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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Increasing substrate concentration increases the rate of reaction because more substrate molecules collide with active sites. The rate levels off when all active sites are occupied: the enzyme is saturated. Adding more substrate beyond this point has no effect unless more enzyme is added.
Substrate concentration is the third factor affecting enzyme activity, tested at Supplement level. The concept of enzyme saturation is important for understanding why reaction rates eventually plateau: a concept that connects to real-world enzyme kinetics and drug design.
How Substrate Concentration Affects Rate
At a fixed enzyme concentration:
Low substrate concentration: There are many unoccupied active sites. Increasing substrate concentration increases the frequency of enzyme-substrate collisions. Rate increases proportionally.
Moderate substrate concentration: More active sites are occupied. Rate still increases but more slowly.
High substrate concentration: All active sites are continuously occupied: the enzyme is saturated. Adding more substrate has no effect on the rate because there are no free active sites. The rate plateaus.
The Substrate Concentration Graph
The graph has a distinctive shape:
Initially, a steep, straight-line increase: rate is directly proportional to substrate concentration.
The line then curves and levels off: rate reaches a maximum (Vmax).
Beyond this point, the line is flat: rate is constant regardless of further increases in substrate.
To increase the rate beyond the plateau: Add more enzyme: this provides more active sites for the substrate to bind to.
Linking to Other Factors
The plateau can also be overcome by:
Increasing enzyme concentration: more active sites are available, so more enzyme-substrate complexes form per unit time.
Increasing temperature (up to the optimum): faster collisions.
Practical application: In industry, enzymes are used at concentrations where substrates are saturated to maximise production rate. Understanding saturation helps design efficient industrial processes.
Exam approach: When describing the graph, always explain in terms of active sites and collisions: not just "there is more substrate". The marks come from the mechanism.
Try a focused question (3 marks)
Explain why increasing substrate concentration beyond a certain point does not increase the rate of an enzyme-catalysed reaction. (3 marks)
Show the answer and marking guidance
All the active sites of the enzyme molecules are occupied / the enzyme is saturated.
No free active sites are available for additional substrate molecules to bind to.
The rate is limited by the number of enzyme molecules, not substrate concentration.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying "the enzyme runs out": the enzyme is saturated, not consumed.
Not mentioning active sites when explaining the plateau: the marks require you to explain the mechanism.
Drawing the graph as a straight line that stops suddenly: it curves into a plateau, not a sharp corner.