Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Enzymes are proteins that act as biological catalysts. They speed up reactions in cells and are not used up.
- Activation energy (Eₐ) is the energy needed to start a reaction. Enzymes lower Eₐ, so many more encounters lead to products.
- Enzymes do not change ΔG, do not supply energy and do not change the amount of product finally made.
- The active site is a pocket lined by R groups. It has a particular shape and a particular pattern of charge and polarity.
- A substrate binds only if it is compatible in shape and charge. Binding forms the enzyme–substrate complex; products are then released and the enzyme is reused.
- Cells regulate processes by controlling which enzymes they make and how active those enzymes are.
Key relationships
| Idea | What it means | Exam signal |
|---|---|---|
| lower Eₐ | smaller energy hill on an energy profile | “explain how the enzyme speeds up the reaction” |
| same ΔG | start and end levels unchanged | energy diagram with two curves |
| shape fit | substrate size and outline match the pocket | “why does the enzyme act on only one substrate?” |
| charge fit | opposite charges attract; like charges repel | substitution of a charged R group in the active site |
| enzyme reused | E appears on both sides of E + S → ES → E + P | “why does the rate fall?” (substrate used up, not enzyme) |
| Equation | Notes |
|---|---|
| rate = change in amount ÷ time | units, e.g. cm³ s⁻¹ or mmol L⁻¹ min⁻¹ |
| relative rate = 1 ÷ time | when you time an end point, such as a colour change |
Assumptions behind the numbers
- The initial rate assumes the substrate concentration has not yet fallen much.
- Comparing rates assumes temperature, pH and volumes were the same in every tube.
- Small trials support or do not support a claim; they do not prove it.
Mistakes to avoid
- Saying the enzyme is “used up” when the rate falls.
- Saying the enzyme “gives energy” to the reaction or makes ΔG more negative.
- Explaining specificity by shape only, and forgetting charge.
- Confusing negative controls (no effect expected) with positive controls (effect expected).
- Reading the final height of a product–time curve as the rate; the rate is the gradient.
Quick self-check
- 18 cm³ of gas is produced in the first 60 s. Initial rate? (18 ÷ 60 = 0.30 cm³ s⁻¹)
- An enzyme lowers Eₐ from 70 to 25 kJ mol⁻¹. By how much is it lowered, and what happens to ΔG? (45 kJ mol⁻¹; ΔG is unchanged)
- Why add a boiled-enzyme tube to an experiment? (It shows whether the effect needs an intact, heat-sensitive protein, ruling out other substances in the extract.)
Next: practice questions.