AP® Biology Free-Response Questions Past Paper 2025 Solution

Ribosomes are cellular organelles responsible for protein synthesis, a process called translation. They read the sequence of codons on a messenger RNA (mRNA) molecule and, with the help of transfer RNA (tRNA), link amino acids together in the specific order dictated by the mRNA sequence to form a polypeptide chain.

The dependent variable in the experiments shown in Figure 1 is the relative amount of Sec62 and SR protein, measured as a percentage of the control.

The researchers included this control to ensure the specificity of the siRNA treatment. By measuring the amount of SR protein in cells treated with Sec62 siRNA, they could verify that the siRNA was only reducing the expression of its target protein (Sec62) and not having unintended, off-target effects on the expression of other proteins like SR. The data show that the Sec62 siRNA did not significantly affect the level of SR protein, confirming the specificity of the treatment.

Based on Figure 1, treating cells with Sec62 siRNA has little to no effect on the production of SR protein. The relative amount of SR protein in these cells remains at approximately 100% of the control level, and the error bars for the control and the Sec62 siRNA-treated SR protein levels overlap.

The independent variable in the second experiment is the type of siRNA (control, Sec62, or SR) added to the cells.

Based on Figure 2, Protein 2 is the only protein that showed an increase in percent transport to the ER when treated with Sec62 siRNA compared with the control.

The difference in the number of amino acids between the two proteins is 87.

The data in Figure 2 support the researchers' claim. The background information states that the Sec62 protein is required for the transport of proteins to the ER *after* their complete translation. The experiment shows that when cells are treated with Sec62 siRNA to reduce the amount of Sec62 protein, the transport of Protein 1 to the ER is significantly inhibited, dropping to approximately 50% of the control level. In contrast, the transport of Proteins 2 and 3 is not inhibited under these conditions. This indicates that Protein 1's transport is dependent on Sec62, and therefore it is the only one of the three proteins tested that uses the post-translational transport pathway.

The researchers' claim is justified because the sequence of amino acids at the amino terminus of a protein often acts as a signal peptide. The specific R-groups of these amino acids determine their chemical properties, such as hydrophobicity and charge. These properties dictate the three-dimensional shape of the signal peptide, which must be chemically and sterically compatible with the binding site of the protein channel (translocon) in the ER membrane. This interaction, based on molecular complementarity, is what allows the channel to recognize the protein and initiate its transport across the membrane. Therefore, the specific amino acids at the N-terminus are a critical factor in determining the efficiency of protein transport into the ER.

The portion of a receptor that is embedded inside the plasma membrane is composed of amino acids with nonpolar R-groups. This is because the interior of the plasma membrane consists of the hydrophobic, nonpolar fatty acid tails of phospholipids. For the receptor to be stable within the membrane, its transmembrane domain must also be hydrophobic and therefore nonpolar, allowing it to interact favorably with the surrounding lipid tails.

(Student response would be a hand-drawn or digitally created bar graph. A description is provided here.)

Graph Title: Effect of DopEcR Inhibition on Moth Activity

Description of the Constructed Graph: The graph is a bar chart with the "Treatment" (Saline Control and siRNA) on the x-axis and "Mean Percent Activity" on the y-axis. For each treatment, two bars are plotted: one for "General Activity" and one for "Oriented Activity," distinguished by a legend. The bars for the saline control are at 95% (general) and 60% (oriented). The bars for the siRNA treatment are at 90% (general) and 25% (oriented). Error bars representing ±2SE are included on each bar.

Oriented activity was the type of activity significantly affected by inhibiting the expression of the DopEcR receptor. While general activity showed only a small decrease from 95% to 90% with overlapping error bars, oriented activity dropped dramatically from 60% in the control group to 25% in the siRNA-treated group, and their respective error bar ranges do not overlap.

The treatment group in which the oriented activity was greater than 50% of the general activity was the male moths injected with saline (control solution).

The mutation will significantly decrease or eliminate oriented activity. According to Figure 1, the G protein must be activated by GTP displacing GDP to initiate the signaling pathway that leads to oriented activity. If the mutation prevents this displacement, the G protein will remain in its inactive state, the signaling cascade will be blocked, and the genes responsible for oriented activity will not be expressed, even when the moth is exposed to the pheromone.

The scientists' claim is supported by the experimental data in Table 1. The data show that inhibiting the DopEcR receptor via siRNA drastically reduces oriented activity (from 60% to 25%), which is defined as movement toward a high concentration of female pheromones. Since finding a female requires moving toward her pheromones, and this behavior is dependent on the DopEcR receptor, the described increase in DopEcR gene expression as moths reach sexual maturity would enhance this critical mate-finding behavior, thus increasing the likelihood of successful mating.

An inhibitor of the DopEcR pathway could protect crops by disrupting the moths' reproductive cycle. Moth larvae are a common cause of crop damage. The signaling pathway shown in Figure 1 is essential for male moths to locate females for mating. By applying a chemical that inhibits this pathway—for example, by blocking the DopEcR receptor so 20E cannot bind—male moths would be unable to exhibit the oriented activity needed to find mates. This would lead to a significant reduction in successful mating events, a lower reproductive rate for the moth population, and consequently, fewer larvae to damage the crops.

Removing a keystone species will have a disproportionately large and often negative effect on its ecosystem relative to its abundance. The removal can lead to a trophic cascade, causing a drastic change in the populations of other species and altering the overall structure and biodiversity of the ecosystem. For example, removing a keystone predator can lead to an explosion in the herbivore population, which can then overgraze and decimate the plant community.

The scientists should include a control group where buffelgrass is grown by itself, without the presence of any native grass species, under both the nondrought and drought watering conditions.

The null hypothesis is that the presence of native grass species will have no significant effect on the height or dry weight of buffelgrass.

The scientists' claim is justified because buffelgrass has two key advantages in a fire-prone environment. First, it is drought-tolerant and can survive wildfires, while many other plants, including young saguaro cacti, cannot. Second, its population growth rate is much faster than that of native grasses after a fire. This means that after a wildfire clears an area of competition, the surviving buffelgrass will be able to repopulate the area and outcompete the slow-growing native species for resources like water and space, thereby increasing its abundance and dominance in the ecosystem.

Genetic evidence that evolution is occurring is a change in the allele frequencies within a population's gene pool over successive generations. This can be measured directly through genetic sequencing of individuals over time.

The formation of a land barrier causes allopatric speciation. The barrier splits an ancestral population into two isolated populations, cutting off gene flow between them. The two populations are then subjected to different selective pressures in their new, distinct environments (e.g., cooler, nutrient-rich Pacific vs. warmer Caribbean). Over time, different mutations arise and are selected for in each population, leading to the accumulation of genetic differences. Eventually, these differences become so significant that the two populations can no longer interbreed, even if the barrier were removed, meaning they have diverged into two separate species.

The formation of the isthmus likely led to a decrease in resource availability for many native South American species.

My prediction is justified because the formation of the isthmus created a land bridge that allowed North American land animals to migrate south. These migrating species would have increased the overall number of animals in South America, leading to increased interspecific competition for resources such as food, water, and territory. Since the North American species occupied similar niches as the native South American species, this new competition would have reduced the amount of resources available for the original inhabitants.

An enzyme's active site has a unique three-dimensional shape and chemical environment that is highly specific and complementary to its substrate. This specificity, often described by the induced-fit model, ensures that only the correct substrate molecule can bind to the active site, allowing the enzyme to catalyze a specific reaction.

The pathway shows that amino acid B, the end product, regulates enzyme 1 through feedback inhibition. Amino acid B binds to an allosteric site on enzyme 1, changing the enzyme's conformation. This change alters the shape of the active site, making it less effective at binding to its substrate, amino acid A. This prevents the pathway from producing more of amino acid B when it is already abundant.

The product of the reaction catalyzed by enzyme 2 is Intermediate Y.

A significant change in pH outside of the optimal range for enzyme 3 could cause the enzyme to denature. This denaturation would alter the three-dimensional structure of the enzyme, specifically changing the shape of its active site. If the active site shape is changed, it will no longer be complementary to its substrate, intermediate Y, and will be unable to catalyze the conversion of intermediate Y to amino acid B, thus halting the production of the final product.

The fly genotype in which the average percent of metaphase cells with ALD-associated filaments is close to 12% is ald1/del.

Based on Figure 1B, flies with the genotype ald3/ald23 produce a visibly greater amount of ALD protein than flies with the genotype ald23/del, as indicated by the thicker protein band for the ald3/ald23 genotype.

The data from the WT/del genotype support the hypothesis. In Figure 1B, the WT/del flies produce about half as much ALD protein as the wild-type (WT/WT) flies. Despite this 50% reduction in protein level, Figure 1A shows that the percent of metaphase cells with ALD-associated filaments in WT/del flies (around 85%) is nearly identical to that of the wild-type flies (around 90%), and the error bars overlap. This indicates that a normal amount of filaments can be produced even with half the amount of ALD protein.

Even though both genotypes produce a similar, reduced amount of ALD protein, the phenotypes differ because the structure and function of the protein produced by the ald1 allele are different from the wild-type (WT) allele. The ald1 allele is a mutation, not a deletion, meaning it produces a full-length but likely altered or non-functional protein. In the WT/del flies, all the protein present is functional wild-type protein, which is sufficient for near-normal filament formation. In the ald1/del flies, the protein produced by the ald1 allele may interfere with the function of the remaining cellular machinery or may be simply non-functional, leading to a much lower percentage of filament formation, even with a similar amount of total protein present.