Digestion

A series of free Science Lessons for 7th Grade and 8th Grade, KS3 and Checkpoint Science in preparation for GCSE and IGCSE Science.

Human digestive system
The food we eat has to be broken down into other substances that our bodies can use. This is called digestion. Without digestion, we could not absorb food into our bodies and use it.
Digestion happens in the digestive system, which begins at the mouth and ends at the anus.
After we swallow, our food passes through these organs in turn:

• oesophagus or gullet
• stomach
• small intestine
• large intestine

Stages of digestion
Different things happen to food as it passes through the digestive system:

• food is digested in the mouth, stomach and small intestine
• digested food is absorbed into the bloodstream in the small intestine
• excess water is absorbed back into the body in the large intestine
• any undigested food passes out of the anus as faeces when we go to the toilet

Digestive Enzymes
Enzymes are not living things. They are just special proteins that can break large molecules into small molecules.
Different types of enzymes can break down different nutrients:

• amylase and other carbohydrase enzymes break down starch into sugar
• protease enzymes break down proteins into amino acids
• lipase enzymes break down lipids (fats and oils) into fatty acids and glycerol

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