Human digestive system

Biology · Human nutrition. 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

Most food molecules, such as starch, protein and fat, are large and insoluble. They cannot pass through the gut wall into the blood. Digestion breaks them into small, soluble molecules that can be absorbed.

Physical digestion breaks food into smaller pieces without changing the molecules: chewing by the teeth, churning by the stomach, and bile emulsifying fat. This gives a larger surface area for enzymes.

Chemical digestion uses enzymes to break large molecules into small ones. Each enzyme acts on one substrate and works best at its own pH. Food is pushed along the gut by peristalsis: waves of contraction of the circular and longitudinal muscles in the gut wall.

Rules to remember

Amylase (salivary glands, pancreas): starch → maltose. Maltase (lining of small intestine): maltose → glucose
Protease: protein → amino acids. Pepsin acts in the stomach (acid); trypsin from the pancreas acts in the duodenum (slightly alkaline)
Lipase (pancreas): lipids → fatty acids + glycerol
Hydrochloric acid in the stomach kills bacteria in food and gives the acid pH that pepsin needs
Bile: made in the liver, stored in the gall bladder; emulsifies fats into small droplets (physical, no enzyme) and neutralises stomach acid
Teeth: incisors cut, canines tear, premolars and molars crush and grind. Enamel outside, dentine under it, pulp with nerves and blood vessels in the centre, cement holding the root in place
Order: mouth → oesophagus → stomach → duodenum → ileum → colon → rectum → anus

Common mistake

Bile is not an enzyme. It does not break fat molecules down; it only splits large fat drops into small droplets so that lipase has a larger surface to work on.

Worked example

Small cubes of cooked egg white (protein) are placed in two tubes at 37 °C. Tube 1 contains pepsin at pH 2. Tube 2 contains pepsin at pH 8. In which tube do the cubes disappear, and why?

Egg white is protein, and pepsin is a protease, so pepsin can digest it to soluble amino acids.
Pepsin works in the stomach, where hydrochloric acid makes the contents acidic, so it works best at about pH 2.
At pH 8 the shape of the pepsin molecule is changed, so it does not fit the protein and very little is digested.

Answer: Tube 1 (pH 2). The cubes stay in tube 2.

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Nutrition in Animals

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