What you need to know
- 2.7(ii–iii)enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. as biological catalysts that lower activation energyThe minimum energy needed to start a reaction. Enzymes lower the activation energy.; intracellular and extracellular action.
By the end, you should be able to…
- Define an enzyme as a biological catalystA substance that speeds up a chemical reaction without being used up..
- Explain activation energy without confusing it with energy released.
- Distinguish intracellular and extracellular enzymes.
Build your understanding
Catalysts made by living systems
enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. are biological catalysts: they increase reaction rate without being used up overall. The enzymes studied here are proteins whose folded structures enable catalysis. They can participate in temporary interactions during a reaction and then become available again.
A substrateThe molecule that binds to an enzyme’s active site and is changed in the reaction. is a reactant on which an enzyme acts; products are the substances formed. An enzyme does not supply the raw material for its product or disappear each time one substrate molecule reacts.
Lower the barrier, not the energy difference
activation energyThe minimum energy needed to start a reaction. Enzymes lower the activation energy. is the minimum energy needed for a reaction to proceed through its transition state. An enzyme provides a reaction pathway with a lower activation-energy barrier. At a given temperature, a greater proportion of molecular encounters can then lead to reaction, increasing rate.
The enzyme does not change the energy levels of the starting reactants and final products, so it does not change the overall energy difference between them. It also does not turn an energetically unfavourable isolated reaction into a favourable one merely by being present.
Lower the barrier, not the energy difference (continued)
On an energy-profile diagram, compare the rise from the reactants to the highest point along each pathway. The catalysed pathway has a lower peak, but the same starting and ending levels. The horizontal axis represents reaction progress, not elapsed time.
Inside or outside a cell
Intracellular enzymes catalyse reactions within cells. Catalase, for example, helps break down hydrogen peroxide inside many cells. Enzymes involved in respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. also act within cells.
Extracellular enzymes are made by cells but act outside the cells that secrete them. Digestive amylase acts in saliva or the gut lumenThe space inside a tube, such as a blood vessel., hydrolysing starchThe storage polysaccharide in plants, made of amylose and amylopectin. It is compact and insoluble, so it does not affect water potential. to smaller carbohydrates. Outside a cell does not necessarily mean outside the organism.
Location suits the task: large food molecules can be broken down outside cells into products that can be absorbed, while intracellular enzymes control reactions in the cellular environment.
NOT: an enzyme does not heat the substrate or add energy to the products.
- Activation energy is the rise from reactants to the transition peak
- Catalysed peak is lower; start and end levels stay the same
- The horizontal axis is reaction progress, not time
- Amylase in the gut lumen is extracellular
Two traps this diagram blocks
- Do not say the enzyme gives the substrate energy or increases the energy released.
- Do not call every enzyme that leaves a cell “outside the organism”.
From the diagram alone
Question
An uncatalysed energy profile rises from 20 units to a peak of 70 and ends at 5. A catalyst lowers the peak to 40. Calculate both activation energies and the overall energy change. [3]
Show a model answer
Uncatalysed activation energy is 70 − 20 = 50 units. Catalysed activation energy is 40 − 20 = 20 units. The overall energy change is 5 − 20 = −15 units for both pathways, because the start and end have not changed.
Make your reasoning visible
Describe the energy change precisely
Write that enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. lower activation energyThe minimum energy needed to start a reaction. Enzymes lower the activation energy.. Do not say they provide energy, increase the energy of substrates or increase the total energy released.
Distinguish outside the cell from outside the body
An enzyme in the gut lumenThe space inside a tube, such as a blood vessel. is extracellular even though the gut is within the organism. State where catalysis occurs, not just where the enzyme was made.
Key explanations
- It speeds both the forward and reverse reactions, so equilibrium is reached sooner.
Avoid this misconception
The claim: An enzyme speeds a reaction by heating its substrateThe molecule that binds to an enzyme’s active site and is changed in the reaction..
The correction: It lowers the activation-energy barrier through a different reaction pathway. It need not increase the temperature.
How does an enzyme speed up a reaction, and what stops it working?
Enzymes are proteins that act as biological catalysts. Each has an active site with a shape that only its substrate fits. The enzyme is not used up, but it can be damaged.
What the model shows
A small sample of enzyme molecules catching substrate in their active sites, next to a graph of how fast the whole mixture reacts. A second view shows the energy needed for the reaction with and without the enzyme.
What to look for
When the curve stops rising as you add substrate. What happens to the rate while the enzyme is hot, and after you cool it down again. Which inhibitor can be beaten by adding more substrate.
What it tells us
Enzymes work by binding substrate at a specific active site and lowering the activation energy. Rate is limited by substrate when it is scarce and by active sites when it is plentiful. Heat and extreme pH change the active site’s shape for good.
Be careful what you claim
This is one made-up enzyme with made-up numbers, chosen so you can watch changes happen. Real enzymes have their own optimum temperature and pH.
Try it in the model: watch one gold substrate molecule go into an active site and come out as two green products. Then slowly drag the substrate slider up.
Enzyme rate lab
Watch substrate bind to active sites, then change the conditions and read the initial rate. Heat damage is permanent: cooling does not bring active sites back.
What you see
- Left: a small sample of enzyme molecules (purple) with active sites, gold substrate, green products and red inhibitors.
- Right: the initial rate against substrate concentration for the whole mixture, a dashed reference curve for fresh enzyme at 40°C and pH 7, and the rate over the last minute.
- The Energy view: the energy along the reaction pathway with and without the enzyme, and how many encounters have enough energy at this temperature.
How to use it
- Change substrate, enzyme amount, temperature and pH with the sliders. Add a competitive or non-competitive inhibitor.
- Heat damage is permanent by default. “Fresh enzyme” replaces the denatured enzyme. “Reversible model” shows the older, simpler model that refolds on cooling, for comparison.
What it shows
- Specificity: substrate fits the complementary active site and forms an enzyme–substrate complex (2.7(i)).
- Initial rate rises with substrate, then levels off when active sites limit it; more enzyme raises the maximum.
- Heat and extreme pH denature the enzyme permanently; cold and mild pH changes do not.
- Enzymes lower the activation energy (2.7(ii)).
Limits of the model
- One imaginary enzyme with an optimum near 40°C and pH 7. Real enzymes have different optima.
- All numbers are illustrative model units, not measurements. The denaturation half-lives (about 19 s at 55°C, 1 s at 65°C) are made up to be watchable.
- The particle view is a small sample drawn to show the mechanism; it is noisier than the curve, which describes the whole mixture.
- Real active sites change shape slightly when substrate binds (induced fit); the drawing uses a simple notch.
Guided tour · step 1 of 13
- Watch the gold substrate molecules in the simulator.
- Each one that fits an active site forms an enzyme–substrate complex, then leaves as two green product pieces.
The active site has a shape and charges complementary to the substrate, so only that substrate binds. The enzyme is not used up: it catalyses again and again.
Conditions
Questions
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
An illustrative profile has reactant energy 20 units, product energy 5 units and an uncatalysed peak of 70 units. A catalyst lowers the peak to 40 units. Calculate both activation energies and the overall energy change.
Show suggested answer
Uncatalysed activation energy = 70 − 20 = 50 units. Catalysed activation energy = 40 − 20 = 20 units. The products minus reactants change is 5 − 20 = −15 units for both pathways.
Practice question
Retrieval and explanation · suggested answer
A pancreatic cell secretes an enzyme that acts in the small-intestine lumen. Is its action intracellular or extracellular? Explain.
Show suggested answer
Extracellular: it catalyses the reaction outside the secreting cell, even though the enzyme was synthesised inside that cell.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two identical reaction mixtures reach the same final equilibrium composition, but one reaches it much sooner after an enzyme is added. Explain why these findings are compatible.
Show a hint
Separate how fast equilibrium is reached from where it lies.
Explore the reasoning
A catalyst increases the rates of the forward and reverse reactions without changing their equilibrium position. It can therefore shorten the time taken to reach the same equilibrium composition without changing the overall energy difference.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare intracellular enzymes with extracellular enzymes.
Enzyme action and location
Practice question
Retrieval and explanation · suggested answer
Compare intracellular enzymes with extracellular enzymes.
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Catalysis in the washing machine
2B · EnzymesEnzyme manufacturers develop detergent enzymes for effective washing at lower temperatures. This is an application of catalysis under conditions where uncatalysed breakdown of stains would be slower. A…Specification 2.7(i) · 2.7(ii) · 2.8Read & use the scienceClose article
Why this matters
Enzyme manufacturers develop detergent enzymes for effective washing at lower temperatures. This is an application of catalysis under conditions where uncatalysed breakdown of stains would be slower. A protease acts on protein stains, while other enzymes have different substrates: a single catalyst does not digest every kind of dirt. Temperature and pH affect performance, and the whole wash includes water, detergent and mechanical action. The manufacturer source describes commercial applications; the American Chemical Society explanation offers an additional chemistry perspective. To assess an environmental claim, compare equivalent cleaning outcomes and the energy actually used. Merely including an enzyme cannot establish the total impact of a product.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.7(i) understand the mechanism of action and the specificity of enzymes in terms of their three-dimensional structure
- 2.7(ii) understand that enzymes are biological catalysts that reduce activation energy
- 2.8 CORE PRACTICAL 4 Investigate the effect of temperature, pH, enzyme concentration and substrate concentration on the initial rate of enzyme-catalysed reactions.
Use the science
A detergent advertisement claims “enzyme-powered” automatically means a lower environmental impact. What additional evidence would you request?
Explore a possible response
Comparable cleaning performance, the temperatures and energy actually used, dosage, manufacture and other lifecycle effects. The presence of a catalyst alone does not quantify the overall impact.
Explore the sources
- Novonesis: enzyme-based laundry applications (manufacturer source)Novonesis
- The Chemistry of Cold-Water WashingAmerican Chemical Society, inChemistry (2024)
Can an enzyme recycle a plastic bottle?
2B · EnzymesResearchers engineered FAST-PETase, an enzyme that breaks bonds in the polyester PET, and tested it on several post-consumer products. The work linked changes in amino-acid sequence with useful…Specification 2.7(i) · 2.7(ii) · 2.8Read & use the scienceClose article
Why this matters
Researchers engineered FAST-PETase, an enzyme that breaks bonds in the polyester PET, and tested it on several post-consumer products. The work linked changes in amino-acid sequence with useful activity under selected temperature and pH conditions. That is an extension of enzyme specificity and experimental rate measurements, not evidence that one enzyme digests every plastic. PET products also differ in structure and accessibility, so results depend on the material tested and any pretreatment. Recovering monomers can make chemical recycling possible, but an industrial process must additionally account for energy, separation, enzyme production and repeated performance.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.7(i) understand the mechanism of action and the specificity of enzymes in terms of their three-dimensional structure
- 2.7(ii) understand that enzymes are biological catalysts that reduce activation energy
- 2.8 CORE PRACTICAL 4 Investigate the effect of temperature, pH, enzyme concentration and substrate concentration on the initial rate of enzyme-catalysed reactions.
Use the science
Why would an enzyme that hydrolyses PET not necessarily break down polyethylene, even though both are called plastics?
Explore a possible response
Their chemical structures and bonds differ. An enzyme’s active site and catalytic mechanism are specific; polyethylene lacks PET’s ester linkages.
Explore the source
- Machine learning-aided engineering of hydrolases for PET depolymerizationLu et al., Nature (2022)