Enzymes and pH

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.13 — How pH affects enzyme function by altering the active site
• 2.14 — Practical: investigate the effect of pH on enzyme activity (bold B statement — Paper 2 only) (required practical)

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

You already know that heat wrecks an enzyme by destroying the shape of its active site. pH does exactly the same damage, and it does it by a completely different route — which is why the two are separate statements and why the two graphs do not look alike.

How an acid gets at a shape

1. An enzyme is a PROTEIN — a chain folded into one particular shape. That is the whole reason it has a shape for the acid to spoil.
2. The acid or alkali acts on the ENZYME, not on the substrate. The substrate is left exactly as it was.
3. The folded shape changes, and the active site — which is nothing more than a shape made by those folds — changes with it.
4. The substrate is no longer complementary, so no enzyme–substrate complex forms and the rate falls.
5. Push it far enough and the change is permanent: the enzyme is denatured, and putting the pH back does not put the shape back.
6. GOING FURTHER, and not needed for the exam: what holds the folds is attraction between charged parts of the chain, and pH is a measure of hydrogen ions, which carry charge — so changing the pH changes the pulls, and the chain settles somewhere else. You will never be asked for this at International GCSE. It is here because it answers the obvious question of how a liquid reaches a shape.

Think of it like a magnetic puzzle

Imagine a shape held together by small magnets rather than glue — the pieces sit exactly where the pulls balance. Now change the magnets: weaken some, reverse others. Nothing has been cut or broken, and yet the whole thing settles into a different arrangement, because the forces holding it were what decided the shape in the first place. That is what changing pH does to a protein. And notice what the picture also explains: there is no one arrangement that is 'correct'. There is only the arrangement you get at a given set of pulls — which is why different enzymes have different best pH values rather than all preferring neutral.

That last point is the one that catches people out, so it is worth stating plainly. There is no such thing as a pH that is good for enzymes. Each enzyme has its OWN optimum, and it is the pH of wherever that enzyme does its job. Protease in the stomach works at about pH 2, which is roughly the acidity of lemon juice, and would be destroyed at pH 7. Amylase in your mouth works at about pH 7 and is destroyed by the stomach acid it meets a few seconds after you swallow — which is precisely why starch digestion stops in the stomach and starts again in the small intestine, where pancreatic enzymes work at about pH 8.

Why the pH graph is a different shape from the temperature graph

1. The temperature graph rises gently and then falls off a cliff, because two different things are happening: faster collisions on the way up, destruction on the way down.
2. The pH graph is roughly symmetrical — it falls away on both sides of the optimum at a similar rate.
3. That is because only ONE thing is happening: moving away from the optimum in either direction distorts the active site, and it does not much matter which direction you move in.
4. There is no pH equivalent of 'more kinetic energy'. Nothing about a low pH speeds the molecules up, so there is no helpful effect to compete with the harmful one.
5. So if you are shown an unlabelled curve: sharply asymmetrical means temperature, roughly symmetrical means pH.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Optimum pH

The pH at which an enzyme's rate of reaction is at its maximum. It is a property of that particular enzyme, and differs from one enzyme to another.

Model answer [3 marks]

Explain why a change in pH reduces the rate of an enzyme-controlled reaction. [3]

A change in pH alters the charges on the groups holding the enzyme's folded shape, so the shape of the active site changes. The substrate is no longer complementary to the active site, so fewer enzyme–substrate complexes form and the rate falls.

Model answer [4 marks]

Amylase in the mouth works at about pH 7. Explain what happens to it when it is swallowed, and what this means for starch digestion. [4]

The stomach contents are about pH 2. At that pH the charges holding amylase in its folded shape are altered, so the active site changes shape and is no longer complementary to starch. The amylase is denatured, and this is not reversed when it passes on. Starch digestion therefore stops in the stomach and only resumes in the small intestine, where pancreatic amylase works at about pH 8.

The words a full-mark pH answer uses

Optimum pH

the pH at which that enzyme's rate is highest; different for each enzyme

Active site

the region of the enzyme the substrate binds to — a shape, made by the folding

Complementary

the substrate's shape fits the active site exactly; this is what is LOST when pH changes

Denatured

the active site has permanently changed shape, so the substrate can no longer bind

Buffer

a solution that holds the pH steady, so the recorded pH is the pH the reaction ran at

Model answer [3 marks]

In a pH investigation a student adds 2 cm³ of buffer to the first tube and 5 cm³ to the last. Explain why their results are invalid. [3]

The buffer adds volume, so the tube with 5 cm³ has its enzyme and starch more diluted than the tube with 2 cm³. A lower concentration of enzyme and substrate gives fewer enzyme–substrate complexes per second and so a lower rate. The difference in rate between the tubes is therefore caused by concentration as well as by pH, so it cannot be attributed to pH alone.

Optimum pH values — learn these three

• Salivary amylase — about pH 7
• Stomach protease (pepsin) — about pH 2
• Pancreatic enzymes in the small intestine — about pH 8

How temperature and pH each reach the active site

A. TEMPERATURE does two things at once: below the optimum it speeds molecules up and damages nothing, and above it the vibration breaks the bonds holding the fold. The graph is asymmetrical because those are two different processes.

B. pH does one thing only: it changes the charges holding the fold, in either direction from the optimum. There is no helpful effect to compete with it, so the graph is roughly symmetrical.

The two graphs side by side — how to tell an unlabelled one apart

Answering any 'explain the effect of pH' question — the four-step chain

1. pH changes the charges on the groups that hold the enzyme's folded shape.
2. The folding changes, so the shape of the ACTIVE SITE changes.
3. The substrate is no longer COMPLEMENTARY, so it cannot bind and no enzyme–substrate complex forms.
4. Fewer complexes per second means a lower rate — and if the change is large, the enzyme is denatured and the loss is permanent.

Required practical 2.14 — the effect of pH on enzyme activity (Paper 2 only)

1. Use the starch–amylase method from the temperature practical, but hold the temperature constant in a water bath at 37 °C.
2. Add a measured volume of buffer solution to set the pH of each tube: pH 3, 5, 7, 9 and 11.
3. The buffer holds the pH steady even though the reaction itself would otherwise shift it slightly.
4. Mix, start the stopwatch, and sample a drop onto iodine solution on a spotting tile every 10 s.
5. The end point is the first drop that stays orange-brown — all the starch has been digested.
6. Calculate rate = 1 ÷ time and plot rate against pH. The optimum is the pH directly beneath the peak.

Variables

Independent (changed) — pH, set by the buffer solution
Dependent (measured) — time for the starch to disappear, converted to rate = 1 ÷ time

Sources of error

Not this: 'Acid speeds enzymes up' — or its mirror image, 'acid destroys enzymes.'

This: Neither is a rule. Acid is the only condition stomach protease will work in and is what destroys salivary amylase. What matters is the distance from THAT enzyme's own optimum, in either direction.

Mark-losing trap. There is no pH that is good for enzymes. Each enzyme has its own optimum, and it matches where in the body it works.

Mark-losing trap. Say the active site is no longer COMPLEMENTARY. 'The enzyme stops working' describes without explaining and does not score.

Mark-losing trap. The pH graph is roughly SYMMETRICAL; the temperature graph is not. If you are shown an unlabelled curve, the shape tells you which it is.

Mark-losing trap. A buffer is not a detail. Without it the pH drifts during the run and the recorded value is not the one the reaction ran at.

Mark-losing trap. Add the same volume of buffer to every tube. Different volumes dilute the reactants differently and the result reads as a pH effect.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — What is the approximate optimum pH of the protease that works in the stomach?
2. Grade 7 · Explain [4 marks] — Explain why moving an enzyme away from its optimum pH reduces the rate of reaction.
3. Grade 8 · Explain [4 marks] — Starch digestion begins in the mouth, stops in the stomach, and starts again in the small intestine. Explain why, in terms of pH.
4. Grade 9 · Analyse [3 marks] — In a pH investigation, starch disappears after 50 s at pH 5 and after 20 s at pH 7. Calculate the rate at pH 7, as 1 ÷ time, and state how many times faster it is than at pH 5.
5. 9+ · Evaluate [4 marks] — A student claims that because stomach protease works best at pH 2 and amylase at pH 7, 'acid speeds up enzymes'. Select every statement that belongs in the reasoning showing this claim is wrong.

The people behind this science

Two ways into the same idea — the one who invented pH in order to study enzymes, and the one who said the whole thing is a matter of shape. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Søren Sørensen — the one who invented pH in order to study enzymes

The pH scale on the x-axis of your graph was invented in 1909 by Sørensen, at the Carlsberg brewery laboratory, and it was invented for exactly this problem: he was studying how enzymes behave in brewing, found that acidity changed everything, and had no way of stating how acidic anything was. This lesson is the reason the scale exists.

• “Why did you need a new scale before you could study enzymes properly?”
• “What did people say about acidity before there was a number for it?”
• “Why is the scale built on powers of ten rather than counting straight up?”
• “What did you see enzymes do as you changed the acidity?”
• “Why does a brewery need a chemist at all?”

Emil Fischer — the one who said the whole thing is a matter of shape

This lesson explains that pH ruins an enzyme by changing the shape of its active site. That only means anything because of Fischer's 1894 proposal that enzyme and substrate fit together like a lock and its key — the idea that makes a change of shape a catastrophe rather than a detail.

• “Why should a change of shape stop a reaction completely?”
• “How did you work out that shape was what mattered?”
• “What did you think held an enzyme in its shape?”
• “Could a lock be bent back into its old shape once distorted?”
• “Why does one enzyme ignore a substance almost identical to its own?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against Søren Sørensen in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course

• Previous lesson: Enzymes: Catalysts With a Shape
• Next lesson: Diffusion, Osmosis and Active Transport
• Levels of Organisation — From organelle to organism — the five-step ladder that every later topic in biology is built on.
• Comparing Cells, and Stem Cells — What plant and animal cells share and where they differ — then why a cell specialising is one of the most powerful ideas in biology.
• Carbohydrates, Proteins and Lipids — The three big food molecules — what they are made of, what they are built from, and how to test for them.
• Enzymes: Catalysts With a Shape — Why a protein with a hole in it speeds up a reaction, and why heat destroys that ability permanently.
• Diffusion, Osmosis and Active Transport — The three ways anything gets into or out of a cell — which need energy, which do not, and what makes each one fast or slow.
• All of Biology · Incandio Science