GCSE · Biology · AQA · Spec 8461

Enzymes

You run an investigation, change nothing but the pH, and the exact same enzyme takes far longer to finish the exact same job. What could possibly have changed?

Biology · Required practical

Investigating how pH affects an enzyme's rate of reaction

Starch and amylase mixture, sampled onto an iodine grid
Iodine test spot

Starch and amylase mixture, sampled onto an iodine grid. pH of the reaction mixture: pH 4 (acidic) pH. How does changing the pH of the reaction mixture affect how quickly amylase breaks down starch?

How does changing the pH of the reaction mixture affect how quickly amylase breaks down starch?

PredictBefore you run it — at which pH do you think amylase will break down starch fastest?

pH of the reaction mixture (pH)

Timer0.0 s

Amylase breaks down starch into sugars. Change only the pH of the reaction mixture — keep the temperature constant using a water bath or electric heater — then sample it with iodine solution every 30 seconds to see how fast the reaction goes. The reaction mixture is kept at a constant temperature in a water bath (or with an electric heater), so pH is the only thing that changes between runs. Every 30 seconds, a drop of the reaction mixture is spotted onto a well of iodine solution. While starch is still present the spot turns blue-black; once the starch has all been broken down, the spot stays iodine's own orange-brown colour.

Watch out: A LONGER time to fully digest the starch means a SLOWER reaction, not a faster one — don't read the results the wrong way round.

Enzyme specificity and denaturation

?

Reason it through

Why does a change in temperature or pH slow down or stop an enzyme-controlled reaction?

Link 1 of 4

First link · your turn

What decides which substrate an enzyme can act on?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Meet the parts: enzyme, active site, substrate and products

Meet the parts: enzyme, active site, substrate and products

Showing 1 layer: Enzyme and substrate

Explore

Tap a part to see what it does

Reading the rate straight off the curve

02.557.5100255075100Time (min)Starch broken down (%)
Time 2 min

Time: 2 min. Rate of reaction: 23.7 % per min

Gradient. The steepness of the tangent at this point is the rate of reaction at that moment — how fast starch is being broken down right now.

Move along the curve to see how the rate of reaction — the gradient — changes as the reaction goes on.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

One shape decision explains why enzymes are picky, and why heat or the wrong pH can stop them.

What you need to know

  • Enzymes are biological catalysts: they speed up reactions without being used up themselves.
  • An enzyme's active site has a shape that fits only one particular substrate — this is enzyme specificity.
  • Lock and key is a simplified model of this fit: the substrate's shape complements the active site.
  • Raising the temperature past an enzyme's optimum, or moving the pH away from its optimum, changes the shape of the active site.
  • A changed active site no longer fits its substrate: the enzyme has denatured, and the rate of reaction falls.
  • Every enzyme has an optimum temperature and an optimum pH at which its rate of reaction is fastest.
  • Enzymes catalyse the reactions of metabolism — the chemical reactions that take place in living organisms.

The big picture

Enzymes are biological catalysts whose active site has a shape that fits only one substrate. That same shape is the reason heat and the wrong pH can slow or stop a reaction: they change the active site's shape rather than 'killing' the enzyme.

Key points

1Enzymes are specific: each one's active site shape fits only one kind of substrate.
2Denaturation is a change in the enzyme's SHAPE, not its death — a denatured enzyme still exists as a molecule, it just no longer fits its substrate.
3Extremes of temperature and extremes of pH both slow or stop a reaction through the same mechanism: they change the active site's shape.
4An enzyme is not used up by the reaction it catalyses: it is unchanged afterwards and free to bind another substrate molecule.

Worked example

Problem

Amylase breaks down starch into simple sugars. A student says amylase should also be able to break down protein, since both are large molecules. Explain why the student is wrong.

⚠ Watch out

Do not say an enzyme 'dies' or is 'killed' by heat. An enzyme is a molecule, not a living thing. Heat changes its SHAPE — it denatures it — which is a different fact from killing it.

🧠

Memory hook

Change the shape, lose the fit, lose the reaction.

✓

Check yourself

Could you explain, in your own words, why a denatured enzyme still exists but no longer works?

Flashcards

(12)
What is an enzyme?
A biological catalyst — a protein that speeds up a reaction without being used up itself.
What is the active site?
The region of an enzyme with a shape that matches one particular substrate.
What does 'enzyme specificity' mean?
Each enzyme's active site fits only one substrate shape, so it only catalyses one reaction.
What is the lock-and-key model?
A simplified model of enzyme action: the substrate's shape complements — fits into — the active site, like a key fitting a lock.
What is an enzyme-substrate complex?
The temporary structure formed while a substrate is bound in the active site, during the reaction.
What happens to an enzyme after it has catalysed a reaction?
It is unchanged and free to bind another substrate molecule — it is not used up.
What does 'denatured' mean for an enzyme?
The enzyme's shape has changed (usually from heat or extreme pH), so its active site no longer fits its substrate.
Is a denatured enzyme dead?
No — an enzyme is a molecule, not a living thing. Denaturing changes its SHAPE; nothing has died.
What is an enzyme's optimum temperature?
The temperature at which its rate of reaction is fastest, before heat starts to denature it.
What is an enzyme's optimum pH?
The pH at which its rate of reaction is fastest; moving away from it in either direction slows the reaction.
Why does the rate of reaction fall at the wrong pH?
The pH change alters the shape of the active site, so fewer substrate molecules can bind and form the enzyme-substrate complex.
On a graph of product formed against time, how do you read off the rate of reaction at a point?
Find the gradient (steepness) of the curve at that point — the steeper the line, the faster the reaction.

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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