Example 1: calculating a rate
Starch is fully digested in 250 s. Calculate the rate using rate = 1000 ÷ time.
- Rate = 1000 ÷ 250.
- = 4.
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IB DemystifiedMYP Sciences
Enzymes
Without enzymes, digesting a meal could take years and your cells could not respire fast enough to keep you alive. Enzymes are protein catalysts that speed up reactions in every living cell, and industries now use them to wash clothes, make food and treat textiles.
Recommended for MYP 3 · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the ideas
- Temperature and pH
- Enzymes in the real world
- Worked examples
- Assessment tips
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- explain how enzymes work as biological catalysts, using the active site
- explain enzyme specificity with the lock and key model
- explain the effects of temperature and pH, including denaturing
- calculate rates of reaction from times
- analyse enzyme data and design fair experiments
- discuss uses of enzymes in homes and industry
Before you start
You will use these skills. If any feel shaky, review them first.
- nutrition and digestive enzymes (see Nutrition and digestion)
- acids, alkalis and pH (see Acids, alkalis and indicators)
- particles and collisions
Key vocabulary
- Enzyme
- A protein that acts as a biological catalyst.
- Catalyst
- A substance that speeds up a reaction without being used up.
- Substrate
- The molecule an enzyme acts on.
- Active site
- The part of an enzyme where the substrate fits and reacts.
- Optimum
- The temperature or pH at which an enzyme works fastest.
- Denatured
- When an enzyme's active site has permanently changed shape, so it no longer works.
Understanding the ideas
-
What enzymes are
Enzymes are proteins folded into a specific shape. Each has an active site where its substrate fits, like a key in a lock, and is changed into products. The enzyme is not used up, so it can be used again and again.
-
Specificity
Because the active site has a specific shape, each enzyme usually works on only one type of substrate: amylase digests starch, protease digests protein and lipase digests fat.
-
Temperature and pH
As temperature rises, molecules collide more often and the rate increases, up to the optimum temperature (about 37–40 °C for human enzymes). Above this, the enzyme is denatured: its active site changes shape and the substrate no longer fits. Each enzyme also has an optimum pH, such as about pH 2 for pepsin in the stomach and about pH 7 for amylase in saliva.
-
Why does it matter?
Enzymes are used in biological washing powders, lactose-free milk, fruit juice production and textile factories, where they can replace energy-hungry and polluting chemical processes.
-
What does it connect to?
Enzymes link to digestion, respiration, photosynthesis and DNA in biology, and to catalysts and rates of reaction in chemistry.
Temperature and pH
- Below the optimum: rate rises with temperature (more frequent collisions).
- At the optimum: fastest rate.
- Above the optimum: the active site is denatured, so the rate falls quickly, usually permanently.
- pH: each enzyme has an optimum pH; far from it, the active site changes shape.
- Rate = 1 ÷ time (or 1000 ÷ time for convenient numbers).
Enzymes in the real world
Biological washing powders remove stains at 30–40 °C, saving energy. Textile factories use enzymes to remove starch from cloth and to soften denim. Fresh pineapple contains an enzyme that tenderises meat.
Worked examples
Example 2: explaining denaturing
Why does an enzyme stop working at 70 °C?
- The high temperature breaks the bonds that hold its shape.
- The active site changes shape (denatured).
- The substrate no longer fits, so no reaction happens.
Assessment tips
Enzyme questions often use graphs of activity against temperature or pH. Expect to:
- Explain enzyme action using the active site and specificity.
- Describe and explain the effects of temperature and pH.
- Calculate rates from times.
- Design fair experiments with controls.
Common mistakes: saying enzymes are "killed" (they are proteins, not living things); saying the substrate is denatured; thinking all enzymes have the same optimum; and forgetting that denaturing is usually permanent.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: pineapple and jelly
Partially guided investigation · about 30 minutes, plus setting time · pairs
- Research question
- Does fresh pineapple stop jelly from setting, and what happens if the pineapple is cooked first?
- Scientific background
- Jelly sets because gelatin, a protein, forms a network. Fresh pineapple contains a protease enzyme (bromelain) that digests gelatin. Heating denatures the enzyme.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Jelly crystals or gelatin, hot water, small cups, fresh pineapple pieces, canned (cooked) pineapple pieces, a kettle, labels, a refrigerator.
- Method
- Make up the jelly and pour equal volumes into three cups.
- Add fresh pineapple to one cup, canned pineapple to another, and nothing to the third (the control).
- Leave all three in the refrigerator for 3 hours.
- Compare how well each cup has set.
Safety. Take care with hot water. Do not eat anything made in the laboratory.
Then evaluate: explain your results using the words protease, substrate and denatured.
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
|---|