Digestive, immune and reproductive systems sample question 1

Secretin, which stimulates bicarbonate secretion by the pancreas

Digestive, immune and reproductive systems sample question 2

Leydig cells, stimulating testosterone production

Digestive, immune and reproductive systems sample question 3

bicarbonate from the pancreas raises the pH of the chyme to about 6–7, far above the pH optimum of pepsin near 2.

MCAT · Biological and Biochemical Foundations · Biology

Digestive, immune and reproductive systems

Digestive, immune and reproductive systems is part of Biology in the MCAT Biological and Biochemical Foundations section. The questions below are written in the style of the real test, each with the answer and a worked explanation.

Question types you will see here include digestive enzymes, sites and pH, absorption and malabsorption, immune cell roles and antibody response and memory.

Common traps: mixing pepsin's acid optimum with pancreatic enzymes in the duodenum; treating bile as an enzyme; reversing the targets of secretin and CCK.

Sample questions

Question 1
Acidic chyme entering the duodenum triggers release of which hormone, and with what main effect?
- A.
  Secretin, which stimulates contraction of the gallbladder
- B.
  Gastrin, which stimulates bicarbonate secretion by the pancreas
- C.
  Cholecystokinin, which stimulates acid secretion by the stomach
- D.
  Secretin, which stimulates bicarbonate secretion by the pancreas
Answer: D. Secretin, which stimulates bicarbonate secretion by the pancreas
Low duodenal pH triggers S cells to release secretin, which causes pancreatic duct cells to secrete bicarbonate-rich fluid that neutralizes the acid. Gallbladder contraction is triggered mainly by cholecystokinin in response to fats and amino acids, not by secretin.
Question 2
In males, luteinizing hormone (LH) acts mainly on which cells, and with what effect?
- A.
  Sertoli cells, stimulating testosterone production
- B.
  Leydig cells, stimulating secretion of inhibin
- C.
  Spermatogonia, directly stimulating mitosis
- D.
  Leydig cells, stimulating testosterone production
Answer: D. Leydig cells, stimulating testosterone production
LH binds receptors on interstitial Leydig cells and stimulates testosterone synthesis. FSH is the hormone that acts on Sertoli cells, which support spermatogenesis and secrete inhibin.
Question 3
Chief cells secrete pepsinogen, which acid from parietal cells converts to pepsin in the stomach, yet pepsin does not go on digesting protein in the duodenum. The best explanation is that:
- A.
  bile salts convert active pepsin back into inactive pepsinogen as soon as the chyme enters the duodenum.
- B.
  bicarbonate from the pancreas raises the pH of the chyme to about 6–7, far above the pH optimum of pepsin near 2.
- C.
  parietal cells in the duodenal wall secrete an inhibitor that binds pepsin and prevents it from cleaving peptide bonds.
- D.
  chief cells occur only in the stomach, so no pepsin can be present in the contents of the duodenum.
Answer: B. bicarbonate from the pancreas raises the pH of the chyme to about 6–7, far above the pH optimum of pepsin near 2.
Bicarbonate from the pancreas and duodenal glands neutralizes the acid chyme, so the pH is far above the optimum of pepsin near 2 and pepsin is inactivated (B). Bile does not revert pepsin to pepsinogen (A), parietal cells make acid, not a pepsin inhibitor (C), and pepsin made in the stomach does travel with chyme, so cell location does not explain its inactivity (D).

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