KS3 · Biology

Digestion and enzymes

However well you chew toast, it can't get into your blood as it is. Something else has to happen first, and it isn't more chewing.

Follow one bite of toast

Stage 1 of 5: A bite of toast. paused

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Take a bite of toast. It's packed with large molecules. Digestion is the process of breaking those large molecules down into small ones. At every stop, ask yourself one question: is this working on the food, or on its molecules?

Is it acting on the food, or on its molecules?

Mechanical or chemical?

Sort every action along the way

Pick an action, then decide: is it mechanical digestion or chemical digestion?

Still to sort

Mechanical digestion (0)

Physically breaks food down by acting on the food directly.

Where the line is: It changes the size of the pieces, not the molecules they're made of.

Chemical digestion (0)

Breaks food down using chemicals, including enzymes.

Where the line is: It breaks the large molecules themselves into small ones.

8 of 8 still to sort.

Some of these happen in the same place, so the place can't be your rule.

Why bother digesting?

Reason it through

Why can't your body just use food the way it arrives?

Link 1 of 4

First link · your turn

Food is a mixture of chemicals. What's in the mix?

Locked — reveal the link above first

Which enzyme breaks what?

Match each enzyme group to its food molecule

For each group of enzymes, tick every large food molecule you think it can break down. Then check the grid.

Carbohydrases (for example amylase)

Breaks down carbohydratesBreaks down proteinsBreaks down lipids (fats)

Proteases

Breaks down carbohydratesBreaks down proteinsBreaks down lipids (fats)

Lipases

Breaks down carbohydratesBreaks down proteinsBreaks down lipids (fats)

What happens to the enzyme?

Is an enzyme used up?

A protease molecule has just broken down one protein molecule. The products have moved away from it.

What happens to that protease now? Pick the idea closest to what you think.

It's used up, so the body has to make a new one for the next protein.It becomes part of the amino acids it has just made.It's unchanged, so it can break down another protein molecule.It can work again, but only on a different kind of food, like fat.

How sure are you?

I'm sureI think soI'm guessing

It's used up, so the body has to make a new one for the next protein.

This is a common misconception

It feels as if something that does work should get worn out, but an enzyme is a catalyst. A catalyst speeds up a reaction without being changed by it. The same protease is still there, ready to go again.

It becomes part of the amino acids it has just made.

This is a common misconception

The amino acids come from the protein, not from the enzyme. The enzyme stays unchanged in the reaction, and once the products have left it, it is free to act again.

It's unchanged, so it can break down another protein molecule.

This matches the evidence

Exactly. Enzymes are biological catalysts: large, complex molecules that speed up reactions. The enzyme remains unchanged, so it can be reused again and again.

It can work again, but only on a different kind of food, like fat.

This is a common misconception

You're right that it can work again, but it won't switch foods. Each enzyme is specific and can only speed up one reaction. A protease goes on breaking down proteins, and fats need lipases.

An enzyme is a biological catalyst: it speeds up a reaction, stays unchanged, and can be used again and again. It's not an ingredient that gets used up.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why food has to be broken into tiny molecules, and the enzymes that do the breaking.

What you need to know

Digestion breaks the large, insoluble molecules in food into small, soluble ones, which can then be absorbed into the blood.
Mechanical digestion acts on the food physically (teeth, tongue, stomach churning). Chemical digestion uses chemicals such as enzymes, found in saliva, the stomach and the small intestine, to break down its molecules.
Carbohydrases, proteases and lipases each break down one kind of food molecule. Enzymes are catalysts, so they stay unchanged and can be reused.

The big picture

Digestion breaks the large, insoluble molecules in food into small, soluble ones that can be absorbed into the blood. Chewing and churning break food up physically. Enzymes, which are specific and reusable biological catalysts, break down the molecules themselves.

Key points

1Digestion is breaking large molecules in food down into small ones.

2Starch (a large carbohydrate), proteins and fats are large and insoluble. Once digested, the nutrients are small and soluble, so they can be absorbed into the blood and used for energy, growth and repair.

3Mechanical digestion (chewing, biting, tearing, crushing, churning) acts on the food. Chemical digestion (enzymes) acts on its molecules.

4carbohydrate → sugars (carbohydrases, e.g. amylase); protein → amino acids (proteases); lipid → fatty acids + glycerol (lipases).

5Enzymes are biological catalysts. Each one is specific to one reaction, and it stays unchanged, so it can be used again and again.

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Worked example

Problem

Jay says: "If I chew my food for long enough, I won't need any enzymes." Is Jay right? Explain your answer.

⚠ Watch out

Saying that digestion just means 'breaking food into smaller pieces'. That is only mechanical digestion. The point of digestion is to break large, insoluble molecules into small, soluble ones, and that takes chemical digestion by enzymes.

🧠

Memory hook

Teeth make pieces; enzymes make molecules. And each -ase sounds like its food: carbohydrase and carbohydrate, protease and protein, lipase and lipid.

✓

Check yourself

Pick one food mentioned in this lesson. Without scrolling up, name the large molecule it contains, the enzyme group that breaks that molecule down and the small molecules that are made.

Flashcards

(14)

Define digestion.

Digestion is the process that turns the big molecules in food into little ones.

Why does food have to be digested?

Nutrients must be small and soluble to be absorbed into the blood. Once absorbed, they are used for energy, growth and repair.

What does 'insoluble' mean?

It does not dissolve in water. Starch (a large carbohydrate), proteins and fats are large, insoluble molecules.

Name the five kinds of chemical in the food mixture from this lesson.

Carbohydrates, proteins, fats, fibre and water.

What is mechanical digestion?

Physically breaking food down by acting on the food directly, for example chewing, biting, tearing, crushing and churning.

Where does mechanical digestion happen?

In several places, including the mouth (teeth cutting and grinding, the tongue rolling food around) and the stomach (churning).

What is chemical digestion, and where do its enzymes act?

Breaking food down using chemicals, including enzymes. Enzymes are in the saliva in the mouth, in the stomach and in the small intestine.

What is a catalyst?

A chemical that speeds up reactions. Enzymes are biological catalysts.

What happens to an enzyme after it has done its job?

It stays unchanged, so it is free to act again and can be reused again and again.

What does it mean that an enzyme is 'specific'?

It can only catalyse one chemical reaction.

Carbohydrases break down carbohydrates into…?

Sugars. For example, starch (in potatoes, pasta, rice and bread) is digested into glucose.

Proteases break down proteins into…?

Amino acids. Proteins are found in meat, fish, nuts and beans.

Lipases break down lipids (fats) into…?

Fatty acids and glycerol.

What ending do most enzyme names share?

-ase, as in amylase.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Follow one bite of toast

Sort every action along the way

Match each enzyme group to its food molecule

Is an enzyme used up?

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