Cell Membranes & Transport (College Board AP® Biology): Exam Questions

Estrogens are small hydrophobic lipid hormones that promote cell division and the development of reproductive structures in mammals. Estrogens passively diffuse across the plasma membrane and bind to their receptor proteins in the cytoplasm of target cells.

Describe one characteristic of the plasma membrane that allows estrogens to passively cross the membrane.

In a laboratory experiment, a researcher generates antibodies that bind to purified estrogen receptors extracted from cells. The researcher uses the antibodies in an attempt to treat estrogen-dependent cancers but finds that the treatment is ineffective. Explain the ineffectiveness of the antibodies for treating estrogen-dependent cancers.

Water potential (Ψ) is described by the following formulas.

Ψ = Ψ_p + Ψ_s

Ψ = −iCRT

Discuss the variables in both formulas and how they affect water potential.  

In an experiment, rats averaging 300 g of body mass were tested several times over a three-month period. For each individual rat, urine was collected over a three-hour period after ingestion of 10 mL of liquid (water, 1% ethyl alcohol solution, or 5% ethyl alcohol solution). The volume of urine was then measured, and the results were averaged for all individuals within each experimental group. The data are shown in the table below. 

THREE-HOUR URINE OUTPUT FOLLOWING FLUID INGESTION

Propose ONE scientific question that the researchers were most likely investigating with the experiment

State a hypothesis that could be tested to address the question you posed in part (a). 

Using the data in the table, describe the effect of ethyl alcohol on urine production. 

A student studying two different aquatic, plant-eating, unicellular protist species (species A and B) designed an experiment to investigate the ecological relationship between the two species (Table 1).

TABLE 1. EXPERIMENTAL TREATMENT GROUPS

In treatment group I, the student placed 10 individuals of species A into a container with liquid growth medium and 10 individuals of species B into a separate container with an equal amount of the same liquid growth medium. In treatment group II, the student placed 5 individuals of each species into a single container with the liquid growth medium. The student then maintained the containers under the same environmental conditions and recorded the number of individuals in each population at various time points. The results are shown in Table 2.

TABLE 2. NUMBER OF INDIVIDUALS IN EACH PROTIST POPULATION IN BOTH TREATMENT GROUPS

Many protists contain an organelle called a contractile vacuole that pumps water out of the cell. The student repeated the experiment using a growth medium with a lower solute concentration. Predict how the activity of the contractile vacuole will change under the new experimental conditions. Justify your prediction.

The petal color of the Mexican morning glory (Ipomoea tricolor) changes from red to blue, and the petal cells swell during flower opening. The pigment heavenly blue anthocyanin is found in the vacuole of petal cells. Petal color is determined by the pH of the vacuole. A model of a morning glory petal cell before and after flower opening is shown in Table 1.

Identify the cellular component in the model that is responsible for the increase in the pH of the vacuole during flower opening AND describe the component’s role in changing the pH of the vacuole.

 A researcher claims that the activation of the K⁺/H⁺ transport protein causes the vacuole to swell with water.

Provide reasoning to support the researcher’s claim.

Cells synthesize proteins in the rough endoplasmic reticulum (RER), package them in vesicles, and transport them to the Golgi apparatus for modification. These proteins are then sent to the plasma membrane for secretion or for use within the cell.

(i) Describe the process by which these proteins are secreted from the cell.

(ii) Explain how the transport of sodium out of the cell differs from the transport of large proteins as described in part (i).

Scientists used yeast cells to investigate the secretion of amylase, an enzyme involved in breaking down starch. They treated some yeast cells in group A with a drug that inhibits vesicle fusion with the plasma membrane and recorded amylase concentration in the culture medium over time. The results are shown in Table 1.

Scientists investigated amylase secretion in yeast using a drug that inhibits cell membrane fusion

Table 1: Amylase secretion in yeast cells with and without an exocytosis inhibitor

(i) Describe the data represented by Table 1.

(ii) Explain the effect that the drug has on amylase secretion.

On the template provided, construct a graph of the data shown in Table 1.

Calculate the average rate of amylase secretion between 0 and 50 minutes for each group.

Cell membranes consist of a phospholipid bilayer with embedded proteins and cholesterol that help regulate membrane stability. Temperature changes can alter membrane fluidity and permeability.

Scientists conducted an experiment to test how temperature affects cell membrane integrity. They used beetroot cells, which contain a red pigment (betacyanin) inside the vacuole. When membranes are damaged, betacyanin leaks into the surrounding solution, indicating increased permeability.

The researchers placed beetroot samples in water baths at different temperatures and measured the absorbance of the surrounding solution at 540 nm (indicating pigment leakage). The results are shown in Table 1.

Table 1: Effect of temperature on membrane permeability

(i) Describe the role of the phospholipid bilayer in maintaining cell membrane function.

(ii) Explain why extreme temperatures might disrupt this function.

Using the template provided, construct a graph representing the data in Table 1.

(i) Describe the trends in your graph.

(ii) Determine the point at which the membrane loses integrity.

(iii) Justify your answer to part (ii).

Cholesterol is a steroid present in the membrane of eukaryotic cells.

(i) Predict how a membrane with more cholesterol would respond to extreme temperatures.

(ii) Justify your prediction.

Water potential (Ψ\PsiΨ) determines the movement of water across membranes and is influenced by solute concentration and pressure potential. Scientists conducted an experiment to measure the water potential of plant cells by placing them in sucrose solutions of different molarities. The change in mass (%) of the plant cells was recorded over time. The scientists' results are shown in Figure 1.

(i) Describe the trend in plant cell mass when placed in solutions of increasing sucrose molarity.

(ii) Identify the water potential of the plant cells using data from Figure 1.

(iii) Justify your answer to part (ii) with evidence from Figure 1.

Describe what the data suggests about the water potential of plant cells relative to different sucrose solutions.

After identifying the sucrose molarity that was isotonic to the plant cells, the scientists conducted a second experiment. In this follow-up, the potato plant was pre-treated with a metabolic inhibitor that blocks ATP production. The cells from this plant were then placed in a 0.0 M sucrose solution. They measured the % mass change in both treated and untreated cells after 30 minutes.

(i) Predict how pre-treatment with a metabolic inhibitor would affect water uptake in cells placed in 0.0 M sucrose solution.

(ii) Justify your prediction based on the role of ATP in membrane transport.

(i) Identify one additional variable (besides ATP inhibition) that the scientists should control in this follow-up experiment.

(ii) Explain why controlling that variable is important for interpreting the results.

Active transport processes, such as the Na⁺/K⁺ ATPase and proton pumps, require ATP to move molecules against their concentration gradient. Temperature can impact enzyme activity, membrane fluidity, and overall cellular function.

To investigate the effect of temperature on active transport, scientists measured the rate of glucose uptake in animal cells at different temperatures. The cells were placed in a solution containing glucose and oxygen, ensuring sufficient ATP production for active transport.

Describe the role of ATP in the Na⁺/K⁺ ATPase cycle.

Active transport of molecules and or ions across the membrane requires a concentration gradient to be established and maintained. The Na⁺/K⁺ ATPase pump is an example of this mechanism.

Figure 1. below shows a visual representation of the Na⁺/K⁺ ATPase.

Explain how the movement of sodium and potassium ions in the model maintains the electrochemical gradient across the membrane.

Active transport of glucose in animal cells depends on membrane transport proteins, such as the sodium-glucose cotransporter (SGLT), which relies on ion gradients maintained by the Na⁺/K⁺ ATPase. These gradients are established using ATP produced through aerobic respiration. Because enzymes and membrane proteins are sensitive to temperature, the rate of active glucose uptake may vary depending on environmental conditions.

To investigate this, scientists placed identical groups of animal cells into solutions containing glucose and oxygen, then incubated them at different temperatures (5°C, 20°C, 37°C, and 45°C). After 10 minutes, the rate of glucose uptake (μmol/min) was measured.

Table 1: Glucose uptake at different temperatures

(i) Describe the relationship between temperature and glucose uptake based on the data.

(ii) Explain a cellular mechanism that accounts for increased uptake from 5°C to 37°C.

(iii) Identify a reason why glucose uptake decreases at 45°C despite a higher temperature.

(iv) Justify your answer based on principles of enzyme activity and membrane structure.

(i) Identify an experimental control that the researchers should include to confirm that glucose uptake is due to active transport rather than passive diffusion.

(ii) Explain how the results from this control would help support the conclusion.