapling density as deer density increased. No severe winters occurred. Which inference best explains the slowing population growth in the last two years?
18. A steroid hormone is lipid-soluble and consists primarily of four fused carbon rings with few polar functional groups. A peptide hormone, in contrast, contains many polar peptide bonds and charged side chains. Both hormones are present outside target cells. Which statement best predicts how the steroid hormone's structure affects its interaction with the plasma membrane?
19. On an island, a lizard population includes two color morphs controlled largely by a single gene: dark (allele d) and light (allele l). After a wildfire in 2012, the ground surface became darker due to ash and char. Researchers estimated allele frequencies from random samples: in 2011, $f(d)=0.22$; in 2013, $f(d)=0.35$; in 2016, $f(d)=0.48$. Sampling methods and population boundaries were consistent across years. Which observation best demonstrates continuing evolution in this population?
20. A cell completes S phase, producing sister chromatids for each chromosome, and enters G2 with 4C DNA content. Just before mitosis, the centrosomes separate and begin forming a bipolar spindle. A toxin is added that prevents sister chromatids from separating by blocking the proteolysis needed to release cohesion at centromeres. The spindle checkpoint is satisfied because kinetochores are attached and under tension at metaphase. Anaphase is initiated, but cohesion is not removed. Which outcome is most likely observed during anaphase in this cell?

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