Practice Test 60 - 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-surface glycoprotein has an attached oligosaccharide whose terminal sugar is sialic acid, which carries a negative charge at physiological pH. In a variant protein, the terminal sialic acid is replaced with a neutral monosaccharide of similar size. The local extracellular environment contains positively charged ions and proteins that can associate electrostatically with negative groups. Which change is most likely at the molecular level when sialic acid is replaced by the neutral sugar?
2. A bacterial population in a hospital sink is sampled monthly for 8 months. At the start, 5% of cells carry a plasmid allele that provides resistance to a disinfectant; by month 8, 72% carry it. The total bacterial count stays similar each month, and the disinfectant is applied daily throughout the period. Which observation best demonstrates evolution occurring in this bacterial population in real time?
3. A scientist claims that the current increase in atmospheric carbon dioxide is acting as a selective pressure on plant populations worldwide. Which of the following observations would provide the strongest evidence for continuing evolution in response to this pressure?
4. Cells use phosphorylation cascades to coordinate checkpoint signals. In a study, a kinase (K1) phosphorylates and activates a second kinase (K2). Active K2 phosphorylates an inhibitor of CDK2, causing the inhibitor to release CDK2. In cells with a loss-of-function mutation in K2, CDK2 remains inhibited even when K1 is active. Which change would most likely increase CDK2 activity in the mutant cells?
5. A grass population includes individuals with either high or low waxy leaf coating. The coating level is heritable. During a multi-year drought, plants with higher waxy coating lose less water and produce more seeds than plants with low coating. Seedlings each year show both coating types. Which outcome is most likely after several drought years?
6. On a rocky shore, mussels occupy most available space when sea stars are absent. In areas where sea stars are common, mussel cover is lower and many other invertebrate species occupy open rock. When sea stars are experimentally removed from a section of shoreline, mussel cover increases over several months and the number of other invertebrate species observed declines. Physical factors (tide height, wave exposure) are similar across sections. Which outcome is most likely explaining the change in community diversity after sea star removal?
7. Different ligands bind different receptors yet cause the same response. What best explains this?
8. Two wetlands of equal area were surveyed. Wetland A contained 10 bird species with 10 individuals each (100 total). Wetland B contained 2 bird species: 95 individuals of species 1 and 5 individuals of species 2 (100 total). Both wetlands had similar water depth and vegetation cover. Which conclusion is best supported about biodiversity in the two wetlands?
9. A scientist compares two leaf samples under identical light and CO2 conditions. Sample X receives a chemical that prevents NADP+ from accepting electrons at the end of the thylakoid electron transport chain. In Sample X, electrons back up in the chain, and fewer additional electrons can be transferred from excited chlorophyll. Proton pumping decreases over time, reducing ATP synthesis as well. Oxygen evolution declines as water oxidation slows. Which outcome is most likely in Sample X compared with the untreated sample?
10. In a lab, two spherical cells are made of the same membrane material and kept in identical solutions. Cell X has radius 5 µm; Cell Y has radius 10 µm. Both rely on diffusion across the plasma membrane for oxygen uptake, and both consume oxygen at the same rate per unit cytoplasmic volume. After several minutes, Cell Y shows lower internal oxygen concentration than Cell X. Which explanation best accounts for the difference in oxygen levels between the cells?
11. A researcher measures initial reaction rate for enzyme E at increasing substrate concentrations. The rate increases and then plateaus, even though substrate continues to increase. The researcher then adds additional substrate after the plateau is reached and observes no immediate increase in rate. Enzyme concentration, temperature, and pH are constant, and product concentration remains low during the initial-rate measurement. Which explanation best accounts for the plateau and lack of response to added substrate?
12. A lab compares two lipids: wax W is made of a long-chain fatty acid esterified to a long-chain alcohol, while triglyceride T is glycerol esterified to three fatty acids. Both are largely nonpolar, but wax W has a single ester linkage and two long hydrocarbon chains that pack tightly, forming a solid, water-resistant layer. Triglycerides can vary in tail composition and often remain less rigid under the same conditions. Which feature best explains why wax W reduces water loss more effectively when coating a surface?
13. Chitin is a structural polysaccharide composed of repeating N-acetylglucosamine monomers linked by $β(1\to4)$ glycosidic bonds. The acetamide (N-acetyl) group on each monomer can participate in hydrogen bonding with neighboring chains, allowing many parallel chains to pack tightly. An arthropod exoskeleton sample made largely of chitin is hard and resists deformation. Which molecular feature best explains this rigidity?
14. In bacteria, a gene is transcribed into an RNA that is immediately usable as mRNA without intron removal. A student claims that bacterial mRNAs commonly undergo spliceosome-mediated intron excision in the cytoplasm. In an experiment, RNA from a bacterial culture is analyzed and shows continuous sequence matching the coding region with no missing internal segments. Which explanation best accounts for the absence of spliceosome-mediated splicing in the bacterial RNA sample?
15. A cell has intracellular K+ of 140 mM and extracellular K+ of 5 mM. A K+-selective channel is opened, and ATP synthesis is inhibited. Immediately after the channel opens, K+ moves out of the cell, and the rate depends on the concentration difference and channel number. No carrier saturation is observed over the tested range. Which mechanism best explains K+ movement?
16. A researcher compares translation of two mRNAs that differ only in the position of a stop codon. mRNA X: 5′-AUG AAA UAA GGG-3′. mRNA Y: 5′-AUG AAA GGG UAA-3′. In both cases, AUG recruits the initiator tRNA, and UAA is recognized by a release factor that triggers polypeptide release. The ribosome translates codons sequentially from 5′ to 3′ until it reaches a stop codon. Which outcome is most likely when comparing the two polypeptides?
17. In an experiment, the initial rate of an enzyme-catalyzed reaction is measured at pH 7 with constant enzyme concentration. As substrate concentration increases from 0.1 mM to 10 mM, the rate increases rapidly at first and then approaches a plateau near 80 units/min. A second trial repeats the substrate series but doubles the enzyme concentration; the plateau rate increases to about 160 units/min while the substrate concentration at which the rate begins to level off is similar. The temperature and pH are unchanged, and no inhibitors are present. Which explanation best accounts for the higher plateau rate when enzyme concentration is doubled?
18. A eukaryotic pre-mRNA intron begins with the conserved 5′ splice site sequence GU in the RNA. A mutation in the DNA changes the transcribed intron start from GU to AU in the pre-mRNA, while all exons remain unchanged. The spliceosome requires the GU sequence to recognize and remove the intron efficiently. Which outcome is most likely for the mature mRNA and resulting protein?
19. In cats, an X-linked gene influences orange fur. Allele O produces orange fur and allele o produces black fur. Female heterozygotes ($X^OX^o$) show both colors in patches, while males show the color of their single X allele. A calico female ($X^OX^o$) is crossed with a black male ($X^oY$). Which outcome best predicts the phenotypes of the male kittens?
20. A red blood cell is placed in a solution containing 300 mOsm nonpenetrating solutes, matching its cytosol. Aquaporins are present in the membrane. When a small amount of penetrating urea is added to the external solution, urea diffuses into the cell through urea transporters. Water is permeable. Which effect is most likely shortly after urea enters the cell?

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