What are enzymes, how do they speed up reactions, and how do temperature and pH affect them?
Describe enzymes as biological catalysts, explain the lock and key model and enzyme specificity, and describe the effects of temperature and pH on enzyme activity
A focused answer to the O-Level Combined Science outcome on enzymes. Enzymes as biological catalysts, the lock and key model and specificity, denaturation, and the effects of temperature and pH on the rate of an enzyme reaction.
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What this dot point is asking
SEAB wants you to define enzymes as biological catalysts, to explain how they work using the lock and key model and why they are specific, and to describe how temperature and pH affect their activity, including denaturation. The lock and key explanation and the temperature/pH graphs are guaranteed exam content.
The answer
What enzymes are
Enzymes are biological catalysts: proteins made by living cells that speed up chemical reactions without being used up. They lower the energy needed for the reaction, so reactions that would otherwise be far too slow can happen quickly at body temperature. The same enzyme can be used over and over.
The lock and key model
Each enzyme has a region called the active site with a particular shape. Only a substrate with a complementary (matching) shape can fit into the active site, like a key fitting a lock. The substrate binds, the reaction happens, and the products are released, leaving the enzyme unchanged.
Enzyme specificity
Because the active site fits only one shape of substrate, each enzyme is specific: it catalyses only one type of reaction. For example, amylase breaks down starch but not protein. This is why the body needs many different enzymes.
The effect of temperature
- At low temperatures the rate is slow, because the molecules have little energy and collide rarely.
- As the temperature rises, the rate increases (faster, more frequent collisions), up to the optimum temperature (about body temperature for human enzymes), where the rate is fastest.
- Above the optimum, the rate falls sharply because the enzyme is denatured: the active site changes shape and the substrate no longer fits.
The effect of pH
Each enzyme has an optimum pH at which it works fastest. Above or below this pH the rate falls, and an extreme pH can denature the enzyme (changing the active site's shape). For example, pepsin works best in the acidic stomach, while enzymes in the small intestine work best in slightly alkaline conditions.
Examples in context
Example 1. Biological washing powders. These contain enzymes such as proteases and lipases that break down protein and fat stains at relatively low temperatures, saving energy. They stop working if the wash is too hot, because the enzymes are denatured, a direct use of enzyme activity and its temperature limit.
Example 2. Why a fever is dangerous. A very high body temperature can begin to denature the body's enzymes, disrupting the reactions that keep cells working. This shows why the optimum temperature matters and why high fevers must be controlled.
Try this
Q1. Define the term enzyme. [2 marks]
- Cue. An enzyme is a biological catalyst (a protein) that speeds up a chemical reaction without being used up.
Q2. Explain why each enzyme can act on only one type of substrate. [2 marks]
- Cue. The active site has a specific shape, so only a substrate with a complementary (matching) shape can fit, making the enzyme specific (lock and key).
Q3. Describe what happens to an enzyme above its optimum temperature. [2 marks]
- Cue. It is denatured: the active site changes shape, so the substrate no longer fits and the enzyme stops working.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksExplain, using the lock and key model, why an enzyme can break down only one type of substrate, and describe what happens to the enzyme if it is heated to a high temperature.Show worked answer →
Each enzyme has an active site with a specific shape. Only a substrate with a complementary (matching) shape can fit into the active site, like a key fitting a lock. Because the active site fits only one shape of substrate, the enzyme is specific and can break down only that one type of substrate.
If the enzyme is heated to a high temperature, the enzyme molecule (a protein) loses its shape: the active site changes shape and is denatured. The substrate can no longer fit, so the enzyme no longer works.
Markers reward the active site with a specific complementary shape (lock and key) explaining specificity, and denaturation (active site changes shape so the substrate no longer fits) at high temperature.
Original4 marksDescribe and explain how the rate of an enzyme-controlled reaction changes as the temperature is raised from low to high.Show worked answer →
At low temperatures the rate is slow because the enzyme and substrate molecules have little kinetic energy and collide infrequently.
As the temperature rises, the rate increases because the molecules move faster and collide more often, so more enzyme-substrate complexes form. The rate is fastest at the optimum temperature.
Above the optimum, the rate falls sharply because the high temperature denatures the enzyme: the active site changes shape and the substrate no longer fits, so fewer reactions occur.
Markers reward the increase in rate up to the optimum (more frequent collisions), the optimum as the fastest point, and the fall above the optimum due to denaturation of the active site.
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