AP Biology : Excretory Physiology

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #181 : Systems Physiology

Nitrogenous wastes are produced from the degradation of DNA and proteins, and released from the body via the excretory system. Which of the following is the primary nitrogenous waste produced by aquatic animals?

Possible Answers:

Urea

Uric acid

Nitric acid

Ammonia

Correct answer:

Ammonia

Explanation:

Ammonia is highly water-soluble and can be toxic to cells at low concentrations due to presence of its ammonium ion, which can interfere with oxidative phosphorylation. Ammonia is small and can easily diffuse through cell membranes, making it easy to excrete. Essentially, there is a trade off of easy excretion and toxicity levels.

For aquatic animals, however, toxicity is negligible due to the large volume of water available to dilute ammonia wastes. The high solubility of ammonia wastes and the abundance of water solvent allow for the ammonia to be transported out of cells in an very dilute concentration, without harming the organism. This allows aquatic organisms to conserve energy, compared to terrestrial organisms that must convert ammonia wastes to other forms.

Amphibians and mammals convert ammonia to urea, which can be excreted with less water, but must still be relatively dilute. These animals release liquid wastes from the body, resulting in water loss, but conserve energy compared to organisms that continue to convert urea into uric acid. Birds and reptiles excrete uric acid, which requires very little water waste, but uses a larger amount of energy in conversion. This is beneficial to animals that may not have ready access to fresh water.

Example Question #21 : Excretory Physiology

Ammonia is a toxic form of excreted nitrogenous waste. What would be a key reason that an organism would preferentially excrete ammonia instead of urea or uric acid?

Possible Answers:

Less water is lost when ammonia is excreted

Ammonia is selectively reabsorbed better than urea or uric acid during excretion

Ammonia excretion benefits its environment and habitat

Energy is required to convert ammonia to a less toxic form

Correct answer:

Energy is required to convert ammonia to a less toxic form

Explanation:

There is a key trade-off between energy consumption and toxicity in the excretion of nitrogenous wastes. Ammonia is the simplest form of the waste product, and requires very little energy to produce; however, it is highly toxic and must be diluted to extremely low concentrations in order to be safe to the cells. Many aquatic animals excrete ammonia because of their proximity to water. Access to large amounts of water means that these organisms can safely excrete dilute ammonia without needing to use energy in conversions.

Terrestrial animals, with less access to water, excrete urea or uric acid. These wastes are derived from ammonia, but require an input of energy for the conversion. They are less toxic and require less water loss for dilution, making them ideal for animals that must conserve fluids. Uric acid is the least toxic of the nitrogenous wastes, but also requires the greatest energy investment.

Example Question #182 : Systems Physiology

The consumption of alcohol has what effect on the human urinary system?

Possible Answers:

It decreases the activity of antidiuretic hormone (ADH), increasing urine production

It completely blocks the flow of fluids through the kidney tubules

It has no effect on the human urinary system

It increases the activity of antidiuretic hormone (ADH), decreasing urine production

It results in dark, concentrated urine

Correct answer:

It decreases the activity of antidiuretic hormone (ADH), increasing urine production

Explanation:

Alcohol decreases the activity of antidiuretic hormone (vasopressin). A diuretic increases the production of urine and thus, inhibition of this antidiuretic hormone results in an increase in the production of highly diluted urine.

Alcohol does not block the flow of fluids through the kidney tubules.

Example Question #1 : Understanding Other Excretory Physiology

Which of the following would be filtered by the kidney and not found in urine?

Possible Answers:

Proteins

Sodium

Potassium

Urea

Water

Correct answer:

Proteins

Explanation:

Proteins would not be found in the urine because these molecules are too large to pass through the glomerulus of the nephron of the kidney. They would be filtered out and remain in the bloodstream. Meanwhile, all of the other compounds would be present in normal urine. 

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