Enzymes

• Core anchor. Heat and extreme pH do not kill an enzyme — they bend it out of shape.
• Extended anchor. One enzyme, one job, because one 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.

• Core 5.1.1 Describe a catalyst as a substance that increases the rate of a chemical reaction and is not changed by the reaction.
• Core 5.1.2 Describe enzymes as proteins that are involved in all metabolic reactions, where they function as biological catalysts.
• Core 5.1.3 Describe why enzymes are important in all living organisms, in terms of a reaction rate necessary to sustain life.
• Core 5.1.4 Describe enzyme action with reference to the shape of the active site being complementary to its substrate, and the formation of products.
• Core 5.1.5 Investigate and describe the effect of changes in temperature and pH on enzyme activity, with reference to optimum temperature and denaturation.
• Supplement 5.1.6 Explain enzyme action with reference to active site, enzyme–substrate complex, substrate and product.
• Supplement 5.1.7 Explain the specificity of enzymes in terms of the complementary shape and fit of the active site with the substrate.
• Supplement 5.1.8 Explain the effect of changes in temperature on enzyme activity in terms of kinetic energy, shape and fit, frequency of effective collisions and denaturation.
• Supplement 5.1.9 Explain the effect of changes in pH on enzyme activity in terms of shape and fit and denaturation.
• Follow the progress of an enzyme-controlled reaction by measuring the reactant used or the product formed, or by timing a reaction to a fixed endpoint judged by a colour change.
• Calculate a rate as amount of product per unit time, and a relative rate as \(1/t\), with the correct unit.
• 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.

Notice the two stages. A small change of pH 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 — this is the pH version of the “slow is not denatured” distinction you met with temperature.

• “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. TierCore 5.1.1
• “High temperature kills the enzyme.” RepairHigh temperature denatures the enzyme: its three-dimensional shape changes, so the active site changes shape and the substrate no longer fits. WhyAn enzyme is a protein molecule, not a living organism. “Killed” contradicts the definition you gave earlier in the same answer. ExtendedWhy the shape changed: the bonds holding the folded chain in its three-dimensional shape are disrupted. TierCore 5.1.5, with an Extended extension
• “Low temperature denatures the enzyme.” RepairLow temperature slows the reaction. The enzyme keeps its shape and is not denatured; its activity is simply lower below the optimum. WhyWarm the mixture back to the optimum and the rate returns — which could not happen if the enzyme had been denatured. ExtendedThe reason the rate is lower: less kinetic energy, so less frequent collisions and fewer effective ones, so fewer enzyme–substrate complexes form per second. TierCore 5.1.5, with an Extended extension
• “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. TierCore 5.1.5
• “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. TierCore 5.1.5
• “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. TierCore 5.1.1
• “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. WhyHeating an enzyme above its optimum changes the enzyme, not the fit of a substrate that is moving too quickly. The word the statement gives you for that change is denatured. ExtendedFaster movement means more collisions, which on its own would raise the rate. The only thing that can make the rate fall is the loss of working enzyme. TierCore 5.1.5, with an Extended extension
• “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. TierCore 5.1.5
• “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 8.5. WhyIf every enzyme shared an optimum, digestion in an acid stomach and an alkaline small intestine could not both work. TierCore 5.1.5
• “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. TierCore 5.1.5 and practical skill
• “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. TierSupplement 5.1.8 — this is an error only in an explain answer
• “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. TierCore 5.1.4 language rule; applies to both routes

• Core never depends on a Supplement block. You can read this chapter skipping every bordered Supplement block and still have a complete, self-sufficient Core course: both definitions, why enzymes matter to a living organism, a full description of enzyme action, a full account of what temperature and pH do including optimum and denaturation, and both practicals with all their calculations. Nothing you need for Core is hidden inside a Supplement box.
• What this block adds, and what it does not. It does not re-teach the sequence above — that sequence is correct and complete as it stands. It gives the state shown in panel 2 of Figure 5.2 its name, and that name is what turns a describe answer into an explain answer. Statement 5.1.6 names four terms, and all four are expected: active site, enzyme–substrate complex, substrate and product.
• Core candidates: the whole of this section is Supplement. Statement 5.1.7 is an explain statement and there is no Core version of it — specificity appears only in the Supplement column of subtopic 5.1. Nothing in your Core course depends on what follows. The Core account of enzyme action is complete in the previous section.
• About the enzymes named in the next two items. Maltase, maltose and sucrose, and amylase, protease and lipase, are used here only as vehicles — unfamiliar substrates to apply 5.1.7 to. They are not Topic 5 vocabulary. Amylase, protease and lipase are named by 0610 at 7.4.3 (Core) and maltase at 7.4.6(b) (Supplement), both in Chapter 7, Human nutrition, where you will learn what each one actually does. Do not memorise them from this chapter; use them to practise the explanation.
• Where it does earn theory marks. Core statement 5.1.5 asks you to investigate and describe the effect of temperature and pH on enzyme activity. You cannot investigate either without measuring a rate, so everything here is the machinery behind that statement — and behind the two investigations later in this chapter. Learn it as a skill, not as a list of facts.
• Two different reasons, and only one of them is Core. Notice the shape of the Core answer: the fall above the optimum is explained — denaturation, named by the statement itself — but the rise below it is only described. That is deliberate. The reason for the rise is kinetic energy and collision frequency, which belongs to Supplement 5.1.8. If a Core question asks you to describe the effect of temperature, “activity increases up to the optimum, then falls rapidly because the enzyme is denatured” is a complete answer.
• Four named mechanisms, and the statement names all four. Statement 5.1.8 lists them: kinetic energy, shape and fit, frequency of effective collisions, and denaturation. The first three explain the rise below the optimum; the last explains the fall above it. This block does not repeat the Core description — it supplies the reasons behind it.
• A Core answer does not have to explain the mechanism. “Activity is greatest at the optimum pH and decreases as the pH moves away from it; at extreme pH the enzyme is denatured” is a complete Core answer, and the word denatured in it is Core vocabulary from 5.1.5 itself. The reason the activity falls — shape and fit — is Supplement 5.1.9.
• Supporting context, not Topic 5 vocabulary. Pepsin and trypsin are named by 0610 at 7.4.7 (Supplement), in Chapter 7, Human nutrition. They are used here only as the evidence that optima differ. You are not expected to recall them, or their pH values, for a Topic 5 question.
• Two mechanisms — and only two. Statement 5.1.9 names shape and fit and denaturation. That is the whole list. Kinetic energy and the frequency of effective collisions belong to 5.1.8, which is the temperature statement, and they are not part of a pH explanation. A pH change does not alter how fast the molecules are moving; it alters the shape of the enzyme. Importing collision language into a pH answer is easy to do, precisely because the temperature explanation is fresh in mind — but it answers 5.1.8, not 5.1.9.
• Only the first two are named 0610 factors. Statement 5.1.5 names temperature and pH, and those are the two you may be asked about in a theory paper. Enzyme concentration and substrate concentration are in the table because they are genuine method reasoning — if you change one you must control the other — and because an unfamiliar Paper 5 or Paper 6 question may set them as the variable. They are not Topic 5 facts to memorise, and no 0610 statement asks you to explain their effect.
• How the tiers are marked in these tables. Rows and terms that belong only to the Extended route are labelled Supplement in the first column. Everything not so labelled is Core, or a practical and mathematical skill required of both routes. A Core candidate can read straight past the Supplement rows.
• Every entry carries the route it belongs to. Most of these errors cost marks on both routes. Two of them — the ones tagged Supplement — are errors only in an explain answer, because the correct version needs vocabulary that a Core candidate is not examined on. If you are on the Core route, read those two for interest and do not worry that your shorter answer is incomplete.
• The first example is given twice. Once as the Core describe question, and once as the Extended explain question, so you can see exactly what the extra mark is for and what the extra vocabulary buys. Work the one for your route first, then read the other.
• Extended candidates: do this check too, then do the next one. The Core content is examined on Paper 2 and Paper 4 as well, so nothing here is optional for you. The Extended retrieval check that follows adds the four Supplement statements; it does not replace this one.
• Do the Core check first. These five questions cover only the four Supplement statements. They are in addition to the Core check above, never instead of it — Paper 2 and Paper 4 examine Core content as well as Supplement content. Notice that every command word below is explain: that is what the Supplement column of subtopic 5.1 consists of.
• Which questions are yours. Every question is labelled Core, Supplement or practical. Core route: do Q1, Q3 (Core version), Q4, Q5 and Q7 — 19 marks. Extended route: do all of them, taking the Supplement version of Q3 in place of the Core one — 28 marks. Q3 is deliberately given twice so you can see what the extra mark is for.

• Topic 5 can appear in every component of Cambridge IGCSE Biology 0610. The biology does not change between papers — only what you are asked to produce with it, and how far the question is allowed to go.
• 45 minutes, 40 marks. Usually a graph to read or a single definition boundary: which axis, which point is the optimum, what a plateau means. Core content only. Read the axis labels before the curve.
• 1 h 15 min, 80 marks. Structured questions asking you to describe enzyme action, or to describe what happens to activity as temperature or pH changes. Core content only.
• 45 minutes, 40 marks. Core plus Supplement. Where a question offers you both "fewer collisions" and "denatured", it is the Supplement explanations that let you tell which one the conditions call for.
• 1 h 15 min, 80 marks. Core plus Supplement. This is where explain appears: the enzyme–substrate complex, specificity, and the two named mechanisms for temperature and for pH.
• 1 h 15 min, 40 marks. AO3. Rates of enzyme-catalysed reactions, including judging end-points is one of the experimental contexts named in the syllabus, so both investigations in this chapter are live.