Enzymes
Cambridge O Level Biology 5090 Topic 5 revision chapter covering enzymes as protein biological catalysts and the two syllabus statements 5.1 enzyme action and 5.2 the effects of temperature and pH. The chapter defines a catalyst as a substance that increases the rate of a chemical reaction and is not changed by the reaction, then establishes an enzyme as a protein that acts as a biological catalyst in metabolic reactions. It teaches the full syllabus enzyme-action sequence: random movement of substrate molecules, collision with the enzyme, a substrate of complementary shape fitting the active site, formation of an enzyme-substrate complex, catalysed conversion of substrate into product or products, departure of products whose shapes no longer fit in the same way, and the enzyme remaining unchanged and available for reuse. The summary word equation enzyme plus substrate gives enzyme-substrate complex which gives enzyme plus products is presented and each label is drawn in scientifically accurate inline diagrams that identify enzyme, active site, substrate, enzyme-substrate complex and product. Specificity is explained through the lock-and-key model: only a substrate with a shape complementary to the active site can form a suitable complex, so each enzyme catalyses a particular reaction. Reaction progress is followed by measuring falling reactant concentration, rising product concentration, product volume per unit time, or the time taken to reach a defined endpoint, and rate is calculated as amount of product formed divided by time, with relative rate calculated as one divided by time for a fixed endpoint. The temperature section builds the required chain from low kinetic energy through fewer collisions and fewer enzyme-substrate complexes per second at low temperature, rising collision frequency and more effective collisions towards the optimum, and vibration disrupting the bonds that maintain the three-dimensional shape above the optimum so that the active site changes shape, the substrate is no longer complementary, and activity falls rapidly through denaturation. The pH section shows that every enzyme has an optimum pH, that moving away from it changes the interactions holding the active site in shape, and that extreme pH denatures the enzyme, with stomach protease and small-intestine protease used only as examples. Two full practical investigations are included: the amylase and pH investigation using iodine in the wells of a spotting tile, buffers, a constant-temperature water bath and relative rate calculated as one over time, and the catalase investigation using hydrogen peroxide with oxygen collected in a gas syringe. Graph work covers the temperature-rate curve, pH-rate comparison and the product-against-time curve, where the initial gradient is the initial rate, the curve becomes less steep as substrate is used and a plateau may be caused by substrate depletion rather than denaturation. Practical evaluation pairs every named weakness with a specific improvement, and a mistake clinic corrects the common errors of saying that enzymes supply energy, that enzymes are living, that high temperature kills an enzyme, or that low temperature denatures one.Show moreShow less
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What is Enzymes about?
Enzymes are proteins that function as biological catalysts: each one increases the rate of a metabolic reaction and is not changed by that reaction, so a single enzyme molecule can catalyse the same reaction again and again. Cambridge O Level Biology (5090) Chapter 5 explains how an enzyme works through its active site, the enzyme-substrate complex and the lock-and-key model; why temperature and pH change the rate of an enzyme-catalysed reaction; what denaturation is and why it is not the same as being killed; and how to investigate amylase and catalase activity and interpret a product-time graph.
An enzyme is a protein that works as a biological catalyst: it increases the rate of a metabolic reaction and is not changed by that reaction. Each enzyme has an active site with a particular shape. A substrate whose shape is complementary to that active site fits into it and an enzyme–substrate complex forms; the reaction happens, the products no longer fit in the same way and leave, and the unchanged enzyme is free to work again. Raising the temperature towards the optimum gives molecules more kinetic energy, so collisions are more frequent and more of them are effective, and the rate rises. Above the optimum, increased vibration disrupts the bonds holding the enzyme's three‑dimensional shape, the active site changes shape, the substrate no longer fits, and the rate falls rapidly — the enzyme has been denatured. pH behaves the same way around an optimum pH that differs from enzyme to enzyme.
Key ideas to remember
- One enzyme, one job, because one shape. Heat and extreme pH do not kill it — they bend it out of shape.
- Below the optimum, the enzyme is fine and the molecules are slow. Above it, the molecules are fast and the enzyme is finished.
- Ten minutes on day 1, fifteen on day 7, twenty on day 30. That is forty-five minutes in total, and it is the difference between recognising this topic and being able to write it.
What you need to be able to do
- State that a catalyst increases the rate of a chemical reaction and is not changed by the reaction.
- Describe enzymes as proteins that function as biological catalysts and are involved in metabolism.
- Explain enzyme action in order: collision, complementary fit into the active site, enzyme–substrate complex, catalysis, products leave, enzyme reused.
- Use the lock-and-key model to explain why one enzyme catalyses one reaction.
- Describe how reaction progress is followed using reactant or product concentration, product volume per unit time, or time to a fixed endpoint.
- Calculate a rate from amount of product and time, and a relative rate as \(1/t\).
- Explain the effect of temperature on enzyme activity below, at and above the optimum, using kinetic energy, collision frequency, effective collisions and denaturation.
- Explain the effect of pH on enzyme activity, including that optimum pH differs between enzymes.
- Describe and evaluate an investigation into the effect of pH on amylase activity.
- Describe and evaluate an investigation into catalase activity using a gas syringe.
- Interpret a product–time graph, including gradient, the falling gradient and the plateau, and give substrate depletion as an alternative to denaturation.
Why Enzymes matters
Notice the two stages. A small change of pH makes the fit worse and slows the reaction; an extreme change denatures the enzyme outright. Both reduce activity, but only the second is denaturation. Reserve the word for the extreme.
Key terms in Enzymes
- Catalyst
- A substance that increases the rate of a chemical reaction and is not changed by the reaction. It is not used up, does not appear among the products, and does not supply energy to the reaction; it only makes the reaction happen faster than it otherwise would.
- Active site
- The region of an enzyme molecule with a particular three-dimensional shape into which a substrate of complementary shape fits. Only a substrate that fits the active site can form an enzyme-substrate complex, so the shape of the active site is what makes an enzyme specific, and any change to that shape reduces the enzyme's activity.
- Optimum pH
- The pH at which an enzyme has its greatest activity. Moving away from it alters the interactions that hold the enzyme's folded shape, so the active site becomes a less good fit for the substrate, fewer enzyme-substrate complexes form and activity falls; at extreme pH the enzyme is denatured. Different enzymes have different optimum pH values.
- Rate of reaction
- A measure of how much reactant is used or product is formed in a given time. In enzyme experiments it is found either by dividing the amount of product formed by the time taken, or, when the reaction is timed to a fixed endpoint, by taking the reciprocal of that time so that a shorter time gives a larger value.
- Enzyme-substrate complex
- The temporary structure formed when a substrate molecule of complementary shape binds to the active site of an enzyme. While it exists the enzyme catalyses the conversion of the substrate into product or products, which then leave because their shapes no longer fit the active site in the same way.
- Optimum temperature
- The temperature at which an enzyme has its greatest activity. Below it the rate is lower because molecules have less kinetic energy and collide less often; above it the rate falls rapidly because the enzyme becomes denatured. The optimum differs from enzyme to enzyme, and the enzyme still works, more slowly, at temperatures either side of it.
- Enzyme
- A protein that functions as a biological catalyst: it increases the rate of a metabolic reaction inside or outside a cell and is not changed by that reaction, so a single enzyme molecule can catalyse the same reaction repeatedly. Its function depends on the shape of a region called the active site.
- Enzyme specificity
- The property that each enzyme catalyses only one reaction or one type of reaction, because only a substrate whose shape is complementary to that enzyme's active site can fit into it and form an enzyme-substrate complex. A substrate of any other shape does not fit sufficiently well for the reaction to be catalysed.
- Denaturation
- A permanent change in the three-dimensional shape of a protein caused by conditions such as temperature above the optimum or extreme pH, which disrupt the bonds holding the folded chain in place. When an enzyme is denatured its active site changes shape, the substrate is no longer complementary to it, fewer enzyme-substrate complexes form and activity falls.
Common mistakes to avoid
- “Enzymes speed up reactions by giving them energy.” RepairAn enzyme increases the rate of the reaction. It does not supply energy to it. WhyThe definition of a catalyst says nothing about energy, and saying it does contradicts “not changed by the reaction”. A source of energy would be used up.
- “High temperature kills the enzyme.” RepairHigh temperature denatures the enzyme: the bonds holding its three-dimensional shape are disrupted and the active site changes shape. WhyAn enzyme is a protein molecule, not a living organism. “Killed” contradicts the definition you gave earlier in the same answer.
- “Low temperature denatures the enzyme.” RepairLow temperature slows the reaction: less kinetic energy, less frequent collisions, fewer enzyme–substrate complexes per second. The enzyme keeps its shape. WhyWarm the mixture back to the optimum and the rate returns — which could not happen if the enzyme had been denatured.
- “The substrate is denatured at high temperature.” RepairThe enzyme is denatured. Denaturation is a change in the shape of a protein. WhyStarch, hydrogen peroxide and most other substrates are not proteins, so the word does not apply to them.
- “Denatured means the active site is blocked or full.” RepairDenatured means the active site has changed shape, so the substrate is no longer complementary to it. WhyNothing has been put into the active site. The site itself is the wrong shape now.
- “The enzyme is used up in the reaction.” RepairThe enzyme is unchanged and is free to catalyse the same reaction again. WhyThis is half of the catalyst definition. It is also why a small amount of enzyme can convert a large amount of substrate.
- “Above the optimum the molecules move too fast to fit into the active site.” RepairThe fall is caused by the active site changing shape through denaturation, not by speed. WhyFaster movement means more collisions, which would raise the rate. The only thing that can make it fall is losing working enzyme.
- “The optimum is the only temperature the enzyme works at.” RepairThe enzyme works over a range of temperatures, with its greatest activity at the optimum. WhyLook at the curve: it is above zero well either side of the peak.
- “All enzymes have an optimum of 37 °C and pH 7.” RepairOptima differ between enzymes. A stomach protease has an optimum of about pH 2; a small-intestine protease about pH 8. WhyIf every enzyme shared an optimum, digestion in an acid stomach and an alkaline small intestine could not both work.
- “The graph plateaus, so the enzyme has been denatured.” RepairA plateau means no further net product is forming. The usual cause in a controlled experiment is that the substrate has been used up. WhyIf the temperature and pH were held constant, nothing has happened that could denature the enzyme.
- “More collisions, so the reaction is faster.” RepairMore effective collisions, so more enzyme–substrate complexes form per second, so the rate increases. WhyTwo separate marking points are hiding in that sentence: effective collisions, and complex formation.
- “The enzyme recognises its substrate and picks it out.” RepairThe substrate reaches the enzyme by random movement; either its shape is complementary to the active site or it is not. WhyA molecule cannot search, choose or recognise. Specificity is a consequence of shape alone.
How Enzymes is examined
- Topic 5 can appear in every component of Biology 5090. The biology does not change between papers — only what you are asked to produce with it.
- Usually a graph to read or a single definition boundary: which axis, which point is the optimum, what a plateau means. Read the axis labels before the curve.
- Structured questions asking you to describe enzyme action or explain a shape of curve. Marks are given for the steps of the chain, so write the chain, not a conclusion.
- The amylase and catalase investigations, plus tables, graphs, \(1/t\) calculations, anomalies and evaluation. Every improvement you offer must be specific enough to actually do.
- Most explanation marks in this topic are awarded for links in a causal chain. A useful test before you move on: could a reader turn each of your sentences into an arrow pointing at the next one?
- higher temperature → more kinetic energy → molecules move faster → more frequent collisions → more effective collisions → more enzyme–substrate complexes form per second → higher rate
Frequently asked questions
What is an enzyme?
An enzyme is a protein that functions as a biological catalyst: it increases the rate of a metabolic reaction and is not changed by that reaction. Each enzyme has an active site with a particular shape. A substrate whose shape is complementary to that active site fits into it and an enzyme-substrate complex forms; the reaction happens, the products leave because they no longer fit in the same way, and the unchanged enzyme is free to work again.
Why does each enzyme catalyse only one reaction?
Because of shape. Only a substrate whose shape is complementary to the enzyme's active site can fit into it and form an enzyme-substrate complex; a substrate of any other shape does not fit well enough for the reaction to be catalysed. This is the lock-and-key model. The substrate reaches the enzyme by random movement — an enzyme does not search for or recognise its substrate. Specificity is a consequence of shape alone.
Why do enzymes stop working at high temperature?
Above the optimum, increased vibration disrupts the bonds holding the enzyme's three-dimensional shape. The active site changes shape, the substrate is no longer complementary to it, fewer enzyme-substrate complexes form and the rate falls rapidly: the enzyme has been denatured. It is not that molecules move too fast to fit — faster movement means more collisions, which would raise the rate. Only the loss of working enzyme can make the rate fall.
Does low temperature denature an enzyme?
No. Low temperature slows the reaction because molecules have less kinetic energy, so they collide less often and fewer enzyme-substrate complexes form each second. The enzyme keeps its shape, and warming the mixture back to the optimum restores the rate — which could not happen if the enzyme had been denatured. Below the optimum the enzyme is fine and the molecules are slow; above it the molecules are fast and the enzyme is finished.
Is it correct to say that heat kills an enzyme?
No. An enzyme is a protein molecule, not a living organism, so it cannot be killed. High temperature denatures it: the bonds holding its three-dimensional shape are disrupted and the active site changes shape. The same care applies to "the enzyme is used up" and "the enzyme gives the reaction energy": both contradict the definition of a catalyst, which is not changed by the reaction and does not supply energy to it.
Do all enzymes have the same optimum temperature and pH?
No. Optima differ between enzymes. A stomach protease has an optimum of about pH 2, while a small-intestine protease has an optimum of about pH 8; if every enzyme shared one optimum, digestion in an acid stomach and an alkaline small intestine could not both work. An enzyme also works over a range of temperatures and pH values either side of its optimum, with its greatest activity at the optimum, not only at it.
When a product-time graph levels off, does that mean the enzyme has been denatured?
Not in a controlled experiment. A plateau means no further net product is forming, and if temperature and pH were held constant nothing has happened that could denature the enzyme; the usual cause is that the substrate has been used up. Read the gradient as the rate: a falling gradient means the rate is dropping as substrate runs out. Give substrate depletion as the explanation unless the question tells you the conditions changed.
Syllabus reference and sources
Written against: Cambridge O Level Biology (5090) 2026–2028 Syllabus (Subject Content, Topic 5: Enzymes).
Written by: Academiq Edu Instructor Panel
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