3.4 Catalysts and Enzymes

A free GCSE Biology lesson for AQA (8461) from OBA School: enzymes as biological catalysts, the active site, the lock and key model and metabolism. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain what a catalyst is and why enzymes are called biological catalysts
• Describe the structure of an enzyme and the role of its active site
• Use the lock and key model to explain why enzymes are specific
• Give examples of reactions in metabolism that enzymes control
• Calculate a rate of reaction from data

Key words

• Catalyst: A substance that speeds up a chemical reaction without being used up or changed.
• Enzyme: A large protein molecule that acts as a biological catalyst.
• Substrate: The molecule that an enzyme acts on.
• Active site: The region of an enzyme with a particular shape where the substrate binds.
• Specific: Each enzyme catalyses only one reaction (or one type of reaction).
• Metabolism: The sum of all the reactions in a cell or the body.

Catalysts and enzymes

A catalyst speeds up a reaction but is not used up, so it can be used again and again. Living cells need reactions to happen quickly at body temperature, so they use enzymes, which are biological catalysts.

Enzymes are proteins. Each one is a long chain of amino acids folded into a particular 3-D shape. Part of this shape forms a pocket called the active site.

Key fact: Enzymes are not living. They are protein molecules made by living cells.

The lock and key model

The substrate has a shape that is complementary to (fits) the active site, like a key fits a lock.

• Step 1: the substrate collides with the enzyme and fits into the active site, forming an enzyme–substrate complex.
• Step 2: the reaction happens. The products no longer fit, so they are released.
• The enzyme is unchanged and its active site is free for another substrate molecule.

Why enzymes are specific

Only a substrate with the right shape fits the active site. So each enzyme catalyses one particular reaction. For example, amylase breaks down starch but has no effect on protein.

Exam tip: In explanations use the words active site, shape and complementary or “fits”. Avoid saying the substrate and active site are the “same shape”.

Enzymes in metabolism

Enzymes control metabolism. Some reactions break large molecules down; others build large molecules up from small ones.

• Building up; Example reaction controlled by enzymes Glucose → starch, glycogen or cellulose
• Building up; Example reaction controlled by enzymes Glycerol + 3 fatty acids → lipid
• Building up; Example reaction controlled by enzymes Amino acids → proteins
• Breaking down; Example reaction controlled by enzymes Respiration (releases energy from glucose)
• Breaking down; Example reaction controlled by enzymes Digestion of large food molecules in the gut
• Breaking down; Example reaction controlled by enzymes Excess amino acids broken down to form urea (for excretion)

Maths skill: rate of reaction

Rate tells us how quickly a reaction happens. Rate = amount of product made (or substrate used) ÷ time.

Worked example: catalase and hydrogen peroxide

Catalase (found in liver cells) breaks down hydrogen peroxide into water and oxygen. A student collects 36 cm³ of oxygen in 1.5 minutes.
1.5 minutes = 90 seconds.
Rate = 36 ÷ 90 = 0.4 cm³ per second.

Answer: 0.4 cm³/s

Common mistakes

• Saying enzymes are “killed” by heat. Enzymes are not alive; they are denatured (see 3.5).
• Saying the substrate is the “same shape” as the active site. It is complementary: it fits.
• Thinking the enzyme is used up in the reaction. It is unchanged and can be reused.
• Forgetting to convert minutes to seconds before calculating a rate in “per second”.

Revision questions

Try these, then check your answers below each one.

Question 1. What is a catalyst? (Choose from: A substance that speeds up a reaction without being used up, A substance that is used up to make a reaction faster, A substance that slows down a reaction, A living cell that carries out reactions.)

Answer: A substance that speeds up a reaction without being used up

Working: A catalyst increases the rate of a reaction and is unchanged at the end.

Question 2. What name is given to the part of an enzyme where the substrate binds?

Answer: active site

Working: The substrate binds to the active site.

Question 3. What type of molecule is an enzyme? (Choose from: Carbohydrate, Lipid, Protein, Nucleic acid.)

Answer: Protein

Working: Enzymes are large proteins folded into a specific shape.

Related lessons

• Previous lesson: 3.3 The Chemistry of Food
• Next lesson: 3.5 Factors Affecting Enzyme Action
• In this chapter: 3.1 Tissues and Organs
• In this chapter: 3.2 The Human Digestive System
• In this chapter: 3.6 How the Digestive System Works

Continue in the full course

This lesson is section 3.4 (AGCBH0304) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.