GCSE · Biology · Edexcel · Spec 1BI0

Enzymes as biological catalysts in synthesis and breakdown

Some of last night's dinner is being turned into you — pulled apart into tiny pieces, then rebuilt as your own molecules. The cutters and the builders? Enzymes.

Follow one bite of bread

Open each stage. Watch where the enzymes cut — and where they build.

Why enzymes?

Reason it through

Why can't living things manage without enzymes?

Link 1 of 4

First link · your turn

Your cells are busy with chemical reactions all the time — building molecules, breaking them down. At what temperature do those reactions have to happen?

Locked — reveal the link above first

What big molecules are made of

Big molecules, small units

Showing 1 layer: Large molecules

Explore

tap a part →

Three families of large molecule. Tap any part to see what it is.

Carbohydrates, proteins and lipids, drawn as their building blocks.

Predict, then check

Enzymes are fussy. But how fussy?

A protease meets a starch molecule. What happens?

Build or break?

Sort the reactions

Is each reaction building a large molecule, or breaking one down?

Is this synthesis (joining small units) or breakdown (splitting a large molecule)?

Still to sort

Synthesis (building) (0)

Small units joined into one large molecule.

Where the line is: Read the arrow: many small things becoming one big thing is synthesis.

Breakdown (splitting) (0)

One large molecule split into its small units.

Where the line is: Read the arrow: one big thing becoming many small things is breakdown.

6 of 6 still to sort.

Sort each card one way, then switch the rule and sort the same cards again.

Watch out: An enzyme's family name tells you what it breaks down: carbohydrases split carbohydrates, proteases split proteins, lipases split lipids.

What do you really think?

Sam's idea about enzymes

Sam says: "Enzymes are for digesting food. They chop it up, and then they're used up."

Which idea is closest to what you think right now?

An enzyme gets used up in the reaction, so each one can only work once.Enzymes are proteins that speed up reactions — building molecules as well as breaking them — and aren't used up.Enzymes only break things down — that's basically what digestion is.Enzymes are tiny living things that eat the food for us.

How sure are you?

I'm sureI think soI'm guessing

An enzyme gets used up in the reaction, so each one can only work once.

This is a common misconception

This is the one Sam got wrong most. An enzyme is a catalyst, and a catalyst isn't used up by the reaction it speeds up. The enzyme comes out of the reaction unchanged, so the same enzyme molecule can catalyse that reaction again and again.

Enzymes are proteins that speed up reactions — building molecules as well as breaking them — and aren't used up.

This matches the evidence

Yes. Protein? Tick. Speeds up reactions? Tick. Builds as well as breaks? Tick. Unchanged at the end? Tick. That's the whole picture Sam was missing.

Enzymes only break things down — that's basically what digestion is.

This is a common misconception

Digestion is the famous job, but it's only half the story. Enzymes in cells also join small units into large molecules — glucose into glycogen, amino acids into proteins. Building needs enzymes just as much as breaking does.

Enzymes are tiny living things that eat the food for us.

This is a common misconception

Enzymes aren't alive. They're protein molecules, made by living cells but not living themselves. They don't eat anything: they speed up a reaction, and it's the molecule they act on that changes.

Enzymes are protein catalysts. They build and they break — and they come out of every reaction unchanged.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Break it down to get it in. Build it up to make it you.

What you need to know

Enzymes are biological catalysts: proteins that speed up reactions in living things without being used up.
Enzymes let the reactions of life happen fast enough at body temperature.
Each enzyme's active site has a shape complementary to one particular substrate, so each reaction needs its own enzyme.
Carbohydrates, proteins and lipids are large molecules built from small units.
Enzymes catalyse synthesis (joining small units) and breakdown (splitting large molecules).

The big picture

Enzymes are biological catalysts: proteins that speed up the reactions of life without being used up, so reactions happen fast enough at body temperature. Each enzyme's active site fits one particular substrate. Enzymes break large molecules (carbohydrates, proteins and lipids) into their small units — which is how digestion makes food small enough to absorb — and they join small units into large molecules that cells need for growth, repair and energy storage.

Key points

1Starch → sugars: catalysed by carbohydrases, such as amylase.

2Protein → amino acids: catalysed by proteases.

3Lipid → fatty acids + glycerol: catalysed by lipases.

4Synthesis runs the other way: sugars into starch (plants) or glycogen (animals), amino acids into proteins, fatty acids and glycerol into lipids.

5Digestion matters because large, insoluble food molecules can't be absorbed; small, soluble units can, and cells then use them to build new large molecules.

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Worked example

Problem

Explain how fat eaten in a meal can end up as fat stored in the body. Use the words breakdown and synthesis in your answer.

⚠ Watch out

Pairing an enzyme with the wrong molecule — for example, saying amylase breaks down protein. Check the name: carbohydrases break down carbohydrates, proteases break down proteins and lipases break down lipids.

🧠

Memory hook

Break it down to get it in; build it up to make it you. And the name gives the target away: carbohydrASE → carbohydrate, protEASE → protein, lipASE → lipid.

✓

Check yourself

A meal contains protein. Explain how amino acids from that protein could become part of a new protein in a muscle cell that is being repaired. Use the words breakdown, absorbed and synthesis.

Flashcards

(12)

What is an enzyme?

A biological catalyst: a protein that speeds up reactions in living things without being used up.

Why do living things need enzymes?

Without them, the reactions of life would be too slow at body temperature. Enzymes speed them up enough to keep the organism alive.

Why does each enzyme catalyse only one reaction?

Its active site has a shape complementary to one particular substrate — only that substrate fits.

What are carbohydrates such as starch and glycogen made of?

Many sugar units, such as glucose, joined together.

What is a protein made of?

A chain of amino acids.

What is a lipid made of?

Glycerol joined to fatty acids.

Starch or glycogen: which is built by plants, and which by animals?

Plants build starch; animals build glycogen. Both are made from sugar units.

What happens in synthesis?

Enzymes join small units into large molecules, which cells use for growth, repair and energy storage.

What do carbohydrases do? Name one.

They break carbohydrates such as starch down into sugars. Amylase is one.

What do proteases do?

They break proteins down into amino acids.

What do lipases do?

They break lipids down into fatty acids and glycerol.

Why must large food molecules be broken down in digestion?

They're too big and insoluble to be absorbed. Small, soluble units can be absorbed into the blood and used by cells.

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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Sort the reactions

Sam's idea about enzymes

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