Enzymes & RP5 — AQA GCSE Biology

SPEC 4.2.2.1 • ORGANISATION — ENZYMES

Enzymes

Biological catalysts, the lock-and-key model, and the one mark-scheme word — denatured, never killed — that decides every enzyme question on Paper 1.

Two Letters Apart, One Mark Away

Students write the same phrase every year on enzyme questions: "the enzyme is killed by heat." Mark schemes explicitly reject it.

Mark scheme — directives

"Do not accept enzyme(s) killed."

(MS Nov 2020 Paper 1, spec 4.2.2.1 — repeated in MS Nov 2021 with "do not accept the amylase is killed / has died.")

The word that scores

The credit-worthy term is denatured. MS Nov 2020 also allows "destroyed / damaged", but "killed" and "died" are the explicitly banned terms. Default to denatured — it scores every time.

In the next 10 minutes you'll know the lock-and-key model, the effect of temperature and pH, and the exact three-step mark-scheme chain that wins denaturation marks.

Learning Objectives

By the end of this lesson, you'll be able to:

Objective 1

Describe enzymes as biological catalysts using the lock-and-key model.

Objective 2

Explain how temperature and pH affect enzyme activity — including the optimum, and what happens above or below it.

Objective 3

Define denaturation in mark-scheme wording — the three-step chain that scores Level 3.

SPEC REF 4.2.2.1 (ENZYMES) • PAPER 1 • HIGHER TIER

What Enzymes Are

ENZYME — DEF

A biological catalyst — a protein that speeds up a chemical reaction without being used up.

Three features the spec wants you to recognise:

1. Proteins

Made of amino acids folded into a specific 3D shape. The shape is what makes the enzyme work.

2. Not used up

The enzyme catalyses the reaction, then releases the products — and is ready to bind another substrate. Reusable.

3. Specific

Each enzyme catalyses one type of reaction. Different active site shape → different substrate.

Synoptic link — Lesson 2.02

You met three digestive enzymes already — amylase, protease, lipase. They're proteins; they're not used up; each one is specific to a different substrate (starch / proteins / lipids).

Synoptic link — Cell Biology (spec 4.1.3)

Enzymes underpin all of metabolism — the sum of every chemical reaction in cells. Every reaction that builds or breaks down biomolecules in a living thing depends on an enzyme. Without enzymes, metabolism would be too slow to sustain life.

The Lock-and-Key Model

The spec calls it a simplified model. It explains why enzymes are specific.

The mark-scheme word

The substrate is specific to the active site. Different enzymes have different active site shapes, so they catalyse different reactions. "Specific" is the mark-scheme word — use it.

The Reaction Cycle

Step by step, here's what happens at the active site:

Step 1

The substrate binds to the active site of the enzyme. Their shapes are complementary — only the right substrate fits.

Step 2

The enzyme catalyses the reaction — turning the substrate into one or more products.

Step 3

The products leave the active site. The enzyme is unchanged and ready to bind another substrate.

Two types of reactions

Enzymes catalyse both kinds of metabolic reaction:

— Building large molecules from smaller ones (e.g. protein synthesis from amino acids)

— Breaking large molecules into smaller ones (e.g. digestive enzymes)

Specificity — Why Each Enzyme Has Its Own Job

Different active site shape → different substrate it fits → different reaction it catalyses.

EnzymeActive site fits…Reaction catalysed

AmylaseStarch moleculesStarch → simple sugars / glucose

ProteaseProtein moleculesProtein → amino acids

LipaseLipid moleculesLipid → fatty acids + glycerol

Mark-scheme rule

An enzyme only catalyses one type of reaction. Amylase cannot break down protein. Protease cannot break down lipids. The shape of the active site determines what fits.

Effect of Temperature on Rate

Enzyme activity follows a bell curve with temperature. There's an optimum — the temperature at which the enzyme works fastest.

Below optimum

As temperature rises, molecules have more kinetic energy — substrate molecules collide with the active site more often. Rate increases.

Optimum (~37 °C)

For human enzymes, the optimum is around body temperature — about 37 °C. This is where rate is at its maximum.

Above the Optimum — Denaturation

Above the optimum, rate drops sharply. Past about 40 °C for human enzymes, activity falls rapidly to zero. The mechanism is denaturation.

NEVER write

"The enzyme is killed." Enzymes are proteins, not living things — they cannot be killed. Mark schemes explicitly reject this term. The credit-worthy word is denatured.

Effect of pH on Rate

Each enzyme has an optimum pH. Above or below it, the enzyme denatures and rate falls.

Mark-scheme pH values to memorise

Pepsin (stomach protease) — optimum pH 2 (acid environment, suits HCl in stomach). Amylase — optimum pH 7 (neutral, mouth and small intestine). Pancreatic enzymes (incl. lipase) — optimum pH 8 (slightly alkaline — bile neutralises stomach acid to create this environment).

Synoptic forward link — Lesson 2.05 (RP5)

The required practical on the effect of pH on amylase uses rate = 1 ÷ time to convert time-to-digest measurements into a rate. You'll plot a curve just like this one — and identify amylase's optimum pH from your own data.

The Denaturation Chain — Mark-Scheme Phrasing

This is the three-step chain examiners want. Memorise it word for word. It applies whenever temperature OR pH moves away from the optimum.

1

The enzyme is denatured. (The bonds holding the protein in its 3D shape are broken / disrupted.)

2

The active site changes shape. (This is the mark-scheme phrase — use it verbatim.)

3

The substrate no longer fits the active site. (So the enzyme can no longer catalyse the reaction → rate decreases.)

Use this exact chain

"Denatured → active site changes shape → substrate no longer fits." That's the canonical Level-3 answer. Three creditable points. Three marks. Every denaturation question.

PEE Worked Example — 3-Mark Explain

An enzyme has an optimum temperature of 37 °C.

Explain why the rate of reaction decreases when the temperature is increased above the optimum. 3 MARKS

Step 1 — Read the command word. "Explain" — give cause-and-effect, not just describe what happens. AO1 recall + reasoning.

Step 2 — Identify the pattern. 3-mark explain → three discrete points. The mark-scheme chain is the perfect template.

Step 3 — Write the answer.

Model answer

"Above the optimum temperature, the enzyme is denatured. This causes the active site to change shape. The substrate no longer fits the active site, so the enzyme can no longer catalyse the reaction and the rate decreases."

Step 4 — Match to the mark scheme.

Mark-scheme tick

✅ "denatured" — present (NOT "killed")

✅ "active site changes shape" — verbatim

✅ "substrate no longer fits" — verbatim

3 marks / 3 marks.

Examiner Alerts

NEVER "killed"

MS Nov 2020 Paper 1 directs examiners: "allow enzyme(s) (start to) denature / allow enzyme(s) destroyed / damaged / do not accept enzyme(s) killed ." MS Nov 2021 repeats the prohibition. So denatured is safest; destroyed / damaged also score; killed / died never do. Enzymes are proteins, not alive.

"Active site changes shape" — verbatim

Don't write "the enzyme breaks" or "the active site disappears." The mark-scheme phrase is exact: "the active site changes shape." Mark schemes also accept "the shape of the enzyme / active site changes" — same thing, different word order.

Wrong enzyme at wrong pH

If a question gives pH and asks which enzyme works best — match the pH to the optimum. pH 2 → pepsin; pH 7 → amylase; pH 8 → pancreatic enzymes. Saying "amylase works in the stomach" is wrong twice over (wrong enzyme AND wrong pH).

Lock-and-key is "simplified"

The spec explicitly calls lock-and-key a simplified model. If asked to evaluate the model, that's the word: "It's a simplified model — useful for showing specificity, but real enzymes flex slightly when binding (the induced-fit model)." At GCSE, lock-and-key is what mark schemes reward.

Common Errors

WrongRight

"the enzyme is killed by heat""the enzyme is denatured by heat"

"the enzyme breaks""the active site changes shape "

"the substrate can't get in""the substrate no longer fits the active site"

"amylase works in the stomach"" pepsin (protease) works in the stomach (pH 2); amylase works in the mouth / small intestine (pH 7)"

"enzymes are used up in the reaction""enzymes are NOT used up — they catalyse the reaction and can be reused"

"enzymes are alive""enzymes are proteins — biological catalysts — not living things"

Action

Screenshot this table. The "killed" and "enzymes alive" errors alone account for thousands of lost marks every exam season.

Practice Question

A scientist measured the rate of reaction of an unknown enzyme at different pH values. The rate was highest at pH 8 and decreased on either side of this value.

(a) State the optimum pH of this enzyme. 1 MARK

(b) Suggest where in the digestive system this enzyme is most likely to work. Give a reason. 2 MARKS

(c) Explain why the rate of reaction is lower at pH 4 than at pH 8. 3 MARKS

a

The optimum pH is pH 8. (1 mark — read directly from the question)

b

In the small intestine. (1 mark) Bile released into the small intestine is alkaline, so it neutralises stomach acid and creates conditions around pH 8 — matching this enzyme's optimum. (1 mark)

c

At pH 4, the enzyme is denatured. (1 mark) The active site changes shape. (1 mark) The substrate no longer fits, so the reaction is not catalysed and the rate is lower. (1 mark)

Total: 6/6. Pattern for (c): the canonical denaturation chain works for both temperature AND pH changes — denatured → active site changes shape → substrate no longer fits.

Recap — Three Takeaways

1. Lock-and-key model

Enzymes are biological catalysts — proteins, not used up. The substrate fits the active site like a key fits a lock. The shape determines specificity.

2. Temperature and pH have optima

Rate rises to the optimum, then falls sharply above it. Human enzyme optimum temperature ~37 °C. Pepsin pH 2; amylase pH 7; pancreatic enzymes pH 8.

3. The mark-scheme chain

Denatured → active site changes shape → substrate no longer fits. Three creditable points for every denaturation question. NEVER "killed."

You're set

Lock-and-key, optima, denaturation chain. You'll handle any enzyme question on Paper 1 — and you're ready for the food-test required practical coming next.

Next Up

Lesson 2.04 — Food Tests (Required Practical 4)

You know what amylase, protease and lipase do. Next: how to identify the food groups they act on.

The four required tests — iodine (starch), Benedict's (sugars), Biuret (protein), ethanol (lipids) — with exact positive-result colour changes the mark scheme demands.

SEE YOU IN LESSON 2.04.