A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Enzyme Action and Factors Affecting Enzyme Activity — mark scheme explained
The short answer
Enzymes are biological catalysts that play a crucial role in living organisms by speeding up chemical reactions without being consumed in the process. This section delves into how enzymes lower activation energy, the induced-fit model of enzyme action, and the factors that affect the rate of enzyme-controlled reactions.
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
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For calculation questions, marks are typically awarded for correct substitution of values into formulas, accurate arithmetic, and providing the final answer with appropriate units. For conceptual questions, marks are given for clear and concise explanations that demonstrate a deep understanding of the topic.
What the command words demand
- Calculate
- Perform a numerical calculation using given data and appropriate formulas.
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
- Determine
- Find or derive a specific value or quantity based on given information.
- Analyze
- Examine and interpret data or graphs to draw conclusions about the behavior of the system.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, perform calculations, and check your work.
- 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 marks
- Draw a smooth curve through the points to represent the relationship between substrate concentration and initial rate of reaction.1 mark
- Observe that as substrate concentration increases, the initial rate of reaction also increases but levels off at higher concentrations.1 mark
- Conclude that this indicates enzyme saturation, where all active sites are occupied by substrates.1 mark
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.
Work through every step correctly and you earn all 5 marks.
Another worked example
A student is investigating the effect of pH on the activity of an enzyme. The following data were obtained: - pH: 4, 5, 6, 7, 8, 9 - Rate of reaction (μmol/min): 2, 4, 6, 8, 6, 4 Plot a graph to show the relationship between pH and rate of reaction. What is the optimal pH for this enzyme?
- Plot the given data on a graph with pH on the x-axis and rate of reaction (μmol/min) on the y-axis.3 marks
- Draw a smooth curve through the points to represent the relationship between pH and rate of reaction.1 mark
- Identify the pH at which the rate of reaction is highest. This is the optimal pH for the enzyme.0 marks
- Conclude that the optimal pH for this enzyme is 7.1 mark
Final answer: Graph showing a bell-shaped curve with the peak at pH 7, indicating the optimal pH for the enzyme.
Work through every step correctly and you earn all 5 marks.
Common mistakes
Confusing enzyme concentration with substrate concentration in terms of their effects on reaction rate.
Why it happens: Students may mix up the concepts, leading to incorrect conclusions about how changes in these concentrations affect the rate of reaction.
Fix: 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.
Why it happens: Students may think that the active site is rigid and does not change shape upon binding to a substrate.
Fix: 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.
Why it happens: Students may not fully grasp the differences in how these inhibitors affect enzyme activity.
Fix: 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.
Why it happens: Students may not understand how changes in these factors can alter the shape and function of enzymes.
Fix: 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.
Why it happens: Students may not understand the concept of enzyme saturation and how it is represented on a graph.
Fix: 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.
Why it happens: Students may struggle with graphing skills or misinterpret the data presented in graphs.
Fix: 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
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Plot Enzyme Graph | Plot the data, draw a curve, and explain what it shows about enzyme saturation. | 5 |
| Plot Graph And Interpret | Plot rate against pH, draw a curve, and read off the enzyme's optimal pH. | 5 |
| Enzyme Graph Plotting | Plot temperature against reaction rate on a graph and identify the enzyme's optimal temperature. | 5 |
| Enzyme Inhibition Graph | Plot rate against substrate concentration for two conditions and interpret the competitive inhibition pattern. | 6 |
| Total across these question types | 21 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.