Physiology, Pepsin

Food digestion is the breakdown of large food particles into smaller absorbable nutrients needed for energy production, growth, and cellular repair. It begins with ingestion and ends with defecation. Digestion takes place in the gastrointestinal tract in two principal forms: mechanical and chemical. Mechanical digestion is the physical degradation of large food particles into smaller pieces that digestive enzymes can access through chemical digestion. Chemical digestion is the enzymatic cleavage of proteins, carbohydrates, and fats into tiny amino acids, sugars, and fatty acids. Once food enters the mouth, it mixes with saliva and gets chewed through the process of mastication. Saliva is rich in mucus and salivary enzymes, and together, with the effects of mastication, it creates a mass called a food bolus. The food bolus then travels down the esophagus via wave-like muscular contractions, called peristalsis, before it reaches the stomach. The stomach plays a critical role in the early stages of food digestion. Asides from squeezing and churning the food bolus, it also secretes a mixture of compounds, collectively known as "gastric juice." Gastric juice comprises water, mucus, hydrochloric acid, pepsin, and intrinsic factor. Of these five components, pepsin is the principal enzyme involved in protein digestion. It breaks down proteins into smaller peptides and amino acids that can be easily absorbed in the small intestine. Specific cells within the gastric lining, known as chief cells, release pepsin in an inactive form, or zymogen form, called pepsinogen. By doing so, the stomach prevents the auto-digestion of protective proteins in the lining of the digestive tract. Since chief cells release pepsin as a zymogen, activation by an acidic environment is necessary. Hydrochloric acid (HCl), another component of the gastric juice, plays a crucial role in creating the pH required for pepsin activity. Parietal cells produce HCl by secreting hydrogen and chloride ions. When pepsinogen and hydrochloric acid exist together in the gastric juice, pepsin takes its active form. Through the actions of pepsin and the squeezing properties of the stomach, the food bolus enters the intestines as a liquidy mixture of partially digested food particles, called chyme.