Who this is for: Cambridge IGCSE Coordinated Science (0654) students who want enzymes — biological catalysts, lock-and-key action and factors affecting rate — to become reliable marks instead of vague “speed up reactions” answers.
What query it owns: how to understand and revise enzymes in Cambridge IGCSE Coordinated Science.
Why this is safe: this page owns the enzymes revision-guide angle, while Tutopiya’s Enzymes subtopic page owns the learning resource and the free Enzymes quiz owns the practice.

Enzymes are protein catalysts that speed up chemical reactions in living organisms without being used up. Cambridge IGCSE Coordinated Science (0654) tests whether you can define them precisely, explain the lock-and-key hypothesis, describe how temperature and pH affect activity, and use the term denaturation correctly. This guide covers the syllabus definitions, the factors that change rate, and the question types that appear every year.

Key takeaways

• Enzymes are biological catalysts made of protein — they speed up reactions but are not used up.
• The active site has a specific shape that fits the substrate (lock-and-key hypothesis).
• Rate increases with temperature up to an optimum, then denaturation causes a sharp drop.
• Each enzyme has an optimum pH; extreme pH also denatures enzymes.
• Exam answers must link shape change to active site when explaining denaturation.

What are enzymes in Cambridge IGCSE Coordinated Science?

Enzymes are proteins that act as biological catalysts, increasing the rate of chemical reactions in cells. The substrate binds to the enzyme’s active site, where the reaction takes place. The products are released and the enzyme can be reused. Examples include amylase (starch digestion), catalase (hydrogen peroxide breakdown), and pepsin (protein digestion in the stomach).

You can read the full explanation, worked examples and notes on Tutopiya’s Enzymes subtopic page before you attempt questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Biological catalyst	Speeds up reaction; not used up	”Define the term enzyme”
Active site	Region where substrate binds	”Explain how enzymes work”
Lock-and-key	Substrate shape fits active site	”Describe the lock-and-key hypothesis”
Denaturation	Active site shape destroyed	”Explain why activity decreases at high temperature”
Optimum	Best temperature or pH for activity	”State the effect of pH on enzyme activity”

Factors affecting the rate of enzyme activity

Factor	Effect on rate	Biological example
Temperature	Increases to optimum, then denaturation	Human enzymes ~37 °C
pH	Optimum pH varies per enzyme	Pepsin in acid stomach
Substrate concentration	More substrate → faster (until saturated)	Amylase and starch
Enzyme concentration	More enzyme → faster (if substrate available)	Digestive juices

Enzymes in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical enzymes stem
Define	Precise syllabus definition	”Define the term enzyme.”
Describe	What happens, step by step	”Describe the lock-and-key hypothesis.”
Explain	Cause and effect	”Explain why enzyme activity decreases above 40 °C.”
State	Short factual answer	”State the substance an enzyme acts on.”

Worked exam-style stems (how to answer the wording)

1. “Define the term enzyme.” An enzyme is a protein that acts as a biological catalyst, speeding up chemical reactions without being used up. Mark-scheme reward: protein, catalyst, speeds up, not used up.
2. “Describe the lock-and-key hypothesis of enzyme action.” The substrate has a complementary shape to the active site of the enzyme → substrate binds → reaction occurs → products released → enzyme unchanged. Reward: named active site + shape complementarity.
3. “Explain why enzyme activity decreases at very high temperatures.” High temperature denatures the enzyme → active site shape changes → substrate no longer fits → rate decreases. Reward: denaturation linked to active site shape.

When you can recognise the wording instantly, work through the Enzymes quiz and link back to Biological Molecules.

How enzymes connect to the rest of Coordinated Science biology

Enzymes are proteins — link to Biological Molecules. Digestive enzymes underpin the Alimentary Canal and Diet. The Cambridge IGCSE Coordinated Science resource hub links every Enzymes subtopic.

Common mistakes students make

• Defining enzymes as “speed up reactions” without protein or catalyst.
• Saying enzymes are used up in reactions (they are reused).
• Explaining high-temperature drop as “enzymes die” instead of denaturation and active site shape change.
• Confusing substrate (what the enzyme acts on) with product.
• Ignoring optimum pH when only temperature is mentioned in notes.

When you need more support

If enzyme questions keep costing marks, work through the Enzymes quiz, then get focused help from a Cambridge IGCSE Coordinated Science tutor.

Frequently asked questions

Is enzymes hard in Coordinated Science? The core ideas are straightforward, but marks are lost when students omit protein/catalyst in definitions or fail to link denaturation to the active site.

What is the lock-and-key hypothesis? The substrate has a shape complementary to the enzyme’s active site, like a key fitting a lock.

What is denaturation? A permanent change in the shape of the enzyme’s active site, usually caused by high temperature or extreme pH, so the substrate no longer fits.

How do I revise enzymes effectively? Learn the definition, sketch lock-and-key steps, practise graph explain questions for temperature, then take the Enzymes quiz.

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