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
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- state that enzymes are globular proteins that catalyse reactions inside cells (intracellular enzymes) or are secreted to catalyse reactions outside cells (extracellular enzymes)
- explain the mode of action of enzymes in terms of an active site, enzyme–substrate complex, lowering of activation energy and enzyme specificity, including the lock-and-key hypothesis and the induced-fit hypothesis
- investigate the progress of enzyme-catalysed reactions by measuring rates of formation of products using catalase and rates of disappearance of substrate using amylase
- outline the use of a colorimeter for measuring the progress of enzyme-catalysed reactions that involve colour changes
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
- › Recall that the concentration of empty active sites is highest at the start (no substrate bound) and at the end (reaction complete) of the process.
- › Explain that the rate levels off because the enzyme active sites are 'saturated' with substrate, making the enzyme concentration the limiting factor.
- › Analyze enzyme graphs carefully to identify where the reaction rate levels off or where substrate availability becomes the limiting factor.
- › Apply colorimetry only to solutions where light passes through a sample; it is not suitable for reflected light from solid surfaces like skin.
- › Use technical terms like 'reaching Vmax' or 'maximum rate of activity' to describe the upper limit of an enzyme-controlled reaction.
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.
More 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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