AS · Practice questions · How enzymes work

Bind, react, release.

Six original Cambridge-style questions on how enzymes work and how their activity is measured, from specificity to initial rates read from a progress curve.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Shape, barrier, rate.

Active site is complementary to the substrate, not the same shape.
Enzymes lower activation energy; they do not supply energy.
Initial rate = gradient of the tangent at t = 0.
Plateau means substrate used up, not enzyme used up.

Recall

[2 marks]

State what is meant by an intracellular enzyme and an extracellular enzyme, giving one example of each.

Intracellular: catalyses a reaction inside the cell that made it, for example catalase (or DNA polymerase, ATP synthase). ✓
Extracellular: secreted from the cell and catalyses a reaction outside it, for example amylase (or trypsin, pepsin). ✓

Explain

[4 marks]

Explain how an enzyme catalyses a reaction, using the induced-fit hypothesis.

The substrate collides with and binds to the active site, which has a shape complementary to it. ✓
As it binds, the active site changes shape slightly to fit more closely around the substrate, forming an enzyme-substrate complex. ✓
This puts strain on bonds in the substrate / holds groups in the right orientation, lowering the activation energy. ✓
The products form and are released; the enzyme is unchanged and its active site returns to its original shape. ✓

Explain

[2 marks]

Suggest why the reactions of a cell need enzymes when the same reactions could be speeded up by heating.

Raising the temperature enough would denature the cell's proteins / damage membranes, so cells cannot be heated much. ✓
Enzymes lower the activation energy so a large proportion of collisions succeed at body temperature. ✓

Data

[3 marks]

Catalase is added to hydrogen peroxide and the oxygen is collected. Volumes: 0 s, 0 cm³; 20 s, 14 cm³; 40 s, 24 cm³; 60 s, 30 cm³; 80 s, 32 cm³; 100 s, 32 cm³. Calculate the mean rate in the first 20 s and explain why the volume stops rising.

Mean rate = 14 ÷ 20 = 0.70 cm³ s⁻¹. ✓
The rate falls because the substrate concentration falls, so there are fewer successful collisions with active sites. ✓
The volume stops at 32 cm³ because all the hydrogen peroxide has been broken down (not because the enzyme is used up). ✓

Analysis

[3 marks]

Outline how a colorimeter could be used to follow the hydrolysis of starch by amylase.

Add samples of the reaction mixture at intervals to iodine solution (or keep iodine in the mixture); use a red/orange filter, complementary to blue-black. ✓
Zero the colorimeter with a blank (water / iodine solution without starch) and record the absorbance of each sample. ✓
Absorbance falls as starch is broken down; convert to starch concentration with a calibration curve made from known starch concentrations. ✓

Explain

[2 marks]

The lock-and-key hypothesis was later replaced by the induced-fit hypothesis. Suggest one observation the lock-and-key hypothesis cannot easily explain.

Measurements show the shape of the enzyme / active site changes when the substrate binds. ✓
A rigid active site would not explain how binding strains the substrate's bonds and so lowers the activation energy (or how some enzymes act on a small range of similar substrates). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.