Stomach structure and function

The stomach is a muscular, bag-like organ that stores food temporarily and begins the chemical digestion of proteins, while also churning food into a semi-liquid mixture.

A ring of muscle guards each end of the stomach, and the wall itself is folded so the organ can stretch to hold a whole meal. Questions on this structure usually test whether you can link the acidic gastric juice and the muscular wall to the digestion of protein, and whether you know why the stomach is not digested by its own juice.

Parts and functions

How structure suits function

The stomach is adapted to store a meal, digest protein and pass the food on gradually. Its muscular wall does the physical work, its glands supply the acidic juice for chemical digestion, and its mucus and sphincters protect the organ and control the flow of food.

Each adaptation links to one part of the stomach's job, and an examiner awards the mark for the link, not for the feature alone.

• Muscular wall with three layers of muscle, it contracts in different directions to churn food into a semi-liquid chyme and mix it evenly with gastric juice.
• Gastric glands throughout the lining, they secrete plenty of gastric juice, so the whole meal is exposed to acid and pepsin.
• Strongly acidic gastric juice, about pH 2, it provides the low pH at which pepsin works best and kills most microorganisms swallowed with food.
• A layer of mucus over the lining, it protects the stomach's own cells from being digested by its acid and pepsin.
• A sphincter at each end, food is held long enough for digestion, and chyme is then released slowly into the duodenum.
• A folded, elastic wall, the stomach can stretch to store a large meal and release it over a few hours.

Related processes

The stomach begins the chemical digestion of protein. Its gastric glands release pepsin, a protease, which breaks long protein molecules into shorter polypeptide chains.

Pepsin is secreted in an inactive form and is activated by the hydrochloric acid, which also gives the acidic pH of about 2 at which pepsin works fastest and kills most microorganisms swallowed with food.

While the enzyme works, the muscular wall churns the food by peristalsis into a semi-liquid paste called chyme, mixing it thoroughly with gastric juice. The stomach also stores food for a few hours, so a meal is released gradually rather than all at once.

Little absorption happens here; the pyloric sphincter releases chyme in small amounts into the duodenum, where bile and pancreatic and intestinal enzymes complete digestion and the products are absorbed at the villi of the small intestine.

Common labelling errors

Worked question

Question. A student is given a diagram of the human stomach and a note that its gastric juice is strongly acidic. (a) Name the enzyme in gastric juice that digests protein and state its product.

(b) Explain two ways in which hydrochloric acid helps digestion in the stomach. (c) Explain why the stomach is not digested by its own gastric juice.

(d) A medicine reduces acid production in the stomach. Suggest how this could slow the digestion of protein.

Model answer. (a) Pepsin; it digests protein into shorter polypeptides. (b) Hydrochloric acid activates pepsin (converting inactive pepsin to its active form) and provides the low pH, about 2, at which pepsin works best; it also kills most microorganisms swallowed with the food.

(c) The stomach lining is covered by a continuous layer of mucus, which protects the living cells from the acid and pepsin so they are not digested. (d) With less acid the pH rises above pepsin's optimum, so pepsin works more slowly, and less inactive pepsin is activated; both slow the digestion of protein into polypeptides.

Source:SRC-DSKP-EN

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