Form 4 · Worked answers

Metabolism and Enzymes, worked answers

Fully worked answers for Metabolism and Enzymes, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Interpreting a graph of enzyme activity against temperature or pH.
Explaining denaturation in terms of the active site.
Designing or evaluating an experiment on enzyme activity (Paper 3).
Sketching and explaining a graph of rate of reaction against substrate concentration.
Identifying an enzyme from its name and stating the substrate it acts on.
Explaining why biological detergents work well at lower washing temperatures.
Explaining why different digestive enzymes have different optimum pH values.
Describing an industrial or medical use of immobilised or purified enzymes.
Distinguishing metabolism from digestion when asked to define the term precisely.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

Define metabolism. Distinguish between anabolism and catabolism, giving one example of each.

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Metabolism is the sum of all the chemical reactions that take place in a living cell. Anabolism is the building up of large, complex molecules from smaller ones and requires an input of energy, for example protein synthesis, which joins amino acids into a protein. Catabolism is the breaking down of large molecules into smaller ones and releases energy, for example respiration, which breaks down glucose. Both must be named with a matching example to gain full credit.

metabolismanabolismcatabolismenergy

Explain, using the lock-and-key model, why each enzyme can catalyse only one reaction.

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Each enzyme has an active site with a specific three-dimensional shape. Only a substrate whose shape is complementary to that active site can fit into it, in the way that one key fits one lock, forming an enzyme-substrate complex. Because no other substrate has the complementary shape, the enzyme cannot bind it, so the enzyme is specific and catalyses only one reaction.

active sitecomplementarysubstrateenzyme-substrate complexspecific

A reaction catalysed by an enzyme slows and then stops when the mixture is heated well above the enzyme's optimum temperature. Explain why, in terms of the enzyme's active site.

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Heating the enzyme well above its optimum temperature gives the molecules more energy and breaks the bonds that hold the protein in its precise folded shape. The active site therefore changes shape and is no longer complementary to the substrate, so enzyme-substrate complexes can no longer form. The enzyme is denatured; because the folding does not reform on cooling, the change is permanent and the reaction stops.

optimum temperatureactive sitedenaturedcomplementarypermanent

Sketch and explain the shape of a graph of rate of reaction against substrate concentration for a fixed amount of enzyme.

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The graph rises steeply at first and then levels off into a plateau. At low substrate concentration, increasing it provides more substrate molecules to collide with the enzymes' active sites, so more enzyme-substrate complexes form each second and the rate rises. Once the substrate concentration is high enough for every active site to be occupied at any moment, the enzyme becomes the limiting factor, so adding more substrate cannot raise the rate further and the curve flattens. The rate is therefore controlled by substrate at low concentration and by the amount of enzyme at high concentration.

plateauactive sitelimiting factorenzyme-substrate complex

Pepsin and trypsin both digest protein, yet they have different optimum pH values. Explain why.

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Pepsin works in the stomach, where hydrochloric acid keeps the contents strongly acidic, so its active site is shaped to work best at a low, acidic pH. Trypsin works in the small intestine, where bicarbonate neutralises the acid from the stomach and the contents become mildly alkaline, so its active site works best at a higher pH. Each enzyme's optimum pH matches the conditions where it acts, which shows optimum pH is a property of the individual enzyme.

pepsinstomachtrypsinsmall intestineoptimum pH

Explain how a biological detergent removes stains and why it can work at a lower washing temperature.

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A biological detergent contains protease and lipase. The protease breaks down protein-based stains such as blood or egg, and the lipase breaks down fat-based stains, turning them into smaller, soluble molecules that rinse away easily. Because these enzymes are most active around body temperature, the detergent cleans effectively at a lower washing temperature, which also saves energy compared with a hot wash.

proteaselipasesolublelower temperatureenergy

State what an immobilised enzyme is and give two advantages of using one in industry.

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An immobilised enzyme is an enzyme fixed onto or trapped within an inert support, such as alginate beads, instead of being free in solution. One advantage is that the enzyme can be recovered and reused repeatedly, which lowers cost. A second advantage is that the product is easily separated from the enzyme and is not contaminated by it; immobilised lactase used to make lactose-free milk is a common example.

immobilisedalginate beadsreusedseparatedlactase

Phrasing that earns marks

Metabolism: Anabolism builds large molecules from small ones (uses energy); catabolism breaks large molecules down (releases energy).
Enzymes as biological catalysts: Enzymes are proteins that speed up reactions without being used up, and are specific to one substrate.
Lock-and-key model: A substrate fits the enzyme's active site like a key in a lock, forming an enzyme-substrate complex.
Effect of temperature: Rate rises to an optimum (around body temperature) then falls sharply as the enzyme denatures at high temperature.
Effect of pH: Each enzyme has an optimum pH; away from it the rate drops, and extreme pH denatures the enzyme.
Applications: Enzymes are used in detergents, food processing, brewing and medicine.
Effect of substrate concentration: As substrate concentration increases, the rate of reaction increases because more substrate molecules collide with the available active sites; beyond a certain point the rate plateaus because every active site is already occupied at any moment.
Effect of enzyme concentration: As enzyme concentration increases, with excess substrate present, the rate of reaction increases because there are more active sites available; the rate eventually levels off once substrate becomes the limiting factor instead.

Frequently asked questions

Why does enzyme activity fall at high temperature?

As temperature rises, activity increases up to an optimum. Beyond that, the heat changes the three-dimensional shape of the enzyme's active site, so the substrate no longer fits. The enzyme is denatured and the reaction rate falls sharply, usually permanently.

What is the lock-and-key model?

It describes how an enzyme is specific. The substrate (key) has a shape complementary to the enzyme's active site (lock), so only the correct substrate can bind to form an enzyme-substrate complex, which is why each enzyme catalyses one type of reaction.

Are enzymes used up when they work?

No. Enzymes are catalysts, so they are not used up or permanently changed by the reaction they speed up. A single enzyme molecule can catalyse the same reaction repeatedly, which is why cells need only small amounts.