2023 AP AP Biology Practice Paper with Answers

In eukaryotic microorganisms, cellular zinc homeostasis is maintained through the regulated expression of specific transport proteins. When extracellular zinc (\(\text{Zn}^{2+}\)) concentrations are low, an intracellular signaling pathway activates the transcription factor ZapA. Activation occurs when the protein kinase KinZ phosphorylates ZapA, inducing a conformational change that allows it to bind to the promoter region of the ZUP1 gene and stimulate transcription. ZUP1 encodes a high-affinity plasma membrane zinc transporter, Zup1. When extracellular \(\text{Zn}^{2+}\) is abundant, zinc directly binds to ZapA, promoting its interaction with a co-repressor complex comprising Cbp1 and Cbp2, which prevents ZapA from activating transcription of ZUP1.

To investigate the individual contributions of these regulatory components, researchers generated two mutant strains from a wild-type yeast strain: a mutant lacking functional KinZ (\(kinZ^{mt}\)) and a mutant lacking functional Cbp1 (\(cbp1^{mt}\)). Cells of the wild-type and mutant strains were cultured in media containing either high or low levels of \(\text{Zn}^{2+}\). The researchers measured Zup1 transport activity and quantified relative ZUP1 mRNA levels relative to wild-type cells grown in high \(\text{Zn}^{2+}\) (Table 1).

**TABLE 1. Zup1 TRANSPORT ACTIVITY AND RELATIVE ZUP1 mRNA LEVELS IN WILD-TYPE AND MUTANT YEAST STRAINS UNDER HIGH- AND LOW-ZINC CONDITIONS**

| Yeast Strain | Mutation | Zup1 Activity in High-\(\text{Zn}^{2+}\) (\(\text{nmol}/\text{min}/\text{mg}\) protein \(\pm 2\text{SE}_{\bar{x}}\)) | Zup1 Activity in Low-\(\text{Zn}^{2+}\) (\(\text{nmol}/\text{min}/\text{mg}\) protein \(\pm 2\text{SE}_{\bar{x}}\)) | Relative ZUP1 mRNA in High-\(\text{Zn}^{2+}\) (\(\pm 2\text{SE}_{\bar{x}}\)) | Relative ZUP1 mRNA in Low-\(\text{Zn}^{2+}\) (\(\pm 2\text{SE}_{\bar{x}}\)) |
| :--- | :--- | :--- | :--- | :--- | :--- |
| Wild-type | None | \(1.2 \pm 0.2\) | \(25.2 \pm 1.4\) | \(1.0 \pm 0.1\) | \(18.0 \pm 1.5\) |
| \(kinZ^{mt}\) | Nonfunctional KinZ kinase | \(1.1 \pm 0.2\) | \(1.4 \pm 0.3\) | \(0.9 \pm 0.2\) | \(1.2 \pm 0.3\) |
| \(cbp1^{mt}\) | Nonfunctional Cbp1 co-repressor | \(23.8 \pm 1.6\) | \(24.9 \pm 1.5\) | \(17.5 \pm 1.3\) | \(18.2 \pm 1.4\) |

(a) Describe the mechanism by which the addition of a negatively charged phosphate group by a kinase can alter the activity of a transcription factor such as ZapA. Explain how a signaling cascade can amplify a cellular signal during signal transduction.

(b) Based on Table 1, identify ONE dependent variable measured in the researchers' experiment. Justify the researchers' decision to use the wild-type yeast strain as the genetic background for engineering the mutant strains. Justify the researchers' testing of single-gene mutant strains rather than strains with multiple simultaneous mutations.

(c) Based on the data in Table 1, identify the yeast strain and growth condition that resulted in the lowest relative ZUP1 mRNA level. Calculate the percent change in Zup1 transport activity in wild-type yeast cells grown in low-\(\text{Zn}^{2+}\) compared to wild-type yeast cells grown in high-\(\text{Zn}^{2+}\).

(d) In a subsequent experiment, researchers generated a yeast strain with a loss-of-function mutation in the gene encoding Cbp2. Predict the effect of this mutation on ZUP1 mRNA expression when the mutant yeast is grown in a high-\(\text{Zn}^{2+}\) environment. Provide reasoning to justify your prediction.

Light availability is a critical factor influencing the rate of photosynthesis and primary productivity in marine ecosystems. Researchers studying photoadaptation examined several species of marine macroalgae to evaluate how changes in irradiance affect cellular photosynthetic capacity. The researchers measured the rate of oxygen evolution (a proxy for the rate of photosynthesis) in six different algal species when cultured under both low-light (​50 \(\mu\text{mol}\cdot\text{m}^{-2}\cdot\text{s}^{-1}\)) and high-light (​400 \(\mu\text{mol}\cdot\text{m}^{-2}\cdot\text{s}^{-1}\)) conditions (Table 1).

TABLE 1. AVERAGE RATE OF OXYGEN EVOLUTION IN SIX MACROALGAL SPECIES EXPOSED TO LOW-LIGHT AND HIGH-LIGHT CONDITIONS

| Species | Oxygen Evolution at Low Light (\(\mu\text{mol } \text{O}_2\cdot\text{g}^{-1}\cdot\text{h}^{-1}\)) \(\pm 2\text{SE}_{\bar{x}}\) | Oxygen Evolution at High Light (\(\mu\text{mol } \text{O}_2\cdot\text{g}^{-1}\cdot\text{h}^{-1}\)) \(\pm 2\text{SE}_{\bar{x}}\) |
| :--- | :--- | :--- |
| 1 | \(4.2 \pm 0.4\) | \(8.6 \pm 0.5\) |
| 2 | \(2.1 \pm 0.3\) | \(4.8 \pm 0.4\) |
| 3 | \(3.0 \pm 0.5\) | \(6.5 \pm 0.6\) |
| 4 | \(1.5 \pm 0.2\) | \(3.2 \pm 0.3\) |
| 5 | \(5.0 \pm 0.6\) | \(5.4 \pm 0.7\) |
| 6 | \(2.8 \pm 0.4\) | \(7.1 \pm 0.5\) |

(a) Describe the role of the thylakoid membrane in the light-dependent reactions of photosynthesis.

(b) Using the grid provided, construct an appropriately labeled graph that represents the data in Table 1. Determine which species show(s) a statistically significant difference in the rate of oxygen evolution between low-light and high-light conditions.

(c) Based on the data in Table 1, describe the relationship between light irradiance and the rate of oxygen evolution in species 1 through 4 and species 6.

(d) In a particular species of macroalga, individual isolates typically display dark green pigmentation, but researchers isolated a pale green mutant strain with severely impaired photosynthetic electron transport. Genetic analysis indicates that this pale green phenotype results from a mutation in chloroplast DNA (cpDNA). In this algal species, chloroplasts are exclusively contributed by the maternal gamete (female strain) during sexual reproduction. Researchers crossed a female pale green mutant with a male wild-type (dark green) strain. Predict the phenotype(s) of the offspring produced from this cross. Provide reasoning to justify your prediction. Explain why genetically identical algal clones grown in environments with different light intensities can exhibit differences in photosynthetic enzyme concentration.

Mayfly nymphs of the genus Baetis are abundant primary consumers in temperate freshwater streams. These aquatic insects graze on benthic periphyton (algae and cyanobacteria) and serve as a vital energy source for secondary consumers such as juvenile trout and predatory stoneflies. Environmental disturbances, such as elevated stream temperatures and nitrate pollution resulting from agricultural runoff, threaten stream macroinvertebrate communities.

Baetis nymphs typically inhabit clear, cool streams where typical baseline conditions are approximately \(12^\circ\text{C}\) with low dissolved nitrate levels (approximately \(0.5\text{ mg/L}\)). To investigate how rising water temperatures and nutrient enrichment affect macroinvertebrate survival, researchers monitored cohorts of Baetis nymphs reared in stream mesocosms under combinations of three temperatures (\(12^\circ\text{C}\), \(16^\circ\text{C}\), and \(20^\circ\text{C}\)) and three dissolved nitrate concentrations (\(0.5\text{ mg/L}\), \(5.0\text{ mg/L}\), and \(15.0\text{ mg/L}\)) over a 60-day period.

(a) Describe the effect of high species diversity among primary consumers on the resilience of an aquatic ecosystem subjected to an environmental disturbance.

(b) Justify the researchers' choice of \(12^\circ\text{C}\) and \(0.5\text{ mg/L}\) dissolved nitrate as the baseline conditions in their experimental design.

(c) State an appropriate null hypothesis for this experiment.

(d) The researchers claim that a substantial decline in the Baetis nymph population will alter the biomass of benthic periphyton in the stream. Provide reasoning to support the researchers' claim.

During aerobic cellular respiration, glucose is catabolized through glycolysis in the cytosol, producing pyruvate, ATP, and NADH. In eukaryotic cells, pyruvate is transported into the mitochondrial matrix, where it is converted into acetyl-CoA by the pyruvate dehydrogenase (PDH) complex, releasing \(\text{CO}_2\) and reducing \(\text{NAD}^+\) to NADH. Acetyl-CoA then enters the citric acid cycle.

Activity of the PDH complex is tightly controlled. Pyruvate dehydrogenase kinase (PDK1) inhibits PDH by phosphorylating a specific serine residue on the enzyme. Conversely, pyruvate dehydrogenase phosphatase removes the phosphate group, reactivating PDH. In many proliferating cancer cells, PDK1 is constitutively active, leading to significant metabolic reprogramming.

(a) Describe the role of the citric acid cycle in generating molecules required for the electron transport chain.

(b) Explain why a high ratio of ATP to ADP in the mitochondrial matrix acts as a signal to activate PDK1 in non-cancerous cells.

(c) Researchers treat cancer cells overexpressing PDK1 with dichloroacetate (DCA), a small-molecule inhibitor that specifically blocks PDK1 activity. Predict the effect of DCA treatment on the rate of mitochondrial oxygen (\(\text{O}_2\)) consumption in these cells.

(d) Justify your prediction in part (c).

Carnivorous plants have independently evolved specialized structures and biochemical adaptations to capture and digest prey, allowing them to survive in nutrient-poor, waterlogged soils. Researchers investigating the evolutionary relationships among five related plant species (Species 1 through 5) within a single family constructed a phylogenetic tree using molecular sequencing data of chloroplast genes (Figure 1). An ancestral non-carnivorous plant, Species 0, was used as the outgroup.

```
Species 0 (Outgroup)
|
+------ Species 1
|
`------+
|------ Species 2
|
`------+
|------ Species 3
|
`------+
|------ Species 4
|
`------ Species 5
```
Figure 1. Molecular cladogram showing the evolutionary relationships among five plant species and an outgroup.

The researchers also cataloged the presence (+) or absence (−) of three key morphological and physiological traits related to carnivory across the five species (Table 1).

TABLE 1. PRESENCE (+) OR ABSENCE (−) OF SPECIALIZED TRAITS IN FIVE PLANT SPECIES
| Trait | Description | Species 1 | Species 2 | Species 3 | Species 4 | Species 5 |
| :--- | :--- | :---: | :---: | :---: | :---: | :---: |
| 1 | Secretion of acid phosphatase and endopeptidase enzymes | + | + | + | + | + |
| 2 | Stalked glandular trichomes producing sticky mucilage | − | + | + | + | + |
| 3 | Rapid leaf-closure movement (active snap-trap) | − | + | − | − | + |

(a) Describe how comparing homologous DNA sequences across different plant species allows scientists to determine their evolutionary relatedness.

(b) Based on the data in Table 1, identify the trait that represents an ancestral characteristic (symplesiomorphy) shared by all carnivorous species in this lineage.

(c) Based on Figure 1 and Table 1, identify the branch or lineage segment on the cladogram where Trait 2 most likely arose.

(d) Based on the cladogram in Figure 1, explain why the presence of Trait 3 in both Species 2 and Species 5 is more parsimoniously explained by convergent evolution rather than inheritance from a shared common ancestor without trait loss.

Heat-shock proteins (HSPs) function as molecular chaperones that stabilize and refold denatured proteins during cellular stress. Researchers investigated the regulation of several genes involved in cellular homeostasis during thermal stress in mammalian cells. Cultured cells grown at a baseline temperature of 37°C were either maintained at 37°C or exposed to elevated heat-shock conditions of 42°C.

The researchers quantified mRNA stability by measuring the mRNA half-life (the time required for 50% of the mRNA molecules to be degraded) and determined the relative protein synthesis rate for three heat-shock genes (HSP70, HSP90, and HSPA8) as well as a reference structural gene (ACTB). The results are summarized in Table 1.

TABLE 1. mRNA HALF-LIFE AND RELATIVE PROTEIN SYNTHESIS AT 37°C AND 42°C

| Gene | mRNA Half-Life at 37°C (hours) $\pm 2\text{SE}_\bar{x}$ | mRNA Half-Life at 42°C (hours) $\pm 2\text{SE}_\bar{x}$ | Relative Protein Synthesis at 37°C (arbitrary units) $\pm 2\text{SE}_\bar{x}$ | Relative Protein Synthesis at 42°C (arbitrary units) $\pm 2\text{SE}_\bar{x}$ |
| :--- | :---: | :---: | :---: | :---: |
| ACTB | $12.0 \pm 0.8$ | $3.2 \pm 0.4$ | $100 \pm 5$ | $22 \pm 4$ |
| HSP70 | $1.5 \pm 0.2$ | $8.5 \pm 0.6$ | $15 \pm 2$ | $240 \pm 12$ |
| HSP90 | $4.0 \pm 0.5$ | $4.2 \pm 0.4$ | $50 \pm 6$ | $55 \pm 5$ |
| HSPA8 | $6.0 \pm 0.5$ | $11.0 \pm 0.7$ | $40 \pm 4$ | $130 \pm 8$ |

(a) Based on the data in Table 1, identify the gene whose mRNA stability shows the greatest increase in response to thermal stress (42°C).

(b) Based on the data in Table 1, describe the relationship between mRNA half-life and relative protein synthesis for HSP70 when cells are transferred from 37°C to 42°C.

(c) The researchers claim that the post-transcriptional regulation of ACTB mRNA contributes to shifting the cell's translational machinery toward protective chaperone production during thermal stress. Use the data in Table 1 to support the researchers' claim.

(d) Explain how an increase in mRNA half-life allows a cell to increase the accumulation of a specific protein without altering the rate of transcription of that gene.