Practice Test 24 - AP Biology

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

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1. In corn, smooth kernels (K) are completely dominant to wrinkled kernels (k). A plant with smooth kernels is self-fertilized, and the offspring show a 3 smooth : 1 wrinkled phenotype ratio. Assume one autosomal gene with complete dominance and no linkage to other genes. Which genotype must the parent plant have had?
2. In an experiment, muscle cells and liver cells from the same mouse are analyzed. Their DNA sequences match. Chromatin immunoprecipitation shows a transcriptional activator binds near a muscle-specific gene promoter in muscle cells but not in liver cells. Correspondingly, the muscle-specific gene's mRNA is abundant in muscle cells and scarce in liver cells. Which explanation best accounts for the difference in gene expression between the two cell types?
3. Two single cells are the same shape and differ only in size. Both exchange water and small solutes across the plasma membrane, and both are in the same environment. The larger cell shows slower changes in internal solute concentration when the external solute concentration shifts. Which explanation best accounts for the slower internal concentration change in the larger cell?
4. A student builds models of phospholipids with identical polar head groups but different tail lengths: Model 1 has two 14-carbon tails; Model 2 has two 20-carbon tails. Longer hydrocarbon tails provide more surface area for van der Waals interactions when packed together in a bilayer's interior. At the same temperature, the bilayers differ in how tightly the hydrophobic core packs. Which statement best predicts the effect of increasing tail length on bilayer fluidity?
5. Plant cell walls contain cellulose, a polysaccharide of glucose monomers linked by β-1,4 glycosidic bonds. Each glucose is rotated relative to the next, producing straight, unbranched chains. Numerous hydroxyl (–OH) groups on adjacent chains form extensive hydrogen bonds, bundling many cellulose molecules into rigid microfibrils that resist stretching and compression. Which feature best explains cellulose microfibrils' high tensile strength in plant cell walls?
6. A spherical cell's radius increases from 5 µm to 10 µm while membrane permeability and external solute concentration remain constant. The rate of diffusion across the membrane is proportional to membrane surface area, and the cell's demand for solute is proportional to cytoplasmic volume. Which explanation best accounts for the change in the cell's ability to meet solute demand by diffusion alone?
7. A cell is in late G2 with 4C DNA; replication is complete and chromosomes are duplicated but not condensed. A compound is added that prevents activation of the proteins required for entry into mitosis, so the G2-to-M transition does not occur. The cell remains viable and does not initiate chromosome condensation or nuclear envelope breakdown. DNA replication does not restart under these conditions. Which observation is most likely after several hours in the presence of the compound?
8. Scientists compared two prebiotic simulations using the same gas mixture and temperature. Setup 1 included continuous electrical discharges; Setup 2 used no discharges. After seven days, Setup 1 contained a higher diversity of small organic molecules, including amino acids, while Setup 2 contained mostly unreacted starting gases. Which conclusion is best supported about early Earth energy sources and abiotic synthesis?
9. Scientists compare the amino acid sequence of a conserved protein in four species: human, chimpanzee, mouse, and chicken. The protein differs at 1 position between human and chimpanzee, 12 positions between human and mouse, and 35 positions between human and chicken. The same gene is present in all four species and performs a similar cellular function. Which conclusion is best supported by these molecular comparisons?
10. A cell is placed in a solution where Na+ concentration is 140 mM outside and 10 mM inside. A membrane protein allows Na+ to cross, and Na+ movement increases when the protein is present but stops when the protein is denatured. When ATP production is blocked, Na+ still moves into the cell at the same rate. Which mechanism best explains Na+ entry into the cell?
11. A plant cell has a large central vacuole whose lumen is more acidic than the cytosol. Transport proteins in the vacuolar membrane move H+ into the vacuole, maintaining this pH difference. An enzyme that breaks down certain toxins shows high activity at low pH but low activity at cytosolic pH. When the vacuolar membrane becomes leaky, the pH difference decreases and toxin breakdown slows. Which conclusion is best supported about how compartmentalization influences function?
12. A DNA-binding protein recognizes the major groove of B-DNA by contacting exposed chemical groups on base pairs. Which feature best explains how sequence specificity is possible without strand separation?
13. A cell with 0.20 M nonpenetrating solute inside is placed in a solution containing 0.05 M nonpenetrating solute. The membrane is permeable to water but not to the solute. After several minutes, the cell's volume increases. Which outcome is most likely?
14. A lizard population includes individuals with short or long hind limbs due to genetic variation. After an invasive plant increases branch density, long-limbed lizards more often fall while foraging, and short-limbed lizards leave more offspring. Over generations, the short-limb phenotype becomes more common. Which statement best explains how variation affected this outcome?
15. A zoologist compares skeletal features in three vertebrates: a salamander, a lizard, and a bird. All three have a vertebral column and a skull with a single occipital condyle, but only the lizard and bird share a particular ankle-bone arrangement in which two specific bones are fused into a single structure. The salamander lacks this fusion. Which inference is best supported by these observations?
16. A plant cell protoplast (cell wall removed) responds to Ligand C with a rapid increase in cytosolic pH. When the same cells are treated with a reagent that crosslinks and immobilizes membrane proteins, Ligand C still binds to the membrane but the pH change is reduced. The ligand does not enter the cell, as shown by fluorescent labeling remaining outside. Which of the following best explains why immobilizing membrane proteins reduces the early response?
17. A membrane protein transports solute Z only when Z binds to a specific site. With 40 mM Z outside and 4 mM inside, net Z influx occurs. When the same cells are treated with a reagent that locks the protein in one conformation, Z influx decreases even though the gradient remains. No ATP consumption is detected. Which explanation best accounts for reduced Z influx?
18. In endothelial cells, nitric oxide (NO) activates soluble guanylyl cyclase to produce cGMP. cGMP activates protein kinase G (PKG). PKG phosphorylates a cGMP-specific phosphodiesterase, increasing its activity and accelerating cGMP breakdown. When the phosphodiesterase is knocked down, cGMP remains elevated longer after a fixed NO pulse. Which outcome best illustrates how the feedback affects pathway behavior?
19. A population of yeast is grown in identical flasks, with one sugar source as the only carbon supply. At a gene influencing sugar transport, allele S is initially at frequency 0.50. After 300 generations, allele S is at 0.93 in the same continuously reproducing population. Which observation best demonstrates evolution occurring in this yeast population?
20. In a plant species, smooth seeds (S) are completely dominant to wrinkled seeds (s). Two plants are crossed, producing 80 offspring: 60 smooth and 20 wrinkled. Assume a single gene controls seed texture and the counts reflect expected Mendelian ratios. Which parental genotypes most likely produced these offspring?

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