AQA A-Level Biology: Enzyme Action and Factors Affecting Enzyme Activity — mark scheme explained

The question

An experiment was conducted to study the effect of substrate concentration on the rate of an enzymatic reaction. The initial rates of reaction at different substrate concentrations are given below: - Substrate concentration (mM): 0, 1, 2, 3, 4, 5 - Initial rate of reaction (μmol/min): 0, 2, 4, 6, 8, 8 Plot a graph to show the relationship between substrate concentration and initial rate of reaction. What does this graph indicate about enzyme saturation? [Paraphrased for study — not reproduced from any exam paper.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  Step 1: Plot the given data on a graph with substrate concentration (mM) on the x-axis and initial rate of reaction (μmol/min) on the y-axis.

• S2

  Step 2: Draw a smooth curve through the points to represent the relationship between substrate concentration and initial rate of reaction.

• S3

  Step 3: Observe that as substrate concentration increases, the initial rate of reaction also increases but levels off at higher concentrations.

• S4

  Step 4: Conclude that this indicates enzyme saturation, where all active sites are occupied by substrates.

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   Step 1: Plot the given data on a graph with substrate concentration (mM) on the x-axis and initial rate of reaction (μmol/min) on the y-axis.

2. S2

   Step 2: Draw a smooth curve through the points to represent the relationship between substrate concentration and initial rate of reaction.

3. S3

   Step 3: Observe that as substrate concentration increases, the initial rate of reaction also increases but levels off at higher concentrations.

4. S4

   Step 4: Conclude that this indicates enzyme saturation, where all active sites are occupied by substrates.

5. Final answer: Graph showing a linear increase in initial rate of reaction up to a certain point, followed by a plateau. This indicates enzyme saturation.

Common mistakes

• Confusing enzyme concentration with substrate concentration in terms of their effects on reaction rate. — Always remember that increasing enzyme concentration increases the number of active sites available for substrates, while increasing substrate concentration increases the number of substrates available to bind to enzymes.
• Misunderstanding the induced-fit model and its implications. — Understand that the active site can change shape slightly to fit more precisely with the substrate, stabilizing the transition state and lowering activation energy.
• Confusing competitive and non-competitive inhibition. — Remember that competitive inhibitors bind to the active site, competing with the substrate, while non-competitive inhibitors bind to a different site on the enzyme, causing a conformational change that reduces activity.
• Misinterpreting the effects of pH and temperature on enzyme activity. — Practice interpreting data showing how pH and temperature affect enzyme activity, and remember that each enzyme has an optimal pH and temperature for maximum activity.
• Failing to recognize enzyme saturation in graphs of substrate concentration vs. reaction rate. — Look for a plateau in the graph, indicating that all active sites are occupied by substrates and further increases in substrate concentration will not increase the rate of reaction.
• Incorrectly plotting or interpreting graphs showing the effects of pH and temperature on enzyme activity. — Practice plotting and interpreting graphs, paying attention to the shape of the curve and identifying the optimal conditions for enzyme activity.

Where the marks go

• Full worked solution (all marking points)5 marks

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