Video Transcript
What is the main function of pepsin
in gastric digestion? (A) To break down polysaccharides
into small sugars. (B) To emulsify fats. (C) To break down proteins into
smaller fragments of polypeptides. Or (D) to initiate peristalsis in
the esophagus.
To correctly determine the function
of pepsin, let’s start by reviewing a key term we are given. You may remember that gastric
digestion refers to mechanical and chemical digestive processes that occur in the
stomach. It is one of the three major
categories based on the location of digestion, with the other two being buccal, the
mouth, and intestinal, the small intestines.
Because we can’t voluntarily
contract them, it’s easy to forget that the stomach wall has three layers of smooth
muscle tissue. However, these muscles are capable
of involuntary contractions. In fact, it is the rhythmic
contraction and relaxation of these smooth muscles, called peristalsis, that is
responsible for mechanical gastric digestion.
Chemical gastric digestion begins
when food mixes with stomach acid, also called gastric juice. Gastric juice is a strong acid, and
the low pH environment it creates stimulates a substance called pepsinogen to be
converted to its active enzymatic form. Pause the video and see if you can
figure out which enzyme pepsinogen is converted to. If you said pepsin, you are
correct.
Pepsin is a type of protease, which
is an enzyme that catalyzes the hydrolysis of proteins into smaller subunits. Pepsin breaks down proteins into
shorter polypeptide chains that will be further broken down in the small intestine
into single amino acids. Now that we understand the function
of pepsin, let’s return to our answer choices.
Polysaccharides are broken down
into smaller sugars by enzymes called carbohydrases. This occurs during buccal and
intestinal digestion, not during gastric digestion. So we can eliminate choice (A). Fats are emulsified by bile, a
substance released by the liver. So we can rule out choice (B) as
well. Finally, we can rule out answer
choice (D), because although food travels through the esophagus to reach the
stomach, the esophagus is not part of gastric digestion.
Therefore, the correct answer to
our question is (C). The function of pepsin in gastric
digestion is to break down proteins into smaller fragments of polypeptides.
