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3.5 Bacteria and enzymes in digestion

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3.5 Bacteria and enzymes in digestion

Summary: Have you seen TV adverts that say yoghurts and yoghurt drinks are good for your digestive system? They contain bacteria, which are important for digestion. Tags: science oxford-science student-book-7 biology Created: 2026-05-19 Last Updated: 2026-05-19

After this topic, you will be able to:

  • describe the role of bacteria in digestion
  • describe the role of an enzyme
  • describe the role of enzymes in carbohydrate, protein, and lipid digestion.

Think back

  1. What is a unicellular organism?
  2. What happens during digestion?
  3. Which food group does starch belong to?

Key idea Bacteria and enzymes are important for digestion in the body. Bacteria make vitamins that we need to stay healthy. Enzymes are biological Catalysts. This means they Speed up the process of digestion. Carbohydrase, protease, and lipase are enzymes.

Key words bacteria, enzyme, catalyst, carbohydrase, protease, lipase, bile

Have you seen TV adverts that say yoghurts and yoghurt drinks are good for your digestive system? They contain bacteria, which are important for digestion.

How do gut bacteria keep you healthy?

Your large intestine contains bacteria. Bacteria feed on the fibre in your diet. They make important vitamins such as vitamin K. (Vitamin K is a group of vitamins that the body needs for blood clotting, helping wounds to repair.) These vitamins are then absorbed into your body and help keep you healthy.

A State why bacteria are important in your digestive system.

Image

Figure 1Some foods, called probiotic foods, contain bacteria that help digestion in the body. Live yoghurts are a good source of these bacteria.

What is in digestive juices?

Your teeth begin the process of digestion. They break down food into smaller pieces. The digestive juices in your gut contain enzymes. Enzymes are special proteins that can break down large Molecules of nutrients into small molecules.

Large molecules in your food like starch, a type of carbohydrate, are made of lots of smaller molecules joined together. Enzymes cut these large molecules into the smaller molecules they are made from. Figure 2 shows this in a simple diagram.

An enzyme cuts a chain of molecules at the point where the smaller molecules connect leading to smaller molecules during digestion.

Figure 2Enzymes cut large molecules into smaller molecules.

Enzymes are biological catalysts – they speed up digestion. However, catalysts do not get used up.

B State the role of enzymes in digestion.

Different types of enzyme

Different types of enzyme break down different nutrients. The three main types of digestive enzymes are carbohydrase, protease, and lipase.

Carbohydrase

Carbohydrase is an enzyme that breaks down carbohydrates into sugar molecules (see Figure 3). Carbohydrates are digested in the mouth, stomach, and small intestine. Carbohydrase in your saliva breaks down the starch in bread into sugar.

Carbohydrase binds to a carbohydrate molecule, breaking it down into simpler sugar molecules during digestion.

Figure 3Starch is broken down into sugar molecules.

Protease

Protease is an enzyme that breaks down proteins into amino Acids (see Figure 4). Proteins are digested in the stomach and small intestine. Acid in the stomach helps digestion and kills harmful microorganisms in food.

Protease binds to a protein molecule, breaking it down into amino acid molecules during digestion.

Figure 4Protein is broken down into amino

Lipase

Lipase is an enzyme that breaks down lipids into fatty acids and glycerol (see Figure 5). Lipids are digested in the small intestine. It is helped by bile, a substance made in the liver. Bile breaks down lipids into small drops that are easier for the lipase enzymes to work on.

Lipase binds to lipid molecules, breaking them down into fatty acid molecules and glycerol molecules during digestion.

Figure 5Lipids are broken down into fatty acids and glycerol.

Literacy skills Describe the journey bread takes through your body and how it is digested. Present its journey as a flowchart. Hint: bread contains a lot of starch.

Stretch zone

  1. Copy and complete the sentences. Carbohydrates/proteins are broken down into sugars by the enzyme lipase/carbohydrase. Proteins are broken down into amino acids/lipase by the enzyme carbohydrase/protease. Lipids are broken down into lipase/fatty acids and glycerol by the enzyme lipase/carbohydrase.
  2. State where bacteria are found in the digestive system.Name the source of nutrition for bacteria in the human digestive system.Describe one benefit of having bacteria in the digestive system.

Stretch zone

  1. Explain the role of enzymes in digestion.

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