What you'll learn
Enzymes are biological catalysts that control virtually every metabolic reaction in living organisms. This topic appears in multiple CIE IGCSE Biology papers, from describing enzyme action to analysing experimental data on factors affecting enzyme activity. Questions typically require you to explain mechanisms, interpret graphs, and design investigations.
Key terms and definitions
Enzyme — a protein that functions as a biological catalyst, speeding up metabolic reactions without being used up in the process.
Catalyst — a substance that increases the rate of a chemical reaction without being changed or used up by the reaction.
Active site — the specific region on an enzyme molecule where the substrate binds, with a complementary shape to the substrate.
Substrate — the specific molecule that an enzyme acts upon during a reaction.
Product — the substance formed when an enzyme catalyses the breakdown or synthesis of a substrate.
Denaturation — the permanent change in an enzyme's three-dimensional structure, particularly the active site, caused by high temperatures or extreme pH values.
Optimum temperature — the temperature at which an enzyme has maximum activity and catalyses reactions at the fastest rate.
Optimum pH — the pH value at which an enzyme shows maximum activity; different enzymes have different optimum pH values depending on their location in the body.
Core concepts
How enzymes work as biological catalysts
Enzymes lower the activation energy required for reactions to occur. Without enzymes, most metabolic reactions would proceed too slowly to sustain life. Every enzyme is specific to its substrate because of the unique shape of its active site.
The mechanism of enzyme action follows these steps:
- The substrate molecule approaches the enzyme's active site
- The substrate binds to the active site, forming an enzyme-substrate complex
- The enzyme catalyses the reaction (breaking bonds or forming new ones)
- Products are released from the active site
- The enzyme remains unchanged and can catalyse further reactions
This process is explained by the lock and key hypothesis, where the substrate (key) has a complementary shape that fits precisely into the active site (lock). Only substrates with the correct shape can bind to a particular enzyme, explaining enzyme specificity.
Enzyme specificity and examples
Each enzyme catalyses only one type of reaction because of the specific shape of its active site. CIE IGCSE Biology commonly tests knowledge of these enzymes:
Amylase — breaks down starch into maltose (a disaccharide sugar). Produced in salivary glands and the pancreas. Works in neutral or slightly alkaline conditions.
Protease — breaks down proteins into amino acids. Examples include pepsin (in the stomach, optimum pH 2) and trypsin (in the small intestine, optimum pH 8-9).
Lipase — breaks down lipids (fats and oils) into fatty acids and glycerol. Produced in the pancreas and works in the small intestine at slightly alkaline pH.
Catalase — breaks down hydrogen peroxide (a toxic by-product of metabolism) into water and oxygen. Present in most living cells, including liver and potato tissue.
Effect of temperature on enzyme activity
Temperature affects enzyme activity in a characteristic pattern that frequently appears in exam questions:
Low temperatures (0-10°C): Enzyme activity is very slow because molecules have low kinetic energy. Fewer successful collisions occur between enzyme and substrate molecules. Enzymes are not denatured at low temperatures.
Increasing temperature (10-40°C): Enzyme activity increases as molecules gain kinetic energy and move faster. More frequent collisions between enzyme and substrate occur. For many enzymes, the rate approximately doubles for every 10°C rise in temperature.
Optimum temperature (typically 37-40°C for human enzymes): Enzyme activity reaches maximum rate. Human enzymes typically have an optimum around 37°C (body temperature), though some bacterial enzymes from thermophilic organisms work best at 70°C or higher.
High temperatures (above 45°C): Enzyme activity rapidly decreases. The enzyme's protein structure begins to break down as heat breaks hydrogen bonds holding the molecule in its specific shape. The active site changes shape permanently — the enzyme is denatured.
Very high temperatures (above 60°C): Most enzymes are completely denatured and show no activity. Denaturation is irreversible; cooling the enzyme will not restore its shape or function.
Effect of pH on enzyme activity
Each enzyme has an optimum pH at which it works most effectively. pH affects the forces holding the enzyme's three-dimensional structure together.
At optimum pH: The active site maintains its precise complementary shape to the substrate. The enzyme shows maximum activity.
At pH values above or below the optimum: The ionic and hydrogen bonds maintaining the enzyme's structure are disrupted. The active site changes shape, reducing how well the substrate fits. Enzyme activity decreases.
At extreme pH values: The enzyme becomes denatured as its structure is permanently altered. The substrate can no longer bind to the active site.
Different enzymes have different optimum pH values reflecting their normal working environment:
- Pepsin (stomach): optimum pH 2 (highly acidic)
- Salivary amylase (mouth): optimum pH 7 (neutral)
- Trypsin (small intestine): optimum pH 8-9 (slightly alkaline)
- Catalase (cells): optimum pH 7 (neutral)
Effect of enzyme concentration on reaction rate
When substrate is in excess and other conditions remain constant:
Increasing enzyme concentration increases the rate of reaction proportionally. More enzyme molecules mean more active sites available for substrate molecules to bind to. The relationship is linear — doubling the enzyme concentration doubles the rate.
The pattern continues until another factor becomes limiting (such as substrate concentration or temperature).
Effect of substrate concentration on reaction rate
When enzyme concentration remains constant:
Low substrate concentration: Increasing substrate concentration increases reaction rate. Many active sites are empty and waiting for substrates. The relationship is approximately linear.
High substrate concentration: The rate of increase slows. Most active sites are occupied most of the time. Eventually, all active sites are continuously occupied.
Very high substrate concentration: Reaction rate reaches a maximum plateau and cannot increase further. All enzyme molecules are working continuously at maximum capacity. The enzyme concentration has become the limiting factor.
This pattern produces a characteristic curve that levels off, frequently shown in exam questions requiring interpretation.
Investigating enzyme activity experimentally
CIE IGCSE Biology papers regularly include questions on experimental design and analysis. Common investigations include:
Investigating amylase activity using starch and iodine:
- Add amylase to starch solution at specific temperature
- Take samples at timed intervals
- Test with iodine solution in a spotting tile
- Starch present: blue-black colour
- Starch absent (completely digested): remains orange-brown
- Time for colour to disappear indicates reaction rate
Investigating catalase activity using hydrogen peroxide:
- Add catalase source (liver, potato) to hydrogen peroxide
- Measure oxygen gas produced using inverted measuring cylinder over water
- Greater volume of oxygen in given time = faster reaction rate
- Can investigate temperature or pH effects by changing one variable
Investigating protease activity:
- Use exposed photographic film (protein coating) or egg white
- Add protease at different conditions
- Measure time for protein to be digested (film clears, egg white becomes liquid)
Controlled variables in enzyme experiments:
- Temperature (use water bath)
- pH (use buffer solutions)
- Enzyme concentration
- Substrate concentration
- Volume of solutions
Only change the independent variable being investigated. Without proper control, results cannot be attributed to the variable being tested.
Worked examples
Example 1: Interpreting enzyme-temperature data
The table shows the results of an experiment investigating the effect of temperature on the activity of the enzyme amylase. The time taken for amylase to completely digest a starch solution was measured at different temperatures.
| Temperature (°C) | Time taken (s) | Rate of reaction (1/time) |
|---|---|---|
| 10 | 420 | 0.0024 |
| 20 | 210 | 0.0048 |
| 30 | 84 | 0.012 |
| 40 | 48 | 0.021 |
| 50 | 180 | 0.0056 |
| 60 | No reaction | — |
(a) Explain why the rate of reaction increases from 10°C to 40°C. [3 marks]
Mark scheme answer: As temperature increases, the enzyme and substrate molecules gain more kinetic energy [1]. This causes more frequent collisions between enzyme and substrate molecules [1]. More enzyme-substrate complexes form per unit time, increasing the rate [1].
(b) Explain what happened to the enzyme at 60°C. [3 marks]
Mark scheme answer: The high temperature denatured the enzyme [1]. Heat broke the bonds (hydrogen/ionic bonds) holding the enzyme in its three-dimensional shape [1]. The active site changed shape permanently, so the substrate could no longer bind [1].
(c) Suggest the optimum temperature for this amylase. [1 mark]
Mark scheme answer: 40°C [1] (shown by fastest rate/shortest time).
Example 2: Experimental design question
A student wants to investigate the effect of pH on the activity of the enzyme pepsin, which digests protein. Describe how they could carry out this investigation. [6 marks]
Mark scheme answer:
- Use buffer solutions at different pH values (e.g., pH 1, 2, 3, 4, 5) [1]
- Add the same volume of pepsin solution to egg white or milk protein [1]
- Keep temperature constant using a water bath [1]
- Keep enzyme concentration and substrate concentration constant [1]
- Measure the time taken for the protein to be digested/become clear [1]
- Repeat at each pH and calculate mean results [1]
Example 3: Graph analysis
The graph shows the effect of substrate concentration on the rate of an enzyme-controlled reaction.
[Graph would show typical curve: linear increase then levelling to plateau]
(a) Describe and explain the shape of the curve at point X (plateau region). [3 marks]
Mark scheme answer: The rate has reached a maximum and does not increase further [1]. All the enzyme active sites are occupied/working continuously [1]. Enzyme concentration has become the limiting factor [1].
(b) State what would happen to the curve if the enzyme concentration was doubled. [2 marks]
Mark scheme answer: The maximum rate would be higher/the plateau would be at a higher level [1]. Because there would be more active sites available [1].
Common mistakes and how to avoid them
Mistake: Stating that enzymes are "used up" or "consumed" in reactions.
Correction: Enzymes are biological catalysts that remain chemically unchanged after catalysing a reaction. A single enzyme molecule can catalyse the same reaction thousands of times. This is why only small amounts of enzymes are needed in cells.
Mistake: Confusing denaturation with breaking down the substrate, or thinking denatured enzymes can recover.
Correction: Denaturation specifically refers to the permanent change in the enzyme's structure, particularly the active site shape. It is irreversible — the enzyme cannot regain its function. This is different from the enzyme's normal action of breaking down substrate molecules.
Mistake: Describing the active site as "destroyed" at high temperatures rather than "denatured" or "changed shape."
Correction: Use precise terminology: the active site changes shape, the enzyme is denatured, or bonds holding the structure are broken. The enzyme protein still exists but is no longer functional.
Mistake: Failing to explain enzyme specificity in terms of the complementary shape between active site and substrate.
Correction: Always link specificity to the lock and key hypothesis: only a substrate with a complementary shape to the active site can bind. This explains why amylase only breaks down starch, not protein or lipids.
Mistake: In experimental questions, only stating results without describing the method or controlled variables.
Correction: For experimental design questions worth 5-6 marks, include: the independent variable being changed, dependent variable being measured, at least three controlled variables, how to make measurements, and repeats/mean calculations.
Mistake: Misinterpreting rate of reaction graphs — confusing time taken with rate, or not recognizing that shorter time means faster rate.
Correction: Rate is inversely proportional to time: rate = 1/time. If time decreases, rate increases. Always check what the y-axis shows before interpreting the graph.
Exam technique for Enzymes
Command word recognition: "Explain" questions require reasons and mechanisms (e.g., explain enzyme activity changes with temperature — must mention kinetic energy, collision frequency, and denaturation). "Describe" questions require observations without explanation (e.g., describe the graph — state the trend, not why it happens). "State" or "Name" questions need brief factual answers only.
Graph analysis questions: Always describe the trend using data from the graph ("increases from X to Y") before explaining. For enzyme-substrate concentration graphs, identify three regions: initial linear increase, curve slowing, and maximum plateau. State what is limiting the reaction at each stage.
Practical investigation questions: Structure answers using independent variable, dependent variable, controlled variables (name at least three specifically), method of measurement, and safety or reliability considerations. Six-mark questions typically award one mark per distinct point.
Mark allocation patterns: Three-mark "explain" questions typically require three separate points: observation/what happens, first reason/mechanism, second reason/consequence. Make distinct points rather than repeating the same idea. For denaturation questions: enzyme denatures/active site changes shape [1], bonds broken by heat/pH [1], substrate cannot bind/enzyme-substrate complex cannot form [1].
Quick revision summary
Enzymes are protein catalysts with specific active sites complementary in shape to their substrate (lock and key hypothesis). They speed up reactions by lowering activation energy. Enzyme activity increases with temperature up to an optimum (typically 37°C for human enzymes), then rapidly decreases as denaturation occurs. Each enzyme has an optimum pH; extreme pH causes denaturation. Increasing enzyme concentration increases rate proportionally when substrate is in excess. Increasing substrate concentration increases rate until all active sites are continuously occupied (plateau). Common examples: amylase digests starch, protease digests protein, lipase digests lipids, catalase breaks down hydrogen peroxide.