---
title: "Enzyme Action and the Active Site"
description: "An enzyme speeds up one reaction because its active site fits one substrate shape. Covers the lock and key model, specificity, and why an enzyme is not used up by the reaction it runs."
canonical: https://lightmysky.com/learn/science/enzyme-action-and-the-active-site-mt__ozLTPNN0H
source: https://lightmysky.com/learn/science/enzyme-action-and-the-active-site-mt__ozLTPNN0H.md
retrieved: 2026-09-12
---

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# Enzyme Action and the Active Site

An enzyme speeds up one reaction because its active site fits one substrate shape. Covers the lock and key model, specificity, and why an enzyme is not used up by the reaction it runs.

Subject: Science · Area: Organisms & Life Processes · Ages 14 to 16
Page: https://lightmysky.com/learn/science/enzyme-action-and-the-active-site-mt__ozLTPNN0H

## Ready when they can

- Explains specificity in terms of the shape of the active site rather than the strength of the enzyme.
- States what happens to an enzyme once its reaction has finished.
- Predicts what a change in active site shape does to the reaction.

## Lesson: One shape, one job

Enzymes are protein catalysts that speed up reactions without being used up, so the enzyme in saliva can work on each bite of bread. Each enzyme holds its reactants, called substrates, in a pocket named the active site. The enzyme lowers the push needed to start the reaction, called activation energy.

**Example.** The active site fits one substrate shape, like a lock fits one key, then hugs it closer to make it react. The starch-breaking enzyme grips starch and ignores protein completely. After the reaction the enzyme walks away unchanged and works again.

Shape decides the job, so a warped active site stops the work. If the pocket changes shape it no longer grips its substrate and the rate falls to nothing. That is why each enzyme runs only its own reaction.

**Tip.** When asked about specificity, talk about shape, never strength. Say the site fits one substrate, the enzyme is not used up, and a changed site means a stopped reaction.

**Recap.** The active site shape picks one substrate, speeds its reaction, and survives to work again.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Digestion & Enzymes](https://lightmysky.com/learn/science/digestion-and-enzymes-mt_0NlbulkB5P)

## Opens up

- [The Digestive System and Testing Food for Nutrients](https://lightmysky.com/learn/science/the-digestive-system-and-testing-food-for-nutrients-mt__mBJCQEqPS)
- [Measuring the Rate of an Enzyme Reaction](https://lightmysky.com/learn/science/measuring-the-rate-of-an-enzyme-reaction-mt_Rmz54ICmRC)