MCAT Biology › Digestive Enzymes and Cell Types
Which of the following cell types in the stomach is not necessary for digestive purposes?
Mucous cells
G cells
Parietal cells
Chief cells
In order to stimulate digestion in the stomach G cells secrete gastrin, which stimulates parietal cells. Pepsinogen is released by chief cells. The parietal cells release hydrochloric acid into the stomach lumen. The lowered acidity cleaves pepsinogen and creates pepsin, which begins to degrade proteins.
Mucous cells have the nondigestive role of lubricating the stomach lumen and protecting the stomach epithelium from degradation by the highly acidic gastric juices.
Which enzyme is involved in protein digestion?
Pepsin
Salivary amylase
Nucleases
Bile
Chyme
Digestion of proteins begins in the stomach. The low pH of the stomach converts pepsinogen into pepsin, which is then used to break down proteins.
The parietal cells of the stomach fundus are responsible for secreting which component of the gastric juices?
Hydrochloric acid
Mucous
Trypsin
Histamine
Parietal cells are located in the body of the stomach, and are responsible for secreting acid when stimulated by a variety of hormones, including gastrin (from G cells) and histamine (from enterochromaffin-like (ECL) cells). The release of acid allows food particles to be broken down in the stomach before they are transported through the antrum to the small intestine.
Chief cells produce pepsinogen, trypsin is released from the pancreas into the small intestine, and ECL cells release histamine to stimulate parietal cells to secrete acid.
Which of the following enzymes would function best at a pH of 2?
Pepsin
Carboxypeptidase
Chymotrypsin
Sucrase
Only one of the enzymes listed functions in the stomach, where high acidity results in a low pH. That enzyme is pepsin. Other enzymes listed function in the small intestine, and will be denatured by high acidity.
Which of the following secrete hydrochloric acid (HCl)?
Parietal cells
Chief cells
Pepsinogen cells
G cells
Pepsin cells
The correct answer is parietal cells. Parietal cells are responsible for HCl secretion in the stomach, which lowers the overall pH of the stomach. Chief cells secrete pepsinogen, which is converted to pepsin and is responsible for digesting proteins. G cells secrete gastrin, which stimulates parietal cells.
Which of the following stomach cells releases pepsinogen?
Chief cells
Parietal cells
Goblet cells
Pepsinogen is not released in the stomach; pepsin is released and converted to pepsinogen
In the stomach, food is broken down using a host of different cells and enzymatic processes. Chief cells release pepsinogen, making it the correct answer. Parietel cells, which release HCl, allow the conversion of pepsinogen (a zymogen) into pepsin. Pepsin actively digests proteins in the highly acidic environment of the stomach.
Goblet cells release mucus which lines the stomach for protection.
Which of the following is not a component of bile?
Proteases
Bilirubin
Cholesterol
Phospholipids
Bile is released from the gall bladder into the small intestine, where it helps with the emulsification of fats. Bile salts and phospholipids are crucial amphipathic compounds that allow lipids to associate while in the aqueous environment of the small intestine. Cholesterol promotes fluidity and prevents the lipids from becoming tightly packed. Bilirubin is a byproduct produced in the liver and secreted in bile; it is later reabsorbed in the ileum and returned to the liver in a cyclic pattern.
Proteases are secreted from the pancreas and assist in protein cleavage in the small intestine. They are not involved in bile or lipid digestion.
Which process of digestion incorrectly pairs the enzyme or compound with its function in the small intestine?
HCO3- lowers the pH in the intestine
Amylase digests starch
Peptidase digests protein
Lipase digests fat
During digestion, the small intestine plays a major role in absorption and digestion of compounds. Because the contents of the stomach are extremely acidic, bicarbonate serves to neutralize hydrochloric acid, and raises the pH to an optimal level at which the enzymes can break down food.
Amylase is present in both saliva and the small intestine and breaks down starches and carbohydrates. Peptidase disrupts peptide bonds and digests proteins into amino acids. Lipase acts to hydrolyze lipids.
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?
Pepsinogen
Pepsin
Hydrochloric acid
Gastrin
Thrombin
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.
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?
No acid secretion
High level of acid secretion
Low level of acid secretion
Moderate level of acid secretion
The level of acid secretion also depends on the distention of the stomach
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.