MCAT Biology : Digestive Enzymes and Cell Types

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #75 : Digestive System

The parietal cells of the stomach are vital for both food digestion and as a defense mechanism against pathogens. When the parietal cells are not functioning properly, diseases such sepsis due to Clostridium difficile and malnutrition may occur. To keep the digestive system healthy, proper nutrition as well as a balanced diet is vital.

The parietal cells of the stomach secrete hydrochloric acid via the hormone gastrin. Gastrin is released when the stomach distends, via the presence of proteins and/or indirectly by the vagus nerve from the parasympathetic nervous system. Hydrochloric acid breaks down certain ingested food as well as activates certain zymogens for further digestion of macromolecules. The high acidity of the stomach due to the release of hydrochloric acid by parietal cells also destroys most pathogens. When the parietal cell is not functioning properly, opportunistic pathogens may create health problems.

Parietal cells also secrete intrinsic factor, a glycoprotein which binds to vitamin B12 to prevent destruction of the vitamin by the hydrochloric acid. Down the gastrointestinal tract, the vitamin is absorbed by the ileum of the small intestine. Vitamin B12 is essential for red blood cell production. A diet low in vitamin B12 may lead to anemia.

James wanted to learn how the vagus nerve is able to promote acid secretion in the stomach. He removed the stomach from an animal model and directly stimulated it. He administered the same voltage the vagus nerve typical sends out during an action potential. Which of the following results are expected? 

Possible Answers:

Low level of acid secretion 

Moderate level of acid secretion 

High level of acid secretion

The level of acid secretion also depends on the distention of the stomach

No acid secretion 

Correct answer:

No acid secretion 

Explanation:

The vagus nerve indirectly stimulates the parietal cells by releasing the hormone gastric releasing peptide (GRP). GRP then acts on the G cells, stimulating them to secrete gastrin. Gastrin then acts on the parietal cells to promote hydrochloric acid secretion. Since there is no stomach, this cascade cannot proceed, and thus no acid would be secreted.

Example Question #71 : Digestive System

Passage:

In a fictional universe, a new life form is discovered that appears to have a number of similarities to humans. Since its discovery by humans, it has been studied with x-ray imaging, magnetic resonance imaging (MRI), computed tomography imaging (CT), as well as with blood chemistries and laboratory studies. Based upon such analyses, scientists have found that both structurally and functionally, this fictional species, called Lorempis marengis, is highly similar to humans. It has structures that appear similar to lungs that are active during respiration. It has a structure that is highly active at all times, especially so in different parts during different activities (similar to the human brain). It also appears to have a digestive tract with a mouth, esophagus, stomach, small intestine, large intestine, and anus that is assumed to carry out the same functions at the cellular level as the parts of the human digestive tract. Scientists are now hoping to continue studying the organism at the cellular level to confirm their assumption that the cellular functions are indeed similar to those of human tissues.

In one cellular study performed on the organism described in the preceding passage, Lorempis marengis, scientists discover cells called "betts cells," that they believe serve a very similar function to chief cells in the human stomach. If this is accurate, which of the following is directly produced by betts cells?

Possible Answers:

Pepsinogen

Pepsin

Thrombin

Hydrochloric acid 

Gastrin

Correct answer:

Pepsinogen

Explanation:

This question tests your ability to incorporate information from a passage about a fictional organism with many structural and functional similarities to humans to answering a question about similar cellular functions between the two organisms. Ultimately, you are told in the question stem that the new organism contains cells within the stomach called "betts cells" that are very functionally similar to chief cells in the human stomach. Thus, the question is testing your knowledge of the function of human chief cells. The question specifically asks to choose the substance that is produced directly by betts cells, which essentially is asking which substance is produced directly by human chief cells. The only substance listed that is directly produced by the chief cells in the stomach is pepsinogen, which is a zymogen, meaning that it is a precursor to pepsin. Pepsinogen is activated to pepsin when it meets acid in the stomach. Because pepsinogen requires activation outside of the chief cells in order to become pepsin, pepsin is not directly produced by the chief cells, making pepsin an incorrect answer to the question.

Hydrochloric acid is produced by the parietal cells of the stomach, not chief cells.

Gastrin is produced by G cells in the stomach and duodenum, but not chief cells.

Thrombin is a clotting factor found in blood. It is not produce by chief cells.

 

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