What are enzymes, how do they work, and how do temperature and pH affect them?
Enzymes as biological catalysts, the lock and key model and the active site, how temperature and pH affect enzyme activity including denaturing, and investigating enzyme activity experimentally.
A focused CCEA GCSE Biology answer on enzymes, covering how they act as biological catalysts, the lock and key model and active site, the effects of temperature and pH including denaturing, and how to investigate enzyme activity.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
CCEA wants you to explain that enzymes are biological catalysts, describe how they work using the lock and key model and the active site, explain how temperature and pH affect enzyme activity (including denaturing), and describe how to investigate enzyme activity.
What enzymes are
Because enzymes are not used up, a small amount can catalyse many reactions. Each enzyme is specific to one substrate.
The lock and key model
Effect of temperature
As temperature rises towards the optimum, molecules move faster and collide more often, so the reaction speeds up. Above the optimum, the heat breaks the bonds holding the enzyme's shape, so the active site changes shape: the enzyme is denatured and the substrate no longer fits, so the reaction stops. Below the optimum the enzyme is not damaged, just slow.
Effect of pH
Each enzyme has an optimum pH. Moving away from it slows the enzyme; an extreme pH denatures it by changing the active site. Stomach protease (pepsin) works best in acidic conditions (about pH 2), while amylase in the mouth works best near neutral (about pH 7).
Examples in context
- Example 1. Biological washing powders
- Biological detergents contain protease and lipase enzymes that break down protein and fat stains. They work best at lower wash temperatures, around 40 degrees, because hotter water would denature the enzymes and stop them working. This is why the box advises a warm, not boiling, wash, and it is a neat real-world example of the optimum temperature idea.
- Example 2. Why your stomach enzymes need acid
- The protease pepsin in your stomach has an optimum pH of about 2, so the stomach lining releases hydrochloric acid to create these acidic conditions. The same acid would denature the amylase that started working in your mouth at pH 7. This shows that each enzyme is matched to the conditions where it works, and moving an enzyme to the wrong pH stops it.
- Example 3. Why a fever is dangerous
- Body temperature is normally about 37 degrees, the optimum for human enzymes. During a high fever the temperature can climb towards 40 degrees and above. At first reactions run a little faster, but as the temperature passes the optimum the enzymes that control vital processes begin to denature, their active sites change shape and they stop working. This is why a very high fever is treated quickly: losing enzyme activity across the whole body would be life-threatening. It is a clear reminder that the temperature graph rises to a peak and then falls sharply, and that the human body is held close to the enzyme optimum on purpose.
Try this
Q1. Why is an enzyme described as specific? [1 mark]
- Cue. Its active site only fits one particular substrate.
Q2. Explain why a reaction slows down when an enzyme is cooled to 5 degrees. [2 marks]
- Cue. The molecules have less energy, so they collide less often and fewer complexes form; the enzyme is not denatured.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20194 marksExplain why an enzyme stops working when it is heated to a high temperature.Show worked answer →
Four marks for a clear chain of reasoning about the active site.
An enzyme is a protein with a specific shape, including an active site that fits its substrate (lock and key).
As the temperature rises towards the optimum (about 37 degrees in humans), the molecules move faster and collide more often, so the rate increases.
Above the optimum, the heat causes the bonds holding the enzyme's shape to break, so the active site changes shape. The enzyme is now denatured.
The substrate no longer fits the active site, so no enzyme-substrate complexes form and the reaction stops.
Markers reward the active site changing shape, the word denatured, and the substrate no longer fitting.
CCEA 20213 marksDescribe how you would investigate the effect of pH on the activity of the enzyme amylase.Show worked answer →
Three marks for a method that changes pH and measures a rate.
Mix amylase with starch solution at a set pH using a buffer, and keep the temperature constant in a water bath at 37 degrees.
Every 30 seconds, remove a drop and add it to iodine on a spotting tile. Record the time for the iodine to stop turning blue-black, which shows the starch has been broken down.
Repeat at a range of pH values (for example 4, 7 and 9). The shortest time shows the optimum pH, where amylase works fastest.
Markers reward keeping temperature constant, using iodine to follow starch breakdown, and repeating at different pH values.
Related dot points
- The organs of the digestive system and their functions, the digestive enzymes amylase, protease and lipase with their substrates and products, the role of bile, and absorption in the villi of the small intestine.
A focused CCEA GCSE Biology answer on digestion, covering the organs of the digestive system, the enzymes amylase, protease and lipase with their substrates and products, the role of bile, and absorption by the villi.
- The components of a balanced diet and their sources and functions, the consequences of an unbalanced diet, the energy content of food, and the chemical food tests for starch, reducing sugar, protein and fat.
A focused CCEA GCSE Biology answer on nutrition, covering the components of a balanced diet and their functions, the effects of an unbalanced diet, energy content of food, and the food tests for starch, reducing sugar, protein and fat.
- The word and symbol equations for photosynthesis, the role of chlorophyll and chloroplasts, the limiting factors of light intensity, carbon dioxide concentration and temperature, and experiments to investigate the rate of photosynthesis.
A focused CCEA GCSE Biology answer on photosynthesis, covering the word and symbol equations, the role of chlorophyll and chloroplasts, the limiting factors of light, carbon dioxide and temperature, and how to investigate the rate.
- Animal and plant cell structures and their functions, examples of specialised cells and their adaptations, the levels of organisation from cell to organism, and using a light microscope including magnification calculations.
A focused CCEA GCSE Biology answer on cell structure, covering the parts of animal and plant cells and their functions, specialised cells and their adaptations, levels of organisation, and using a light microscope with magnification calculations.
- How the base sequence of a gene codes for the order of amino acids in a protein, the roles of transcription and translation, the part played by mRNA and ribosomes, and how mutations can change a protein.
A focused CCEA GCSE Biology answer on protein synthesis, covering how the base sequence of a gene codes for amino acids, the roles of transcription and translation, mRNA and ribosomes, and how mutations can change a protein.
Sources & how we know this
- CCEA GCSE Biology specification — CCEA (2017)