Practice Test 66 - AP Biology

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

A researcher adds oligomycin, a drug that blocks the proton channel of ATP synthase, to mitochondria actively consuming oxygen. Electrons normally flow from NADH through the electron transport chain to oxygen, and the associated proton pumping builds a gradient. With ATP synthase blocked, protons cannot return to the matrix through the enzyme. Over a short time, the proton gradient becomes very steep, making it harder for the electron transport chain to pump additional protons outward. Which outcome is most likely after oligomycin addition?
A freshwater snail species includes two populations living in the same river system. Population M reproduces by internal fertilization and releases juveniles; population N releases eggs and sperm into the water. When adults from M and N are placed together, courtship occurs, but mating attempts do not result in transfer of sperm because reproductive openings do not align. Genetic data show divergence between the populations despite overlapping ranges. Which prezygotic barrier most directly prevents gene flow between M and N?
A membrane receptor has an extracellular ligand-binding domain and a cytosolic domain. A mutation truncates the cytosolic domain but leaves the ligand-binding domain intact. Ligand still binds, but no cellular response is detected. Which of the following best explains the lack of response?
A phospholipid bilayer without proteins separates two chambers. Equal concentrations of glyceraldehyde (90 Da, polar uncharged) and O $_2$ (32 Da, nonpolar) are placed on one side. Which statement best explains which solute accumulates on the opposite side first?
In a fish species, blue scales (B) are completely dominant to gold scales (b). Two gold-scaled fish are crossed (bb × bb). Each offspring receives one allele from each parent. Assume a single gene controls scale color and inheritance is Mendelian with complete dominance. Which outcome is expected for the offspring phenotype?
In an epithelial cell line, ligand A binds a GPCR and activates Gq. Gq activates phospholipase C (PLC), which cleaves PIP2 in the membrane into IP3 and DAG. IP3 diffuses to the endoplasmic reticulum (ER) and opens IP3-gated Ca2+ channels, raising cytosolic Ca2+. Ca2+ binds calmodulin, and the Ca2+-calmodulin complex activates a kinase that phosphorylates a cytosolic target protein. In a variant line, the IP3 receptor on the ER cannot bind IP3, but PLC activity and PIP2 levels are unchanged. Which change is most likely in the variant cells after exposure to ligand A?
A scientist performs Southern blotting to test whether a 1.2 kb DNA sequence is present in genomic DNA. Genomic DNA is cut with a restriction enzyme, separated by gel electrophoresis, and transferred to a membrane. A labeled single-stranded DNA probe complementary to the 1.2 kb sequence is added under conditions that allow base pairing. After washing, a single band is detected on the membrane. Which explanation best accounts for the detected band?
A biologist compares five vertebrate taxa using shared derived traits relative to a jawless fish outgroup. All five taxa have jaws. Only taxa M, N, and O have a bony skeleton. Only taxa N and O have four limbs. Only taxon O has an amniotic egg. Assume each derived trait evolved once and was not lost. Which pair of taxa shares the most recent common ancestor?
A diploid cell with $2n=6$ completes DNA replication before meiosis, producing sister chromatids for each chromosome. After meiosis I, two cells form, and each proceeds through meiosis II. During meiosis II, sister chromatids separate and are distributed to different gametes. Which statement best describes the change in chromosome number across meiosis I and meiosis II?
In a cell extract, a molecule is found that includes a five-carbon sugar, a phosphate group, and a nitrogen-containing ring. Many of these units connect into a chain when the phosphate of one unit forms a covalent bond with the sugar of the next, producing a repeating sugar-phosphate backbone. The order of the nitrogen-containing rings varies along the chain and can store information at the molecular level. Which statement best describes the polymerization that creates this macromolecule?
A biologist analyzes a double-stranded nucleic acid and finds that the amount of adenine equals the amount of thymine, and the amount of guanine equals the amount of cytosine. The molecule forms a uniform helix in which each base pair has the same overall width. This uniform width results from pairing a purine with a pyrimidine across the two strands. Which statement best explains how base structure contributes to a consistent helix diameter?
Genetically identical cuttings from one coleus plant were grown for 3 weeks under either high light (full sun) or low light (shade) with equal water and nutrients. Leaves from high-light plants developed deeper purple coloration than leaves from low-light plants. DNA sequencing of a pigment-pathway gene from both groups showed identical nucleotide sequences. Which explanation best accounts for the difference in leaf color between the two groups?
In an experiment, vesicles are made from phospholipids with two saturated fatty acid tails instead of unsaturated tails. Saturated tails contain no C=C double bonds, so their hydrocarbon chains remain relatively straight and can pack closely. Unsaturated tails contain one or more cis C=C double bonds that introduce kinks, reducing packing efficiency. Both vesicle types are placed at the same temperature in water, where phospholipids orient with hydrophilic phosphate heads toward water and hydrophobic tails away from water. Which feature best explains the expected difference in membrane fluidity between the two vesicle types?
A plant is shifted from low light to high light while CO $_2$ remains constant. Shortly after the shift, ATP and NADPH production increase, but carbon fixation does not increase proportionally. Which outcome is most likely inside the chloroplast stroma?
A student compares mitosis and meiosis in a species with $2n=16$ . In meiosis I, homologous chromosomes separate into two cells, and in meiosis II sister chromatids separate into four cells. In mitosis, sister chromatids separate in a single division. Which outcome is most likely when comparing chromosome number per daughter cell after meiosis II versus after mitosis?
A lake food web includes phytoplankton, zooplankton, small fish, and large fish. Net primary production by phytoplankton is 9,000 kJ/m $^2$ /yr. Biomass energy measured in zooplankton growth is 900 kJ/m $^2$ /yr, and in small fish growth is 90 kJ/m $^2$ /yr. Assume large fish feed mainly on small fish. Which outcome is most likely for annual large-fish biomass production in this lake?
A cell is heterozygous for two genes located on the same homologous chromosome pair in the arrangement FG on one homolog and fg on the other. During prophase I, a single crossover occurs between the loci on non-sister chromatids. After meiosis II, four gametes are produced. Assume no other recombination events occur and chromosome segregation is normal. At the chromosomal level, the crossover exchanges corresponding segments between homologous chromatids, changing which alleles are physically linked on a chromatid. Which process best explains how this event increases genetic variation among the gametes produced?
Two disaccharides are compared: Disaccharide X contains glucose and fructose linked by a glycosidic bond that uses the anomeric carbon of each monosaccharide, leaving no free anomeric carbon. Disaccharide Y contains two glucose monomers linked so that one anomeric carbon remains free. In a test solution, Y can convert between ring and open-chain forms, whereas X cannot. Which statement best predicts a consequence of these structural differences at the molecular level?
A mainland bird population has allele A at frequency 0.70. A small island is colonized by 12 birds from the mainland. In the first breeding season on the island, allele A frequency in the island population is 0.33. Over the next two generations, the island population remains isolated and grows to 300 birds, with allele A staying near 0.33. No trait linked to A is associated with survival or reproduction in field observations. Which factor most directly explains the initial allele-frequency difference on the island?
Two populations of the same bird species are monitored in separate, enclosed habitats of equal size. Habitat X has abundant nesting sites; Habitat Y has limited nesting sites. Both start at 100 birds. After several breeding seasons, X increases to 380 and continues rising, while Y increases to 190 and then remains near 185–195. Food availability is similar in both habitats, and no migration occurs. Which explanation best accounts for the difference between habitats X and Y?

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