Digestive enzymes and absorption

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Key points

  • The human body digests carbohydrates, proteins and fats.
  • These carbohydrates, proteins and fats are digested into nutrients and absorbed into the body.
  • Enzymes are biological catalysts which speed up reactions including digestion.


Video - Types of enzyme

Can you answer these questions after watching the video?

1. What are the three types of enzymes mentioned in the video?

2. What's in the bread that the presenter eats? What is this broken down into?

  1. Carbohydrase, protease and lipase enzymes.
  1. Starch is in the bread that the presenter eats. This is then broken down into sugar.

Enzymes are protein molecules which act as to speed up reactions. They are not used-up in these reactions. Enzymes can be grouped into two types:

  • Those that break larger molecules apart (like digestive enzymes).
  • Those that join larger molecules together (like plants making glucose in photosynthesis).

Enzymes have a specific shape. This shape fits into the molecule it will break apart or join together. The part of the enzyme where the molecule fits is called the .

The molecules that enzymes act upon are called substrates. An enzyme is specific for its like a key is for its lock. This is called the .

An illustration of how a substrate collides with the active site of an enzyme.
The process of an enzyme breaking a substrate molecule apart.

If enzymes are heated too much or put into a higher or lower pH, their shape can change. The enzyme undertakes a process of denaturation, meaning it will not fit its substrate or substrates. The enzyme cannot speed up the reaction anymore.

Video - Turning starch into glucose

How to turn starch into glucose.

Can you answer this question based on the video?

What does the amylase enzyme break down into?

A sugar called glucose.

Location of enzymes in the digestive system

This table shows where the types of digestive enzyme are found.

EnzymeLocation in digestive system
CarbohydraseMouth, pancreas and small intestine
ProteaseStomach, pancreas and small intestine
LipasePancreas and small intestine
The key parts of the digestive system shown in a table
The key parts of the digestive system.


Chemical digestion occurs when enzymes digest food into nutrients.

a carbohydrase and starch molecule being digested into sugar molecules


Carbohydrates are digested in the mouth, stomach and small intestine. Carbohydrase enzymes break down starch into sugars.

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Digested food molecules are absorbed in the small intestine. This means that they pass through the wall of the small intestine and into the bloodstream. Once there, the digested food molecules are carried around the body to where they are needed.

Only small, soluble substances can pass across the wall of the small intestine. Large insoluble substances cannot pass through.

food in the small intestine on the left, the wall of the small intestine and the blood stream on the right

Food molecules in the small intestine are too large to pass across its wall and into the bloodstream

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Test your knowledge

Quiz - Digestive enzymes and absorption

Test questions

Write an answer to the questions below. Tap 'Show answer' to see the points you could have included.

1. Name the three digestive enzymes and their functions.

2. Describe the lock and key model for the action of enzymes.


  • Carbohydrase enzymes break down carbohydrates into sugars.

  • Protease enzymes break down protein into amino acids.

  • Lipase enzymes break down lipids into fatty acids and glycerol.


  • Enzymes are protein molecules which have a specific shape.

  • This fits together with the molecules they are going to break apart of join together.

  • This area of an enzyme is called an active site.

  • The molecules that enzymes act upon are called substrates.

  • An enzyme is specific for its substrate or substrates like a key is for its lock.