Distinguish intracellular and extracellular enzymes
Enzymes are globular protein catalysts that act within cells or are secreted to act outside them.
Section 3.1 · Mode of action of enzymes
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons teach enzyme concepts and experimental reasoning. Written practice is self-checked; it does not demonstrate supervised practical competence or replace teacher-reviewed assessment.
Before you begin
- Recognise that proteins have specific three-dimensional shapes and that reaction rates can be compared using controlled measurements.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a biological catalyst?
What you will learn
- State that enzymes are globular proteins and biological catalysts
- Distinguish intracellular enzyme action from extracellular enzyme action
Catalysts change reaction rate
An enzyme is a biological catalyst: it increases the rate of a reaction without being used up overall. It can be reused in further reaction cycles, although conditions may alter or damage its structure.
Enzymes are globular proteins. Their three-dimensional shape is important to how they bind substrates. A catalyst provides an alternative reaction route with a lower activation energy; it does not change the overall energy difference between reactants and products.
Where the reaction takes place
An intracellular enzyme catalyses a reaction inside a cell. A catalase molecule acting in a cell is an example of intracellular activity.
An extracellular enzyme is secreted from a cell and catalyses a reaction outside it. Digestive enzymes such as amylase can be secreted into a gut lumen. The location of the reaction, not merely where the enzyme was made, determines the term.
Pause and explain
A secreted enzyme catalyses a reaction in the gut lumen. Where is its action classified?
Worked example
A cell secretes amylase into a digestive space, where the enzyme breaks down starch. Classify the enzyme and state its general role.
Show the worked solution
- The reaction occurs outside the cell that secreted the enzyme.
- The enzyme is a biological catalyst and is not used up overall.
Answer This is extracellular enzyme action because amylase catalyses starch breakdown outside the secreting cell. Amylase is a biological catalyst, so it increases the reaction rate without being used up overall.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define an enzyme and state its general molecular type.
Give me a hint
Include catalyst behaviour and the enzyme type.
Compare my reasoning
- An enzyme is a biological catalyst.
- Enzymes are globular proteins and are not used up overall.
An enzyme is a globular protein that acts as a biological catalyst, increasing reaction rate without being used up overall.
Look for these in your work
- Catalyst role stated
- Globular protein and not-used-up idea included
A reaction occurs in a vesicle inside a cell and is catalysed by an enzyme there. Classify the location.
Give me a hint
Classify where the catalysed reaction occurs.
Compare my reasoning
- The reaction is inside the cell.
- An enzyme acting inside a cell is intracellular.
This is intracellular enzyme action because the catalysed reaction occurs within the cell.
Look for these in your work
- Location identified
- Correct term used
A plant releases an enzyme onto material outside its cells. The enzyme hydrolyses molecules there. Explain why this is extracellular action.
Give me a hint
Use the site of catalysis, not the site of enzyme synthesis.
Compare my reasoning
- The enzyme acts outside the cell.
- Secreted enzymes acting beyond a cell are extracellular.
The enzyme catalyses hydrolysis outside the plant cells, so this is extracellular enzyme action, even though the plant produced and secreted the enzyme.
Look for these in your work
- Reaction site identified
- Production and action locations distinguished
Common mistakes
- Calling every enzyme secreted by a cell intracellular
- Saying the enzyme is permanently consumed in one reaction
- Claiming an enzyme changes the final equilibrium or overall energy difference
What can you explain now?
What is a biological catalyst?
Compare with the explanation
A substance that increases reaction rate without being used up overall
This describes a catalyst.
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Make it stick
What does an enzyme do, what type of protein is it, and how do intracellular and extracellular action differ?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.