Digestion is the breakdown of food into smaller components, until they can be absorbed and assimilated into the body.
The parts of our body that are involved in the process of digestion, together, make up the digestive system.
Mouth
Digestion starts as soon as food enters your mouth. The saliva contains an enzyme called amylase which breaks starch into sugar.
Mechanical digestion also occurs in the mouth. The action of teeth breaking a large piece of food into smaller bits aids amylase action. By breaking large pieces of food into smaller ones, we increase the total surface area of the food pieces. Therefore, the amylase is able to efficiently break starch into sugars.
The small pieces of food that we swallow is called a bolus.
The food then enters the oesophagus. The oesophagus is a muscular tube and its function is to transport food and liquid from the mouth to the stomach.
No absorption takes place in the oesophagus.
Stomach
The stomach is a sac-like organ which is made up of strong muscular walls. It holds the food for some time. The food is churned and mixed in the stomach.
The stomach also secretes hydrochloric acid (HCl) and an inactivated enzyme called pepsinogen.
Function of HCl:
HCl is an acid which plays an important role in the immune system. It kills any harmful bacteria and parasites which are ingested with the food.
It also activates pepsinogen into pepsin.
Function of Pepsinogen
Pepsinogen is activated by the HCl into pepsin. Pepsin is a protease enzyme which breaks large protein molecules into amino acids.
Pancreas
The pancreas secretes lipase, carbohydrase and protease enzymes into the small intestine. They continue the digestion process.
Small Intestine
In the small intestine, bile from the gall bladder and enzymes from the pancreas come to continue the digestion of food.
Bile contains bile acids which are critical for digestion and absorption of fats and fat-soluble vitamins .
The digested molecules of food are then absorbed by the villi present in the small intestine.
Villi are finger-like projections which increase the surface area for the absorption of digested food.
The food then enters the large intestine,
Large Intestine
Faeces or food waste from the digestive process travels across the large intestine by contractions of the tube.
Faeces is then stored in the last part of the large intestine, which is the rectum.
The faeces is stored by closing the muscular sphincters.
Anus
The brain decides whether the rectal contents can be released or not. If they can, the sphincters relax and the rectum contracts, expelling its contents.
Food is egested from the body via the anus.
| Enzyme | Action | Location |
| Carbohydrase | Starch to Sugars | Salivary glands, Pancreas |
| Protease | Proteins to Amino acids | Stomach, Pancreas |
| Lipase | Fats to fatty acids and glycerol | Pancreas |
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