Alimentary canal and enzymes

MDCAT Biology · Biology · Digestion. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Digestion breaks large, insoluble food molecules into small, soluble ones that can be absorbed. It happens in stages along the alimentary canal: mouth, oesophagus, stomach, small intestine (duodenum, jejunum, ileum) and large intestine. Food is moved along by peristalsis, waves of muscle contraction.

Each region has its own pH and its own enzymes. The mouth is about neutral, the stomach is strongly acidic (about pH 2) and the small intestine is slightly alkaline. An enzyme works only near its own best pH, so salivary amylase stops in the stomach and pepsin stops in the duodenum.

Bile contains no enzyme. It emulsifies fats and helps to neutralise the acid.

Rules to remember

Salivary amylase (salivary glands) and pancreatic amylase (pancreas): starch → maltose
Pepsin (gastric glands; secreted as pepsinogen and activated by HCl): proteins → shorter polypeptides
Trypsin (pancreas; secreted as trypsinogen and activated by enterokinase): proteins and polypeptides → smaller peptides
Lipase (mainly pancreas): fats → fatty acids + glycerol
Intestinal wall: maltase gives glucose; sucrase gives glucose + fructose; lactase gives glucose + galactose; peptidases give amino acids
HCl kills microbes and activates pepsinogen; mucus protects the stomach wall; gastrin stimulates the release of gastric juice
Bile is made in the liver and stored in the gall bladder; the colon absorbs water and its bacteria make vitamin K

Common mistake

Bile is not an enzyme and does not digest fat chemically. It only breaks fat into tiny droplets (emulsification) so that lipase has a larger surface to work on.

Worked example

A tumour blocks the pancreatic duct, so no pancreatic juice reaches the duodenum. The digestion of which food class is affected most: starch, protein or fat?

Starch: salivary amylase has already started the work, and the intestinal wall still supplies maltase.
Protein: pepsin in the stomach and peptidases from the intestinal wall still act.
Fat: nearly all lipase comes from the pancreas, so very little fat is digested.

Answer: Fat digestion is affected most, because the pancreas is the main source of lipase.

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