Mode of action of enzymes
Cambridge A-Level Biology (9700) · Unit 3: Enzymes · 8 flashcards
Mode of action of enzymes is topic 3.1 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 3 — Enzymes , alongside Factors that affect enzyme activity. In one line: Intracellular enzymes catalyse reactions inside cells (.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 8 flashcards — 3 definitions and 5 key concepts — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Intracellular and extracellular enzymes, and give an example of each
Intracellular enzymes catalyse reactions inside cells (
What the Cambridge 9700 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- State that enzymes, which are globular proteins, act as catalysts either inside cells (intracellular enzymes) or outside cells after being secreted (extracellular enzymes).
- Explain how enzymes work in terms of the active site, formation of an enzyme-substrate complex, reduced activation energy and specificity, covering both the lock-and-key and induced-fit hypotheses.
- Investigate how enzyme-catalysed reactions progress by measuring how fast product forms with catalase and how fast substrate disappears with amylase.
- Outline how a colorimeter is used to track how far an enzyme-catalysed reaction has gone when the reaction involves a colour change.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Mode of action of enzymes
- › Use the term 'complementary' to describe the relationship between a substrate and an active site, acknowledging the induced fit model.
Define intracellular and extracellular enzymes, and give an example of each.
Intracellular enzymes catalyse reactions inside cells (
Explain how enzymes speed up biochemical reactions.
Enzymes lower the activation energy of a reaction. By forming an enzyme-substrate complex, the reaction pathway is altered, reducing the energy required for the reaction to proceed, thus increasing the reaction rate.
Describe the 'lock-and-key' hypothesis of enzyme action.
The lock-and-key hypothesis proposes that the active site of an enzyme has a rigid shape complementary to a specific substrate. The substrate fits perfectly into the active site like a key in a lock, forming the enzyme-substrate complex.
Describe the 'induced-fit' hypothesis of enzyme action.
The induced-fit hypothesis proposes that the active site of the enzyme is not a rigid shape, but rather changes shape to fit the substrate. This conformational change stresses the substrate's bonds, facilitating the reaction.
What is an enzyme-substrate complex?
An enzyme-substrate complex is the intermediate structure formed when a substrate molecule binds to the active site of an enzyme. This interaction facilitates the chemical reaction by bringing reactants together and/or straining chemical bonds.
Outline the steps involved in investigating the rate of amylase activity.
Mix amylase with a starch solution. At regular intervals, remove samples and add them to iodine solution. Measure the time taken for the iodine solution to stop turning blue-black, indicating starch has been broken down. Control variables like temperature and pH.
How can a colorimeter be used to measure the progress of an enzyme-catalysed reaction that involves a colour change?
A colorimeter measures the absorbance or transmission of light through a solution. As a coloured product is formed (or a coloured reactant is used up), the absorbance will change. This change can be used to quantify the rate of the reaction.
Explain what is meant by 'enzyme specificity'.
Enzyme specificity refers to the ability of an enzyme to catalyse a reaction with only one particular substrate or a small number of structurally similar substrates. This is due to the unique shape of the enzyme's active site which is complementary to the shape of the specific substrate.
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Read NotesMore topics in Unit 3 — Enzymes
Mode of action of enzymes sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Mode of action of enzymes deck
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