FRQ AP Bio Midterms

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Describe one characteristic of a membrane that requires a channel to be present for chloride ions to passively cross the membrane. Explain why the movement of chloride ions out of intestinal cells leads to water loss.

Chloride ions require a channel protein to cross the membrane because the hydrophobic interior of the membrane prevents their free diffusion. As chloride ions are transported out of intestinal cells, the space outside becomes hypertonic, so water moves out of the cells, leading to water loss.

Identify an independent variable in the experiment. Identify a negative control in the experiment.

An independent variable in the experiment is the presence of cholera toxin. A negative control in the experiment is sample I, which does not receive GTP or cholera toxin. 

Describe one characteristic of a membrane that requires a channel to be present for chloride ions to passively cross the membrane. Explain why the movement of chloride ions out of intestinal cells leads to water loss.

Chloride ions require a channel protein to cross the membrane because the hydrophobic interior of the membrane prevents their free diffusion. As chloride ions are transported out of intestinal cells, the space outside becomes hypertonic, so water moves out of the cells, leading to water loss.

Identify an independent variable in the experiment. Identify a negative control in the experiment.

An independent variable in the experiment is the presence of cholera toxin. A negative control in the experiment is sample I, which does not receive GTP or cholera toxin. 

Justify why the scientists included sample III as a control treatment in the experiment.

Sample III serves as a control to compare cAMP production with sample IV, which contains both cholera toxin and GTP. This allows scientists to evaluate whether cholera toxin’s effect on cAMP production requires GTP and the G protein pathway. 

Based on the data, describe the effect of cholera toxin on the synthesis of cAMP. Calculate the percent change in the rate of cAMP production due to the presence of cholera toxin in sample IV compared with sample II.

Cholera toxin increases cAMP production in the presence of GTP, but has no effect on the production of cAMP in the absence of GTP. 

The percent change in the rate of cAMP production due to the presence of cholera toxin in sample IV compared with sample II is 1,170% [(127−10)/10 = 11.7 × 100] 

A drug is designed to bind to cholera toxin and prevent the toxin from crossing the intestinal cell membrane. Scientists mix the drug with cholera toxin and then add this mixture and GTP to a sample of intestinal cell membranes. Predict the rate of cAMP production in pmol per mg adenylyl cyclase per min if the drug binds to all of the toxin. In a separate experiment, scientists engineer a mutant adenylyl cyclase that cannot be activated by Gsα. The scientists claim that cholera toxin will not cause excessive water loss from whole intestinal cells that contain the mutant adenylyl cyclase. Justify this claim.  

If the drug binds to all cholera toxin, the rate of cAMP production will remain at the baseline of 10 pmol per mg adenylyl cyclase per minute. Even with the toxin present, cAMP will not be produced because the mutant adenylyl cyclase cannot be activated by Gsa. Without activation, chloride ions will not be secreted. 

Describe the immediate effect of the neurotoxin on the number of action potentials in a postsynaptic neuron. Predict whether the maximum membrane potential of the postsynaptic neuron will increase, decrease, or stay the same.

Describe: The neurotoxin will increase the number of action potentials by causing more acetylcholine to bind to receptors, leading to a greater influx of sodium ions.

Predict: The maximum membrane potential will increase due to a stronger depolarization from the increased acetylcholine release.

The research proposes two models, A and B for using acetylcholinesterase (AChE), an enzyme that degrades acetylcholine, to prevent the effect of the neurotoxin. In model A, AChE is added to the synapse. In model B, AchE is added to the cytoplasm of the postsynaptic cell. Predict the effectiveness of each proposed model. Justify your predictions.

Model A: Effective, because acetylcholine is in the synapse and can break it down before it binds to receptors.

Model B: Not effective, because acetylcholine is not in the cytoplasm of the postsynaptic cell, so it cannot degrade it in the synapse.

Using Graph 1, determine whether corn strains I, II, and III differ in their average number of crossovers.

There is no (statistical) difference (in the average number of crossovers) between strains II and III. Strain I is higher/different (in the average number of crossovers) compared with strains II and III.

Based on Graph 1, describe the relationship between the average number of double-strand breaks and the average number of crossovers in the strains of corn analyzed in the experiment.

(In general) there is a direct correlation/positive relationship (between the number of double-strand breaks and the number of chromatid crossovers).

Crossing over (Figure 1A) creates physical connections that are required for proper separation of homologous chromosomes during meiosis. A diploid cell with four pairs of homologous chromosomes undergoes meiosis to produce four haploid cells. Crossing over occurs between only three of the pairs. Predict the number of chromosomes most likely present in each of the four haploid cells. Provide reasoning to justify your prediction.

Two haploid cells will have three chromosomes, and two haploid cells will have five chromosomes. During meiosis I, three homologous pairs separate normally, but one pair experiences nondisjunction and does not separate. As a result, one of the daughter cells will end up with an extra chromosome, while another will have one fewer. In meiosis II, the sister chromatids separate normally.