3.5 Factors Affecting Enzyme Action

A free GCSE Biology lesson for AQA (8461) from OBA School: how temperature, pH and substrate concentration change the rate of enzyme-controlled reactions. 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 how temperature affects the rate of an enzyme-controlled reaction
• Explain what happens when an enzyme is denatured
• Describe and explain how pH affects enzymes, with examples of optimum pH
• Explain the effect of substrate concentration on rate
• Calculate rates from times and read rate graphs

Key words

• Optimum: The temperature or pH at which an enzyme works fastest.
• Denatured: The shape of the active site has changed so much that the substrate no longer fits; this is permanent.
• Rate: How fast a reaction happens: amount of change per unit time.
• Collision: When substrate and enzyme molecules meet. More collisions mean more enzyme–substrate complexes.
• Limiting factor: The factor that is in shortest supply and so stops the rate going higher.

Temperature

• Up to the optimum: molecules have more kinetic energy, so they move faster and collide more often. More enzyme–substrate complexes form, so the rate increases. (A rise of 10 °C roughly doubles the rate.)
• At the optimum (about 37 °C for most human enzymes): the rate is fastest.
• Above the optimum: the enzyme vibrates so much that bonds holding its shape break. The active site changes shape, so the substrate no longer fits. The enzyme is denatured and the rate falls quickly to zero.

Key fact: Denaturing is permanent. Cooling a denatured enzyme does not bring back its activity. At low temperatures enzymes are not denatured – they just work slowly.

pH

Each enzyme has an optimum pH. Away from it, the forces holding the protein in shape are disrupted, the active site changes shape, and the rate falls. Very acidic or alkaline conditions denature the enzyme.

• Pepsin (a protease); Where it works Stomach (hydrochloric acid present); Optimum pH about 2 (acidic)
• Salivary amylase; Where it works Mouth; Optimum pH about 7 (neutral)
• Trypsin (a protease); Where it works Small intestine (alkaline from bile and pancreatic juice); Optimum pH about 8–9 (slightly alkaline)

Substrate concentration

At first, more substrate means more collisions with active sites, so the rate rises. Eventually all active sites are occupied at any moment, so adding more substrate cannot raise the rate. Now the amount of enzyme is the limiting factor. Adding more enzyme would raise the rate again.

Calculating rates

Often we time how long a reaction takes. A shorter time means a faster rate, so rate = 1 ÷ time. Because these numbers are small, they are often multiplied by 1000.

Worked example: rate from time

At 30 °C, amylase takes 50 s to digest the starch. At 40 °C it takes 40 s.
Rate at 30 °C = 1 ÷ 50 = 0.020 s⁻¹ (× 1000 = 20).
Rate at 40 °C = 1 ÷ 40 = 0.025 s⁻¹ (× 1000 = 25).
Percentage increase = (25 − 20) ÷ 20 × 100 = 25%.

Answer: the rate is 25% higher at 40 °C

Exam tip: When describing a graph, quote numbers from it: “The rate increases from 10 °C to a peak at 40 °C, then falls to zero at 60 °C.”

Common mistakes

• Saying enzymes are “killed”. Enzymes are not alive; they are denatured.
• Saying enzymes are denatured at low temperatures. They work slowly but recover when warmed.
• Saying the substrate “cannot fit” without saying why: the active site has changed shape.
• Thinking all enzymes have an optimum pH of 7. Pepsin works best at about pH 2.
• Forgetting that a longer time means a slower rate.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the approximate optimum temperature for most human enzymes? (Choose from: 20 °C, 37 °C, 60 °C, 100 °C.)

Answer: 37 °C

Working: Most human enzymes work fastest at about 37 °C.

Question 2. What word describes an enzyme whose active site has permanently changed shape so the substrate no longer fits?

Answer: denatured

Working: The enzyme is denatured.

Question 3. True or false? Enzymes are denatured at low temperatures such as 5 °C.

Answer: False

Working: False. At low temperatures enzymes work slowly because molecules collide less often, but they are not denatured.

Related lessons

• Previous lesson: 3.4 Catalysts and Enzymes
• Next lesson: 3.6 How the Digestive System Works
• In this chapter: 3.1 Tissues and Organs
• In this chapter: 3.2 The Human Digestive System
• In this chapter: 3.3 The Chemistry of Food

Continue in the full course

This lesson is section 3.5 (AGCBH0305) 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.