Chemical digestion and absorption

Last time

Before we start: last time, in Photosynthesis, the leaf and the starch test, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Too big to get in

A starch molecule is far too big to pass through the wall of your gut into your blood. Chemical digestion uses enzymes to break large insoluble molecules into small soluble ones. Then the small intestine, lined with millions of villi, absorbs them.

Chemical digestion

Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules, by enzymes. Small soluble molecules can be absorbed into the blood and used by cells. Breaking starch into sugars is chemical digestion. Chewing and churning are not.

Amylase

Amylase breaks down starch into maltose, a sugar. It is made in the salivary glands and the pancreas, and works in the mouth and duodenum. Maltase, on the membranes of the small intestine lining, then breaks maltose into glucose.

Proteases

Proteases break down protein into amino acids. Pepsin works in the stomach. Trypsin is made in the pancreas and works in the small intestine. The stomach makes hydrochloric acid, which kills bacteria in food and gives pepsin its optimum p H, about two.

Lipase

Lipase breaks down fats and oils into fatty acids and glycerol. It is made in the pancreas and works in the small intestine. Bile helps lipase by emulsifying the fat first.

Where the enzymes act

Here is the table to learn. Each enzyme, where it is made, where it acts, and the products. Notice how much happens in the small intestine.

Absorption

Absorption happens mainly in the ileum. Its wall is folded into millions of villi, each covered in microvilli. Inside each villus are blood capillaries and a lacteal.

How villi are adapted

Villi and microvilli give a very large surface area for absorption. The wall is only one cell thick, so the distance for diffusion is short. A rich network of capillaries carries absorbed food away, keeping a steep concentration gradient. The lacteal absorbs fatty acids and glycerol into the lymph.

Water absorption

Most water is absorbed in the small intestine. The colon absorbs much of what is left, so the faeces become more solid. Glucose, amino acids, mineral ions and vitamins are absorbed in the small intestine.