Enzyme = biological catalyst, unchanged
An enzyme is a biological catalyst that increases the rate of a chemical reaction and is not changed by it — three separately marked elements. "A chemical that speeds things up" misses biological and catalyst; "makes the reaction happen" misses rate. Enzymes are proteins, and the folded 3-D shape forms the active site where the substrate binds; once the products leave, that site is unchanged and binds another substrate — so a catalyst is never used up.
Temperature — rise, peak, then zero
Below the optimum, higher temperature gives enzyme and substrate more kinetic energy: they move faster, collide more frequently and form more enzyme-substrate complexes per unit time, so the rate rises. The optimum is about 37 °C for most human enzymes (higher for thermophilic bacteria). Above it, thermal energy breaks the bonds (e.g. hydrogen bonds) holding the protein in its tertiary structure, the active site changes shape, the enzyme is denatured, and activity falls to zero.
pH — every enzyme has its own optimum
Each enzyme works fastest at its optimum pH, slows either side of it and is denatured at extreme values. Moving away from the optimum alters the charges on the amino-acid side-chains, breaking the bonds that hold the active site in shape, so the substrate can no longer bind. Optima match the working environment: pepsin ~pH 2 (acidic stomach), salivary amylase ~pH 7 (mouth), pancreatic enzymes ~pH 7.5-8 (small intestine).
Drawn from real examiner reports.
Enzymes are not used up
An enzyme is a catalyst, so by definition it is not consumed. It forms an enzyme-substrate complex, releases the products, then binds another substrate and repeats — one molecule catalyses reaction after reaction. "Enzymes are used up", "the enzyme is broken down" and "the enzyme digests itself" all score zero. Write: enzymes are not changed by the reaction and can be reused.
Flagged w22 P12 Q10; P13 Q10
Complementary is not "the same shape"
(Extended) The active site is complementary to the substrate — the two fit together like a key in a lock, but they are not identical. "The active site is the same shape as the substrate" is the single most penalised enzyme wording across the reports. Because the site has one unique shape, only one substrate fits it, and that is what makes each enzyme specific: amylase digests starch but never protein or fat.
Flagged s22 P41 Q1b · s23 P41 Q1d · w22 P42 Q1c
Not every enzyme peaks at pH 7
Assuming pH 7 suits every enzyme wrecks any question about a stomach enzyme — predicting pH 7 as the optimum for pepsin is the classic loss. Optima differ because they match where the enzyme works: pepsin ~pH 2 (acidic stomach), salivary amylase ~pH 7 (mouth), pancreatic enzymes ~pH 7.5-8 (slightly alkaline small intestine). Read the location in the question before you commit to a value.
Flagged s23 P11/21 Q9-10
The activity curve must reach zero
Drawing or reading an activity-against-temperature graph, candidates produce a bell curve that levels off above the x-axis at high temperature. The line must come down to the x-axis: once every enzyme molecule is denatured the active site is gone and no activity at all remains. A curve floating above zero claims the enzyme still works at temperatures that have destroyed its shape.
Flagged s23 P41 Q1c
Denatured is not destroyed or killed
Denaturation is a change of shape: heat or extreme pH breaks the bonds holding the tertiary structure, so the active site no longer fits the substrate and activity is lost. The molecule itself is still there — it is not digested, dissolved, temporarily deactivated or "killed" (it was never alive). Examiners reject "destroyed", "broken down" and "deactivated" as substitutes for denatured.
Flagged w22 P41 Q1a iii
Active site is on the enzyme, not the substrate
Asked to identify parts of an enzyme diagram, many candidates point at the substrate and call it the active site. The active site is the notch or groove in the enzyme molecule; the substrate is the separate molecule that fits into it and is changed into products. Check which molecule the label line actually touches before you write the name.
Flagged w22 P32 Q3a · s23 P32 Q3a
pH answers: charges, not kinetic energy
Do not recycle the collision argument for pH. The chain is: pH moves from the optimum, side-chain charges change, the bonds holding the active site break, the site changes shape, the substrate cannot bind, so the rate falls — and at extreme pH the enzyme is denatured.
Temperature answers: collisions, then shape
Describe the trend first ("the rate rises to the optimum, then falls to zero"), then explain both halves. Below the optimum: more kinetic energy, more frequent enzyme-substrate collisions, more complexes per unit time. Above it: bonds break, the active site changes shape, the enzyme is denatured.
"Why can X not digest Y" — three steps
(Extended) Name the active site, say its shape is complementary to the intended substrate, then say it is not complementary to Y — so Y cannot fit, no enzyme-substrate complex forms and no catalysis occurs. Describing what the enzyme does do ("lipase breaks fat down") answers a different question and scores nothing.
Use the exact words examiners want
Marks hang on four phrases: denatured (not killed, destroyed or deactivated); the active site changes shape (not "the enzyme breaks down"); more frequent collisions (not "it works faster"); and, on Extended papers, complementary. Build the sentence around them.
Enzyme — a biological catalyst that increases the rate of a chemical reaction and is not changed by the reaction.
Every word in the definition is mark-worthy:
| Element | Common wrong version |
|---|---|
| biological catalyst | "a chemical" or "a substance that speeds things up" |
| increases the rate | "makes the reaction happen" (vague) |
| not changed | "used up," "broken down," "consumed" |
Enzymes are proteins. Their three-dimensional shape — determined by the sequence of amino acids — gives each enzyme its unique properties.
Define enzyme.
(a) State what is meant by the term enzyme.
(b) State what type of biological molecule all enzymes are.
