Enzymes

This guide covers Topic 5, Enzymes, in full — subtopics 5.1 Enzyme action and 5.2 Effects of temperature and pH — for Cambridge O Level Biology 5090, 2026–2028 series.

Where this fits in 5090

Enzymes are proteins (from Biological Molecules) that control the rate of essentially every reaction in a living cell — the syllabus returns to enzyme-controlled reactions repeatedly in later topics (digestion, respiration, photosynthesis), which makes this topic’s core model worth having genuinely secure before moving on.

Syllabus coverage

CAMBRIDGE O LEVEL BIOLOGY 5090

• Describe a catalyst as a substance that increases the rate of a chemical reaction and is not changed by the reaction (5.1)
• Describe enzymes as proteins that function as biological catalysts and are involved in all metabolic reactions (5.1)
• Explain enzyme action with reference to the substrate, active site, enzyme-substrate complex, and product (5.1)
• Explain the specificity of enzymes in terms of the complementary shape and fit of the active site with the substrate (the “lock and key” hypothesis) (5.1)
• Understand that the progress of enzyme-catalysed reactions can be followed by measuring the concentrations of reactants and products (5.2)
• Investigate and describe the effects of temperature and pH on enzyme activity (5.2)
• Explain the effect of changes in temperature and pH on enzyme activity in terms of kinetic energy, shape and fit, denaturation and the frequency of effective collisions (5.2)

5090 is not tiered — every candidate covers all of the above.

Enzymes as catalysts

A catalyst is a substance that increases the rate of a chemical reaction without itself being used up or changed by the reaction — it can be used repeatedly. Enzymes are the biological version: proteins that act as catalysts for essentially every metabolic reaction that happens inside a living organism.

The lock-and-key model

An enzyme’s action centres on its active site — a specifically-shaped region of the enzyme molecule. The reacting molecule, the substrate, fits into the active site, forming a temporary enzyme-substrate complex; the reaction then occurs and the product is released, leaving the enzyme unchanged and free to bind another substrate molecule.

The “lock and key” hypothesis explains why a given enzyme only works on one substrate (or a small group of very similar substrates): the active site’s shape is complementary to the shape of its specific substrate — like a key that only fits one lock — so a differently-shaped molecule simply won’t fit into the active site and no reaction occurs. This explains enzyme specificity directly from molecular shape, without needing any further mechanism.

Following reaction progress

An enzyme-catalysed reaction’s progress can be tracked by measuring how the concentration of reactants decreases, or how the concentration of products increases, over time — this is the basis of most practical/experimental investigations of enzyme activity in this syllabus.

Effects of temperature on enzyme activity

Enzyme activity depends on kinetic energy and shape and fit together, giving a characteristic pattern as temperature rises:

• At low temperatures, molecules have low kinetic energy, so enzyme-substrate collisions are infrequent and slow — the reaction rate is low.
• As temperature increases toward the optimum, kinetic energy increases, collisions between enzyme and substrate become more frequent and more forceful, and the reaction rate increases — this is described as an increased frequency of effective collisions.
• Beyond the optimum temperature, the enzyme’s shape begins to break down — the active site’s structure distorts, so it no longer fits its substrate. This is denaturation, and it is permanent: once denatured, an enzyme cannot return to normal function even if the temperature drops again.

Effects of pH on enzyme activity

Each enzyme has an optimum pH at which its active site holds its correct shape and activity is highest. Moving away from this optimum pH in either direction — too acidic or too alkaline — distorts the active site’s shape in the same way excess heat does, reducing the fit between enzyme and substrate and, at extremes, causing denaturation.

Common mistakes

• Saying an enzyme is “destroyed” or “killed” by heat. The precise term is denatured — its shape has permanently changed, so it can no longer function, but the molecule itself hasn’t been chemically destroyed.
• Describing denaturation as reversible. Once an enzyme’s active site shape has been permanently distorted by excessive heat or extreme pH, it does not return to normal.
• Explaining the rise in rate with temperature using “more energy” alone, without mentioning collision frequency. The syllabus specifically wants the explanation in terms of kinetic energy and the frequency of effective collisions between enzyme and substrate.
• Forgetting that a catalyst is unchanged by the reaction it speeds up — an enzyme is reused, not consumed.
• Mixing up substrate and product, or describing the active site as matching the product’s shape rather than the substrate’s.

Quick revision checklist

• Catalyst vs enzyme: what each is, and how they relate
• Substrate, active site, enzyme-substrate complex, product
• The lock-and-key hypothesis, and how it explains enzyme specificity
• How temperature affects rate on both sides of the optimum, including denaturation, kinetic energy and effective collision frequency
• How pH affects enzyme activity, and its link to shape and denaturation

Related resources

• Biological Molecules — enzymes as one specific class of protein
• Cell Structure and Organisation — where these enzyme-catalysed reactions take place
• Cambridge O Level Biology subject hub

Written against Cambridge O Level Biology 5090, 2026–2028 series. Always check the current syllabus for your examination year.