Form 4 · Paper 2 essay guide

Metabolism and Enzymes, Paper 2 essay guide

How Metabolism and Enzymes appears in Paper 2 essays: themes, a planning grid, a model answer structure and the keyword list.

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Essay themes this chapter supports

Describing the properties of enzymes and explaining the lock-and-key model: Define an enzyme as a protein that acts as a biological catalyst. → State that it lowers the activation energy of a reaction and is not used up. → Explain that each enzyme is specific to one substrate or reaction. → Describe the active site and how a substrate forms an enzyme-substrate complex. → Use the lock-and-key comparison to explain why only the correct substrate fits.
Explaining how temperature and pH affect the rate of an enzyme-catalysed reaction: Describe how the rate rises with temperature up to the optimum. → Explain the rise in terms of more kinetic energy and more collisions. → Explain denaturation above the optimum temperature in terms of the active site. → State that each enzyme has an optimum pH and that extreme pH denatures it. → Contrast pepsin and trypsin as examples of different optimum pH values.
Explaining how substrate and enzyme concentration affect the rate of reaction: Describe the graph of rate against substrate concentration, rising then plateauing. → Explain the rise by more collisions with active sites. → Explain the plateau using the idea of a limiting factor. → Describe how increasing enzyme concentration raises the rate when substrate is in excess. → Explain why the rate levels off once substrate becomes limiting.
Describing the uses of enzymes in daily life and industry: Explain how protease and lipase work in biological detergents. → State why such detergents are effective at lower washing temperatures. → Describe at least two uses of enzymes in the food industry, such as pectinase and rennin. → Explain what an immobilised enzyme is and the advantages of immobilisation. → Give a named example such as immobilised lactase used to make lactose-free milk.

How to structure your essay

Open with one or two sentences that define the topic and set the scope.
Devote one paragraph to each key concept, in a logical order.
Use precise biological terms and, where relevant, a labelled diagram.
Give an example or state where the process happens in the body or plant.
Close by linking the ideas back to the question.

Planning grid

Question	Answer
Introduction	State what the question asks and define the key term it uses, such as 'enzyme', 'metabolism' or 'optimum temperature', in one sentence.
Point 1	Name the structures or factors involved, the enzyme, the active site, the substrate, or the factor being changed, and state clearly what each is.
Point 2	Develop the explanation the question asks for, such as how a factor changes the rate, or how the active site and substrate interact.
Point 3	Support the answer with a named example, such as pepsin and trypsin, a biological detergent, or immobilised lactase, explained fully.
Conclusion	Draw the ideas together, linking the enzyme's structure or the factor to the effect on the rate of reaction.

Key terms to include

enzyme
substrate
active site
enzyme-substrate complex
catalyst
activation energy
specific
denature
optimum temperature
optimum pH
lock-and-key
anabolism
catabolism
metabolism
substrate concentration
limiting factor
biological detergent
protease
lipase
immobilised enzyme

A model essay outline

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.

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.