Temperature, pH and substrate concentration (Biology HL)

The big idea: An enzyme is a biological catalyst — it speeds up a reaction without being used up.

How fast an enzyme works (its rate) depends on three things you must be able to explain and read from a graph:

1. Temperature, 2. pH, and 3. substrate concentration.

Each one has its own shaped graph, and each shape has a story about what is happening to the enzyme.

Enzyme

A protein that acts as a biological catalyst, speeding up a specific reaction without being used up.

Active site

The part of the enzyme with a specific shape that the substrate fits into. Its shape is what gives the enzyme its job.

Substrate

The molecule an enzyme acts on. It binds to the active site, where it is changed into product.

Optimum

The temperature or pH at which an enzyme works fastest — the peak of the graph.

Denaturation

A permanent change to the SHAPE of an enzyme's active site (caused by high temperature or extreme pH) so the substrate no longer fits and the enzyme stops working.

Saturation

The point where every active site is occupied by substrate, so adding more substrate cannot increase the rate.

Why shape is everything: An enzyme only works because its active site has a shape that the substrate fits into.

Anything that changes that shape — like too much heat or the wrong pH — stops the enzyme working. This single idea explains almost every enzyme graph.

Each factor changes the rate for a different reason. Work through them one at a time, always asking: what is happening to the active site or to the collisions?

Temperature — up to a point: As temperature rises, molecules move faster, so the substrate collides with the active site more often — the rate increases.

This continues only up to the optimum temperature, where the rate is highest (about 37 degrees C for many human enzymes).

Above the optimum the rate falls sharply: the heat denatures the enzyme — the active site changes shape, so the substrate no longer fits and the reaction stops.

pH — a peak either side of which it falls: Every enzyme has an optimum pH where its active site is the right shape and the rate is highest.

Move the pH too far either way (too acidic or too alkaline) and the active site is distorted — the enzyme is denatured and the rate drops.

Different enzymes have different optimum pHs: for example, a stomach enzyme works best in very acidic conditions, while many others prefer a near-neutral pH.

Substrate concentration — until the sites are full: Adding more substrate gives more molecules to collide with active sites, so at first the rate increases.

Eventually the rate levels off — a plateau. At this point every active site is occupied (the enzymes are saturated), so the substrate has to wait for a site to become free.

Adding even more substrate cannot speed things up — the number of enzymes is now the limiting factor, not the substrate.

A memory hook: Denatured = shape destroyed (heat or pH wrecks the active site → rate falls).

Saturated = sites all seated (every active site is taken → rate plateaus, but the enzyme is fine).

How this is tested: This is the most-examined part of enzymes, and it is a data-question favourite. On Paper 2 you may be given a rate-versus-temperature graph and asked to label the point of highest rate (the optimum), then outline why the rate falls above it — the answer is denaturation.

On Paper 1B / Paper 2 you may be handed a graph (for example, enzyme activity at different storage temperatures) and asked to explain the relationship shown — read the trend off the graph, then give the biological reason.

On Paper 1 a 1-mark question often asks you to explain the plateau on a rate-versus-substrate-concentration graph — the answer is saturation of the active sites.

Why this scores full marks: It explains all three regions of the curve — the rise, the peak and the fall — and for the fall it gives both the keyword (denaturation) and the mechanism (active-site shape changes so the substrate cannot fit).

Saying only 'the enzyme denatures' without explaining the active site usually loses the final mark.