Practice Test 69 - 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 a solution with 9 mM molecule U outside and 3 mM inside. The membrane has U carriers that bind and release U without ATP hydrolysis. If the number of carriers is reduced by half, the initial rate of U uptake decreases. Which explanation best accounts for the reduced uptake rate?
2. A researcher adds a compound to muscle cells that specifically inhibits the Na+/K+ ATPase, the membrane pump that hydrolyzes ATP to move Na+ out of the cell and K+ into the cell against their concentration gradients. The inhibitor does not act on ion channels directly. Over the next several minutes, the Na+ gradient across the plasma membrane decreases. What is the most direct effect of the inhibitor on the rate of ATP hydrolysis by the Na+/K+ ATPase?
3. A student examines red blood cells (RBCs) and typical epithelial cells. RBCs lack a nucleus and most membrane-bound organelles, while epithelial cells contain a nucleus, endoplasmic reticulum, and mitochondria. When both cell types are placed in a solution containing a fluorescent lipid that inserts into membranes, epithelial cells show labeled internal membranes in addition to the plasma membrane, but RBCs show labeling only at the cell surface. Which feature best explains the difference in fluorescence patterns?
4. A bacterial population uses a secreted molecule Q to coordinate behavior. Q binds to a receptor protein embedded in the plasma membrane. Within seconds of adding Q, a cytosolic response protein becomes phosphorylated. If the receptor's extracellular binding site is blocked by an antibody, phosphorylation does not occur. However, if purified active kinase (the enzyme that phosphorylates the response protein) is introduced into permeabilized cells, phosphorylation occurs without Q. Which of the following best explains how signal Q is received and initiates the intracellular response?
5. In mice, black fur (B) is completely dominant to brown fur (b). Two black mice are crossed, and genetic testing shows each parent is heterozygous (Bb). Each parent produces gametes carrying either B or b with equal probability. The offspring fur color depends on whether the genotype includes at least one B allele. Which outcome is most likely for the offspring phenotypes?
6. A researcher isolates nuclear RNA from eukaryotic cells and treats it with an enzyme that specifically removes the 5′ cap but does not affect the RNA backbone elsewhere. The researcher then repeats the treatment on cytosolic RNA from the same cells. Both samples contain transcripts produced by RNA polymerase II. The 5′ cap is normally added co-transcriptionally after the nascent RNA reaches about 20–30 nucleotides in length. Which explanation best accounts for why most cytosolic RNA molecules resist this decapping treatment compared with nuclear RNA molecules?
7. A plant population contains a rare mutation that causes some individuals to produce unreduced (2n) gametes. When 2n gametes fuse with normal n gametes, resulting offspring have 3n and are largely sterile. When two 2n gametes fuse, resulting offspring have 4n and can produce fertile 2n gametes with each other but not with the original n plants. Over several generations, the 4n plants increase in frequency in one area of the population. Which process most directly generated reproductive isolation between the 4n plants and the original plants?
8. A scientist studies transcription of a protein-coding gene in the nucleus. After transcription begins, the nascent RNA is rapidly modified at its 5′ end by addition of a 7-methylguanosine cap through an unusual 5′–5′ triphosphate linkage. In an experimental condition, the capping enzyme is inhibited, but RNA polymerase II still initiates and elongates normally. When RNA is extracted, uncapped transcripts are detected but are much less abundant than capped transcripts from control cells. Which explanation best accounts for the lower abundance of uncapped transcripts?
9. A cell's plasma membrane is the only site for exchange of a particular solute with the environment. Two cells have the same membrane permeability and are in the same solution. Cell U has volume $V$ and surface area $SA$; Cell V has volume $8V$ and surface area $4SA$ (same shape). Both cells consume the solute at the same rate per unit volume. Cell V shows a lower internal solute concentration. Which explanation best accounts for Cell V's lower concentration?
10. A student observes that a thin stream of water can momentarily bridge a small gap between two closely spaced glass surfaces. Water molecules are polar: oxygen has a partial negative charge and hydrogens have partial positive charges. Hydrogen bonds form between neighboring water molecules, creating cohesion, and water molecules can also form hydrogen bonds with polar groups on glass, producing adhesion. Together, these interactions can hold water in a continuous shape across a short distance, resisting separation. Which feature best explains the ability of water to bridge the gap between the glass surfaces?
11. In a lab, a cell type shows abundant rough ER membranes studded with ribosomes and a large Golgi apparatus. When these cells are treated with a drug that prevents vesicles from fusing with the Golgi, newly made proteins accumulate in small transport vesicles near the ER, and very little protein appears outside the cell. Which feature best explains why secretion decreases when Golgi fusion is blocked?
12. Two closely related frog populations occupy the same forest. Males call from similar locations, and females approach calling males. Playback experiments show that females from population 1 approach only the call frequency pattern typical of population 1, while females from population 2 approach only the pattern typical of population 2. When researchers place males and females together in an enclosure, mating occurs mostly within populations even though breeding seasons overlap and hybrids, when produced, develop into viable and fertile adults. Genetic analyses show increasing divergence in allele frequencies over time. Which prezygotic barrier most directly maintains reproductive isolation?
13. A bacterial capsule is composed of a polysaccharide with many carboxyl (-COO$^-$) groups on its repeating sugar units, giving the polymer a high negative charge density at physiological pH. Water molecules form hydration shells around the charged groups, and the polymer chains repel one another, creating a swollen, slippery layer on the cell surface. Which statement best describes how the capsule's carbohydrate structure contributes to this physical property?
14. In fruit flies, white eyes are caused by an X-linked recessive allele $w$; the dominant allele $W$ produces red eyes. A red-eyed female heterozygote ($X^W X^w$) is crossed with a red-eyed male ($X^W Y$). Which offspring outcome is expected?
15. Researchers map RNA transcripts produced from a eukaryotic gene and find that many transcripts extend far beyond the usual 3′ end of the mature mRNA. In cells treated with a drug that inhibits cleavage at the polyadenylation site, the extended transcripts accumulate, and few RNAs contain a poly(A) tail. Transcription initiation is unaffected by the drug. Which explanation best accounts for the accumulation of extended transcripts in treated cells?
16. At the G1/S transition, a cyclin–Cdk complex phosphorylates a protein that promotes assembly of replication machinery at origins. A specific phosphatase counteracts this by removing those phosphates, reducing origin activation. A researcher overexpresses the phosphatase's catalytic activity via microinjection of active enzyme, without altering cyclin–Cdk levels. Which outcome is most likely following microinjection?
17. A researcher synthesizes a polymer by repeatedly joining monomers that each have one amino group and one carboxyl group. Each joining reaction removes a molecule of water and forms a covalent bond between the carbonyl carbon of one monomer and the nitrogen of the next. The polymer's properties change when the monomers' side groups differ. Which statement best describes the chemical basis for polymer diversity in this macromolecule class?
18. A coral reef and a nearby seagrass bed are surveyed for fish. Reef: 30 fish species, but 60% of individuals are one damselfish species. Seagrass: 18 fish species, with individuals spread more evenly among species; total fish counted is the same at both sites. Which statement is best supported about comparing biodiversity between these communities?
19. A eukaryotic pre-mRNA is transcribed from a protein-coding gene. When analyzed, the mature mRNA has an added stretch of adenine nucleotides at its 3' end that is not encoded in the DNA template. The 5' end of the mature mRNA also contains a modified guanine nucleotide linked by an unusual bond. These features are absent in newly made nuclear RNA but present after processing. Which explanation best accounts for how these end modifications arise?
20. A single-stranded RNA folds into a hairpin when regions of the same molecule base-pair. Which feature best explains this folding at the molecular level?

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