Enzymes (Years 9–11) — Science

Enzymes are biological catalysts whose specific active-site shape speeds up reactions and is affected by temperature and pH.

Inside every cell, thousands of chemical reactions happen every second, yet body temperature is far too cool for most of them to run on their own. The trick is enzymes — protein molecules that act as biological catalysts, speeding up reactions without being used up. A single enzyme molecule can process thousands of substrate molecules per second.

Why this matters Digestion, respiration, photosynthesis, copying DNA — none of it works without enzymes. They are also big business: biological washing powders, cheese-making and medicines all depend on them, and CAIE loves to test the temperature and pH graphs.

The lock-and-key model

An enzyme is a protein folded into a precise 3D shape. Part of it is a dent called the active site. Only a molecule with a matching shape — the substrate — fits into it, like a key into a lock. Once bound, the enzyme lowers the energy needed for the reaction, the products form and are released, and the enzyme is free to work again.

Specific — each enzyme catalyses only one reaction, because only one substrate fits its active site.
Catalyst — it speeds the reaction up but is not used up, so tiny amounts do a lot.
Named by their job — many end in -ase: amylase breaks starch, protease breaks protein, lipase breaks fats.

The substrate fits the active site like a key in a lock, then is split into products.

Temperature and pH

Because the active site depends on a delicate protein shape, conditions matter enormously.

Low temperature — molecules move slowly, few collisions, low rate.
Rising temperature — more kinetic energy, more successful collisions, rate rises up to the optimum (around 37°C in humans).
Too hot — above the optimum the bonds holding the shape break; the active site changes shape and the enzyme is denatured. The substrate no longer fits and the rate crashes to zero. Denaturing is permanent — cooling does not fix it.

Enzyme / site	Optimum pH
Salivary amylase (mouth)	~7 (neutral)
Pepsin (stomach)	~2 (acidic)
Enzymes in small intestine	~8 (slightly alkaline)

Each enzyme has an optimum pH; move too far either side and the active site distorts, so the enzyme denatures and stops working.

Rate rises to the optimum, then falls sharply as the enzyme denatures.

Worked example. A student measures how fast amylase digests starch at 10°C, 30°C and 60°C. At 60°C the reaction stops completely.
Explain (3 marks): (1) rate rose from 10°C to 30°C because higher temperature gives molecules more kinetic energy and more successful collisions; (2) at 60°C the high temperature broke the bonds holding the enzyme's shape; (3) the active site changed shape so the substrate could no longer bind — the enzyme was denatured.

Common exam mistake Never say an enzyme is “killed” — enzymes are not alive. Say denatured, and always explain it as the active site changing shape, not just “the enzyme breaking”.

Memory hook “Right key, right lock.” One substrate, one active site. And remember: heat and extreme pH bend the lock so the key won't fit — denatured, not dead.