Chemical digestion begins in the mouth and the stomach

The enzyme amylase is secreted by the salivary glands and mixed with food as it is chewed. Amylase hydrolyzes the bonds between the glucose monomers that make up starch molecules. The action of amylase is what makes a chewed piece of bread or cracker taste slightly sweet if you hold it in your mouth long enough.

Most vertebrates can rapidly consume a large volume of food, but digesting that food is a slower process. The stomach stores the consumed food until it can be digested. The se

The larynx is pulled up and forward and is covered by the epiglottis. The esophageal sphincter relaxes. the bolus of food enters the esophagus.

cretions of the stomach kill microorganisms that are taken in with the food and begin the digestion of proteins.

The major enzyme produced by the stomach is an en-dopeptidase called pepsin. Pepsin is secreted as a zymogen called pepsinogen by cells in the gastric glands that are deep folds in the stomach lining (Figure 50.13). Other cells in the gastric glands produce hydrochloric acid, and still others near the openings of the gastric glands and throughout the stomach mucosa secrete mucus.

Hydrochloric acid (HCl) maintains the stomach fluid (the gastric juice) at a pH between 1 and 3. This extremely low pH activates the conversion of pepsinogen to pepsin, and provides the right pH for pepsin's enzymatic action.

(a) Stomach

Lower esophageal sphincter

Pyloric sphincter

Gastric glands

Gastric mucosa

(a) Stomach

Lower esophageal sphincter

Gastric glands

Gastric mucosa

Gastric Glands

Pyloric sphincter

50.13 The Stomach (a) The human stomach stores and breaks down ingested food. (b) Cells in the gastric glands secrete hydrochloric acid and pepsin. Both the gastric glands and the gastric mucosa secrete mucus that protects the stomach.

Acid-

secreting cell

Enzyme-secreting cell

In a process called autocatalysis, newly formed pepsin activates other pepsinogen molecules. Low pH converts pepsinogen to pepsin.

In a process called autocatalysis, newly formed pepsin activates other pepsinogen molecules. Low pH converts pepsinogen to pepsin.

Acid-

secreting cell

Enzyme-secreting cell

Pepsin Activated Pepsinogen
Gastric gland

Inactive zymogen: pepsinogen

Active enzyme: pepsin

Inactive zymogen: pepsinogen

Active enzyme: pepsin

Pepsin Activation

50.14 Activating a Zymogen Low pH in the stomach stimulates cleavage of a masking sequence of amino acids, transforming the zymogen pepsinogen into the active digestive enzyme pepsin. Pepsin itself also activates pepsinogen, through autocatalysis.

The pepsinogen molecule is activated by the cleavage of a masking sequence of 44 amino acids from the its N-termi-nal end, exposing the pepsin active site (Figure 50.14). The newly formed pepsin activates other pepsinogen molecules in a positive feedback process called autocatalysis. Hydrochloric acid also helps dissolve the intercellular substances holding the ingested tissues together. The breakdown of these tissues exposes more food surface area to the action of pepsin and, eventually, other digestive enzymes in the small intestine.

Mucus secreted by the stomach mucosa coats the walls of the stomach and protects them from being eroded and digested by HCl and pepsin. Sometimes, however, the walls of the stomach are exposed, and the resulting damage is called an ulcer. It was once thought that ulcers were caused by stress and oversecretion of digestive juices. However, it turns out that the basis for most ulcers is an infectious bacterium called Helicobacter pylori, which has the remarkable ability to live in the highly acidic environment of the stomach. Damage to the stomach lining initiated by the bacterial infection is made worse by HCl and pepsin.

Contractions of the smooth muscles in the walls of the stomach churn its contents, thoroughly mixing them with the stomach secretions. The acidic, fluid mixture of gastric juice and partly digested food in the stomach is called chyme. A few substances can be absorbed from the chyme across the stomach wall, including alcohol (hence its rapid effects), aspirin, and caffeine, but even these substances are absorbed in rather small quantities in the stomach.

Peristaltic contractions of the stomach walls push the chyme toward the bottom end of the stomach. These waves of peristalsis cause the pyloric sphincter to relax briefly so that little squirts of the chyme can enter the small intestine. In this manner, the human stomach empties itself gradually over a period of approximately 4 hours. This slow introduc tion of food into the small intestine enables the small intestine to work on a little material at a time.

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Responses

  • helena larnia
    How is pepsinogen activated to pepsin?
    8 years ago
  • leland
    How is pepsin activated and what is it activating enzyme called?
    8 years ago
  • Ville-Veik Lahtinen
    How is pepsin formed in the stomach?
    8 years ago
  • rudolph
    How is pepsin activated in the stomach?
    8 years ago
  • robert pratt
    Can pepsin molecules activate pepsinogen?
    8 years ago
  • Amaranto Cremonesi
    Does the gastric gland secret pepsinogen?
    8 years ago
  • h
    What enzyme turns pepsinogen into pepsin?
    8 years ago
  • gaudenzia
    What chemical digestion begins in the mouth?
    8 years ago
  • danyl gray
    What is the substance in the stomach that activates pepsin from pepsinogen?
    8 years ago
  • samira
    What substance will begin a chemical digestion in mouth?
    7 years ago
  • LINDA
    Which of these substances are actively absorbed by the stomach?
    7 years ago
  • sandra
    What substance can originally lower the pH OF THE CHYME?
    7 years ago
  • klaudia
    What activate pepsinogen into active pepsin in stomach?
    7 years ago
  • maria
    What substance originally lowered the pHof chyme?
    6 years ago
  • EMMA
    How pepsinogen is converted to pepsin?
    2 years ago

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