B3.4 Catalysts and Enzymes

A free GCSE Combined Science: Trilogy lesson for AQA (8464) 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 role of the active site in an enzyme
• Use the lock and key model to explain why enzymes are specific, and evaluate the model
• 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.
• Product: The substance made in an enzyme-controlled reaction.
• Active site: The region of an enzyme with a particular shape where the substrate binds.
• 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. Cells need reactions to happen quickly at body temperature, so they use enzymes: 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: bonds in the substrate are broken (or new bonds made). 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, carbohydrases break down carbohydrates but have no effect on fats.

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”.

Evaluating the lock and key model

The lock and key idea is a model: a simple way to explain something we cannot see.

• Explains why each enzyme is specific to one substrate; Weaknesses A real enzyme is not rigid like a lock: its active site changes shape slightly as the substrate binds
• Explains why the enzyme can be reused; Weaknesses Does not show how bonds in the substrate are broken or made
• Explains denaturing: if the “lock” changes shape the “key” no longer fits; Weaknesses Does not show the effect of collisions, temperature or concentration

Enzymes in metabolism

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

• Building up; Example reaction controlled by enzymes Glucose → starch, glycogen or cellulose; amino acids → proteins; glycerol + fatty acids → lipids
• Breaking down; Example reaction controlled by enzymes Digestion of large food molecules in the gut; respiration (releases energy from glucose)

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 potato and liver cells) breaks down hydrogen peroxide into water and oxygen. A student collects 27 cm³ of oxygen in 1.5 minutes.
1.5 minutes = 90 seconds.
Rate = 27 ÷ 90 = 0.3 cm³ per second.

Answer: 0.3 cm³/s

Common mistakes

• Saying enzymes are “killed” by heat. Enzymes are not alive; they are denatured (see B3.5).
• 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 “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 speed up a reaction, A substance that slows down a reaction, A product of a reaction.)

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

Working: A catalyst increases the rate of a reaction and is not changed, so it can be reused. Enzymes are biological catalysts.

Question 2. What type of molecule are enzymes?

Answer: protein

Working: Enzymes are proteins folded into a particular shape.

Question 3. True or false? An enzyme is used up in the reaction it catalyses.

Answer: False

Working: False. The enzyme is unchanged and its active site is free to bind another substrate molecule.

Related lessons

• Previous lesson: B3.3 The Chemistry of Food
• Next lesson: B3.5 Factors Affecting Enzyme Action
• In this chapter: B3.1 Tissues and Organs
• In this chapter: B3.2 The Human Digestive System
• In this chapter: B3.6 How the Digestive System Works

Continue in the full course

This lesson is section B3.4 (AGCSC0304) of our free GCSE Combined Science 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 Combined Science course.