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10.2 What is the job of the digestive system and how does it do that job? (Higher)
Digestive SystemDigestionAbsorption

Digestive System

The functions of the digestive system are to digest and absorb foods.




Digestion is the breakdown of large, insoluble food molecules into small, soluble molecules so that they can be absorbed into the bloodstream.

Digestion can be mechanical, for example chewing in the mouth and churning in the stomach to break the food into smaller pieces.

Chemical digestion involves the use of special chemicals called enzymes.  These are catalysts which speed up the digestion of large molecules into small ones. 

The table summarises the types of enzymes and the products of digestion:

 Enzyme  Where it is produced  Food molecule it acts on    Products of digestion
 Amylase Salivary glands
Wall of small intestine 
 Starch (a carbohydrate)  Sugars
 Protease Stomach
Wall of small intestine
 Proteins  Amino-acids
 Lipase Pancreas
Wall of small intestine 
 Fats and oils (lipids)  Fatty acids and glycerol

Food is digested in the mouth, stomach and small intestine.

Hydrochloric acid is also produced in the stomach.  This provides the correct pH for the protease enzyme to work and kills most of the bacteria taken in with food.

The liver produces bile. 

This is stored in the gall bladder before being released into the small intestine. 

It neutralises the acid that was added to food in the stomach. 

This provides alkaline conditions for enzymes in the small intestine to work most efficiently. 

Bile also emulsifies fats (breaks large drops of fat into smaller droplets). 

This increases the surface area for lipase enzymes to act on.


Absorption of the small soluble products of digestion occurs in the small intestine.

The small intestine is adapted for this.

It is very long and its surface is covered in tiny finger-like projections called villi. 

These increase the surface area for absorption.

The lining is only one cell thick so food does not have far to diffuse into the blood.

Each villus has a good blood supply to collect the food.  It is then transported around the body.

Undigested food and liquid passes into the large intestine

Water is absorbed into the bloodstream here. 

The indigestible food which remains makes up the bulk of the faeces.

Faeces leave the body via the anus.

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