Practice Test 84 - AP Biology

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

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1. In a mountain rabbit population, ear length varies. After several colder winters, rabbits with shorter ears leave more surviving offspring than rabbits with longer ears. Over six generations, the mean ear length decreases and the allele associated with shorter ears rises in frequency from 0.40 to 0.68. Which outcome best describes the evolutionary change occurring in the population?
2. A nucleotide includes a phosphate group (P with multiple O atoms), a sugar (C, H, O), and a nitrogenous base (contains N). In a DNA double helix, the bases pair in the interior, while the sugar-phosphate backbone faces the surrounding aqueous environment. The phosphate groups carry negative charges due to electronegative oxygen atoms, making the backbone highly polar. The nitrogenous bases contain regions that can act as hydrogen-bond donors or acceptors, allowing specific base pairing. Which statement best describes why the phosphate-containing backbone is oriented outward in water?
3. Two forest plots were surveyed for tree species. Plot A had 12 species, each contributing about 8–10% of total stems. Plot B had 12 species, but one species contributed 70% of stems and the remaining 11 species were rare. Both plots were the same size and sampled with identical methods. Which explanation best accounts for the difference in biodiversity between plots?
4. A population of mosquitoes is exposed to insecticide-treated bed nets starting in 2011. A sodium-channel allele (kdr) confers reduced sensitivity to the insecticide. In 2011, $f(kdr)=0.08$ in the village population; in 2013, $f(kdr)=0.21$; in 2016, $f(kdr)=0.39$; in 2020, $f(kdr)=0.52$. Mosquitoes were sampled from the same set of households each year. Which observation best demonstrates continuing evolution in this mosquito population?
5. In a fish population, body depth varies. In a lake with gape-limited predators, fish with deeper bodies are harder to swallow and leave more offspring than shallow-bodied fish. Across five generations, the mean body depth increases and shallow-bodied individuals become less common. Which explanation best accounts for the observed evolutionary change?
6. A student compares two nucleic acids: one contains ribose, the other deoxyribose. Which feature best distinguishes their chemical stability at the molecular level?
7. A population of mice has two alleles for coat color, and both alleles are present at the start of a long-term study. When the habitat shifts from light sand to dark volcanic rock, owls capture light-coated mice more often than dark-coated mice. Over ten generations, the frequency of the dark-coat allele increases. Which statement best explains how variation affected evolutionary processes in this population?
8. A phospholipid bilayer has hydrophilic heads facing outward and hydrophobic tails forming the membrane's interior. Some transmembrane proteins act as channels with a hydrophilic pore. A toxin binds to the external surface of a specific channel protein and physically blocks its pore without altering the bilayer. After toxin exposure, a particular ion's movement across the membrane decreases, but movement of other ions and small nonpolar molecules is unchanged. Which feature best explains the specificity of the toxin's effect?
9. In a population of mice, coat thickness is influenced by multiple genes and shows continuous heritable variation. A region experiences colder winters for several decades. Over time, the distribution of coat thickness shifts so that thicker coats become more common in the population. Which statement best explains how variation contributed to this population-level evolutionary change?
10. Galactose and glucose are monosaccharides with the same molecular formula but differ in the orientation of a hydroxyl group on one carbon (a stereoisomer difference). A membrane transporter in intestinal epithelial cells binds glucose strongly but binds galactose weakly, even though both sugars are similar in size and polarity. The binding pocket forms multiple hydrogen bonds with specific hydroxyl positions on the sugar. Which feature best explains the lower binding of galactose to the transporter?
11. A researcher compares two membrane bilayers: one enriched in phospholipids with two saturated fatty acid tails and another enriched in phospholipids with one cis-unsaturated tail. Cis double bonds create kinks that prevent tight tail packing, reducing van der Waals interactions among tails. Saturated tails remain straight and align closely, increasing packing. At the same temperature, the bilayers differ in how easily phospholipids move past one another within the plane of the membrane. Which feature best explains the bilayer that shows greater lateral fluidity at the same temperature?
12. In a shorebird population, beak length varies continuously. During several years with abundant medium-sized prey, birds with very short or very long beaks produce fewer surviving offspring than birds with intermediate beaks. After eight generations, the mean beak length remains similar, but the proportion of birds with extreme beak lengths decreases and overall variation narrows. Which explanation best accounts for these changes in the population?
13. A polymer is synthesized from monomers that each contain a central carbon bonded to an amino group, a carboxyl group, a hydrogen, and a variable side chain. Polymerization occurs when the carboxyl group of one monomer reacts with the amino group of another, releasing water and forming a covalent bond. Changing the order of monomers changes the chemical properties of the polymer because side chains differ in polarity and charge. Which statement best connects monomer structure to polymer function at the molecular level?
14. An enzyme binds double-stranded DNA by interacting with the major groove, where chemical groups from base pairs are exposed. In a particular 12-bp region, the enzyme recognizes the pattern of hydrogen-bond donors and acceptors presented in the groove. A lab-made duplex has the same base sequence but is synthesized with the complementary strand reversed so that the phosphodiester backbone runs 5'→3' in the same direction as the original strand (parallel rather than antiparallel). The bases are still capable of hydrogen bonding, but the geometry of the grooves changes. Which statement best predicts the consequence for enzyme binding?
15. A lab compares gene expression in adipose cells and osteoblasts from the same adult human. Whole-genome sequencing shows identical DNA sequences. Adipose cells contain abundant mRNA for a lipid-storage protein, while osteoblasts contain abundant mRNA for a bone matrix protein; each cell type has low levels of the other transcript. Which explanation best accounts for how these cell types maintain different expression patterns?
16. A cell completes S phase and enters G2 with duplicated chromosomes. During prophase, chromosomes condense and the spindle begins to form. In prometaphase, the nuclear envelope breaks down and spindle microtubules attach to kinetochores. One chromosome fails to attach to spindle microtubules at either kinetochore, while all other chromosomes achieve bipolar attachment. The cell remains in a mitotic state with condensed chromosomes and does not begin chromatid separation. Which event is most likely being prevented in this cell?
17. A conservation team monitored two populations of a freshwater snail in separate but similar streams. Stream 1 averaged 5 snails per square meter, and Stream 2 averaged 40 snails per square meter. Over six weeks, water flow rate and temperature were similar, and no flooding occurred. Stream 2 showed a larger fraction of snails with shell damage and reduced egg production per adult compared with Stream 1. Which factor is most likely density dependent in this case?
18. A muscle fiber contracts when myosin binds actin and performs a power stroke. In vitro, myosin heads bind tightly to actin when ATP is absent. When ATP is added, myosin detaches from actin; after ATP is hydrolyzed to ADP + $P_i$, the myosin head changes conformation and can rebind actin in a high-energy state. The energy from ATP hydrolysis is stored temporarily in the altered protein conformation and then transferred to movement during the power stroke. Which process most directly explains how ATP hydrolysis contributes to mechanical work by myosin?
19. In a eukaryotic cell, some enzymes that modify newly synthesized proteins are embedded in the membrane of the rough endoplasmic reticulum (RER), where the lumen contains high concentrations of specific modifying molecules. When these enzymes are experimentally relocated to the cytosol, the modification rate of the same proteins decreases even though enzyme amount is unchanged. Which feature best explains how compartmentalization in the RER increases modification efficiency?
20. Cells A and B are the same shape and have identical membrane permeability to ions. Cell B is twice the linear dimension of Cell A. Both cells are placed in a solution where an ion enters passively down its gradient. After equal time, the ion concentration change per unit cytoplasmic volume is smaller in Cell B. Which explanation best accounts for the result?

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