# Enzymes: Action, Temperature and pH

> Biology · CIE IGCSE 0610
> Source: https://www.owlsprep.com/study/cie-0610-u3-enzymes-action-temperature-and-ph/

This guide covers Core and Extended CIE IGCSE Biology 0610 content for enzyme action, plus the effects of temperature and pH on enzyme activity, including graph interpretation and exam-standard worked examples.

**Prerequisites:** [Knowledge of protein structure as biological molecules](https://www.owlsprep.com/study/cie-0610-u3-biological-molecules/); Basic understanding of chemical reaction rates

## Learning objectives

- Define enzymes as protein biological catalysts for Core level
- Explain enzyme specificity using the lock and key hypothesis for Extended level
- Describe and explain the effect of temperature on enzyme activity
- Describe and explain the effect of pH on enzyme activity
- Interpret and draw graphs of enzyme activity against temperature and pH
- Distinguish between Core and Extended content for exam answers

## Core: Enzyme Definition and Basic Action

**Enzyme** — A biological catalyst made of protein that speeds up chemical reactions in living organisms, without being changed or used up in the reaction.

Enzymes control all metabolic reactions in cells, including digestion, respiration, and photosynthesis. Each enzyme is only able to speed up one specific reaction, as they are highly specific to their target substrate.

**Worked example:** State two defining features of an enzyme for a Core level 2-mark question.

1. 1. First feature: Enzymes are biological catalysts made of protein.
2. 2. Second feature: They speed up metabolic reactions without being used up or altered in the process.

> **Exam tip:** Core questions often ask for 2-3 features of enzymes. Memorise the definition word-for-word to guarantee full marks for these simple recall questions.

## Extended: Enzyme Action: Lock and Key Mechanism

**Lock and Key Hypothesis** — The model explaining enzyme specificity, where the shape of the enzyme's active site is exactly complementary to the shape of its specific substrate, so they fit together like a key into a lock.

When a substrate binds to the enzyme's active site, an enzyme-substrate complex forms. The reaction takes place at this complex, before the products are released. The unchanged enzyme is then free to bind another substrate molecule.

**Worked example:** Explain why amylase only breaks down starch but not sucrose, for a 3-mark Extended question.

1. 1. Amylase is an enzyme with an active site of a specific fixed shape.
2. 2. Starch has a shape complementary to amylase's active site, so it can bind to form an enzyme-substrate complex, allowing the reaction to occur.
3. 3. Sucrose has a different shape that does not fit amylase's active site, so no enzyme-substrate complex forms, and no reaction takes place.

> **warning**
>
> Extended learners must reference the enzyme-substrate complex and lock and key hypothesis for explanation questions about enzyme specificity to earn full marks.

## Effect of Temperature on Enzyme Activity

Enzyme activity changes with temperature, following a consistent pattern: <br>1. Below optimum temperature: Reaction rate increases as temperature rises, because molecules gain more kinetic energy, leading to more frequent collisions between enzymes and substrates, and more successful enzyme-substrate complexes. <br>2. At optimum temperature: Reaction rate is at its maximum. <br>3. Above optimum temperature: Reaction rate drops rapidly, as high heat breaks the bonds holding the enzyme's shape together, denaturing the active site so the substrate can no longer bind.

**Denaturation** — A permanent change in the shape of an enzyme's active site, caused by high temperature or extreme pH, that stops the enzyme from working.

**Worked example:** Describe and explain the shape of a graph showing the effect of temperature on human amylase, which has an optimum temperature of 37°C, for a 4-mark question.

1. 1. From 0°C to 37°C, the graph slopes upwards: increasing temperature gives amylase and starch molecules more kinetic energy, leading to more frequent collisions, so reaction rate increases.
2. 2. At 37°C, the graph peaks: this is the optimum temperature, where reaction rate is highest.
3. 3. Above 37°C, the graph slopes steeply downwards: high temperatures denature amylase's active site, so starch can no longer bind, and reaction rate falls to zero.

> **Exam tip:** Always explicitly label the optimum temperature on any enzyme activity graph you draw, and state clearly that denaturation is a permanent change.

## Effect of pH on Enzyme Activity

All enzymes have an optimum pH where they work fastest. Extreme pH values (either too acidic or too alkaline) break the bonds holding the enzyme's shape together, denaturing the active site and stopping the reaction. Different enzymes have different optimum pH values to match their environment: for example, stomach enzyme pepsin has an optimum pH of 2, while salivary amylase has an optimum pH of 7.

**Worked example:** Pepsin is a protein-digesting enzyme found in the stomach (pH 1-2). Explain why pepsin stops working when it moves into the small intestine, which has a pH of 7-8, for a 3-mark question.

1. 1. Pepsin's optimum pH is ~2, which matches the acidic environment of the stomach.
2. 2. The small intestine has a neutral/alkaline pH that is far from pepsin's optimum pH.
3. 3. This extreme pH denatures pepsin's active site, so it can no longer bind to protein substrate, and stops working.

**Check your understanding**

1. What term describes the permanent change to an enzyme's active site caused by extreme pH or high temperature?

   *Why:* Correct! Denaturation permanently alters the shape of the active site, so the enzyme can no longer bind its substrate.

2. What is the name of the temperature or pH where an enzyme works at its maximum rate?

   *Why:* Correct! The optimum value is where enzyme activity is highest.

## Common pitfalls

- **Wrong:** Saying enzymes are "killed" by high temperature
  - Why it fails: Enzymes are not living organisms, so they cannot be killed. This answer will lose marks in exams.
  - Correct: Use the term "denatured" to describe the permanent change to the enzyme's active site at high temperatures or extreme pH.
- **Wrong:** Claiming all enzymes have an optimum temperature of 37°C
  - Why it fails: 37°C is only the optimum for human enzymes. Enzymes from other organisms (e.g., thermophilic bacteria) have much higher optimum temperatures.
  - Correct: Always refer to the optimum temperature given in the question, unless you are specifically asked about human enzymes.
- **Wrong:** Extended learners explaining enzyme specificity without referencing the lock and key hypothesis
  - Why it fails: Extended level mark schemes require specific terminology including enzyme-substrate complex and lock and key model for full marks on explanation questions.
  - Correct: Always include the lock and key hypothesis and enzyme-substrate complex when explaining enzyme specificity for Extended questions.
- **Wrong:** Stating denaturation is reversible
  - Why it fails: For IGCSE level, denaturation is defined as a permanent change to the enzyme's active site that cannot be reversed.
  - Correct: Explicitly state that denaturation is permanent in all exam answers.
- **Wrong:** Confusing low temperature and high temperature effects on enzymes
  - Why it fails: Low temperatures only slow enzyme activity by reducing kinetic energy and collision rate, they do not denature enzymes. High temperatures above optimum cause denaturation.
  - Correct: Clearly distinguish the two effects: low temperature = reduced collisions, high temperature above optimum = permanent denaturation.

## Cheatsheet

| Concept | Core Content | Extended Only Content |
| --- | --- | --- |
| Enzyme definition | Protein biological catalyst, speeds up reactions, not used up | Same as Core, plus active site and enzyme-substrate complex |
| Enzyme specificity | Enzymes only work on one substrate | Explain via lock and key hypothesis, complementary active site shape |
| Temperature effect | Rate increases up to optimum, then falls as enzyme denatures | Same as Core, plus explain kinetic energy and collision rate changes |
| pH effect | Enzymes have optimum pH, extreme pH denatures enzymes | Same as Core, plus link pH change to active site shape alteration |
| Graph interpretation | Identify optimum value, describe trend above/below optimum | Explain trend using enzyme-substrate complex and denaturation |

## What's next

Now you have mastered enzyme action, temperature and pH effects, you are ready to progress to related CIE IGCSE Biology 0610 topics. Next, you will learn about enzyme practical investigations, which are frequently tested in Paper 3 (Core) and Paper 4 (Extended) practical and theory papers. You will also apply this enzyme knowledge to the human digestive system, where different enzymes break down specific food molecules in different pH environments. Make sure you practise drawing and interpreting enzyme activity graphs, as these appear in almost every exam series for both tiers. Extended learners should practise writing 3-4 mark explanations of enzyme specificity using the lock and key hypothesis to prepare for longer exam questions.

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