A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Light-Dependent and Light-Independent Reactions in Photosynthesis — mark scheme explained
The short answer
Photosynthesis is a fundamental biological process that converts light energy into chemical energy, which plants use to grow and survive. This section focuses on the detailed mechanisms of both the light-dependent and light-independent reactions, highlighting key steps, molecules involved, and their interconnections.
The question
Explain how chlorophyll absorbs light and initiates the light-dependent reaction.
[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 descriptive questions, marks are typically awarded for accurate and detailed explanations that include key steps and molecules. For evaluation questions, marks are given for clear reasoning and evidence-based assessments of agricultural practices.
What the command words demand
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
- Describe
- Give a clear account of the process or mechanism, step-by-step.
- Identify
- Recognize and name specific factors or components involved in the process.
- Evaluate
- Assess the effectiveness or impact of different practices or conditions.
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, provide a thorough answer, and check your work.
- Chlorophyll molecules in the thylakoid membranes absorb light energy.1 mark
- The absorbed light energy excites electrons within the chlorophyll molecule, leading to photoionisation.1 mark
- The high-energy electrons are released and transferred to an electron acceptor.1 mark
- These electrons then enter the electron transfer chain (ETC).1 mark
Final answer: Chlorophyll absorbs light energy, which excites electrons and leads to photoionisation. The high-energy electrons are then transferred to an electron acceptor and enter the ETC.
Work through every step correctly and you earn all 4 marks.
Another worked example
Describe the role of ATP synthase in the production of ATP during the light-dependent reaction.
- The electron transfer chain (ETC) generates a proton gradient across the thylakoid membrane.1 mark
- Protons (H + ) accumulate on one side of the membrane, creating a concentration gradient.1 mark
- ATP synthase, an enzyme embedded in the thylakoid membrane, allows protons to flow back down their gradient.1 mark
- The energy from this proton flow is used by ATP synthase to catalyse the formation of ATP from ADP and Pi.1 mark
Final answer: ATP synthase uses the energy from the proton gradient generated by the ETC to catalyse the formation of ATP from ADP and Pi.
Work through every step correctly and you earn all 4 marks.
Common mistakes
Confusing the roles of ATP and NADPH in the light-dependent reaction.
Why it happens: Students may mix up the functions of these molecules, leading to incorrect answers about their production and use.
Fix: Remember that ATP is produced via chemiosmosis using the proton gradient, while NADPH is formed by reducing NADP + with high-energy electrons from the ETC.
Misunderstanding the steps of the Calvin cycle.
Why it happens: Students may struggle to remember the sequence of events in the light-independent reaction, leading to confusion about how carbon dioxide is fixed and reduced.
Fix: Practice the steps of the Calvin cycle: CO 2 fixation by rubisco, reduction of GP to TP using ATP and NADPH, and regeneration of RuBP.
Failing to recognize how environmental factors affect photosynthesis.
Why it happens: Students may not fully understand the impact of light intensity, CO 2 concentration, and temperature on the rate of photosynthesis.
Fix: Review how each factor limits a specific part of the process: light affects the light-dependent reaction, CO 2 affects carbon fixation, and temperature affects enzyme activity.
Confusing the location of the light-dependent and light-independent reactions.
Why it happens: Students may mix up where these reactions occur within the chloroplast, leading to incorrect answers about their locations.
Fix: Remember that the light-dependent reaction occurs in the thylakoid membranes, while the light-independent reaction (Calvin cycle) takes place in the stroma.
Incorrectly describing the role of rubisco in carbon fixation.
Why it happens: Students may not fully understand the specific function of rubisco, leading to vague or incorrect explanations.
Fix: Practice explaining that rubisco catalyses the reaction between CO 2 and RuBP to form two molecules of GP.
Failing to explain how agricultural practices can overcome limiting factors in photosynthesis.
Why it happens: Students may struggle to connect specific practices with the environmental factors they address, leading to incomplete or incorrect answers.
Fix: Review common practices like supplemental lighting, CO 2 enrichment, and temperature control, and explain how each addresses a limiting factor.
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 |
|---|---|---|
| Light-Dependent Reaction | Describe how chlorophyll absorbs light and triggers the start of the light-dependent reaction. | 4 |
| Describe ATP Synthesis | Describe how ATP synthase produces ATP using the proton gradient in the light-dependent reaction. | 4 |
| Describe A Process | Explain the sequence of steps that produces NADPH in the light-dependent reaction. | 3 |
| Describe Enzyme Role | Describe rubisco's function and explain how it fixes carbon dioxide in the Calvin cycle. | 4 |
| Total across these question types | 15 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.