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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Chlorophyll molecules in the thylakoid membranes absorb light energy.
- S2
Step 2: The absorbed light energy excites electrons within the chlorophyll molecule, leading to photoionisation.
- S3
Step 3: The high-energy electrons are released and transferred to an electron acceptor.
- S4
Step 4: These electrons then enter the electron transfer chain (ETC).
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Chlorophyll molecules in the thylakoid membranes absorb light energy.
- S2
Step 2: The absorbed light energy excites electrons within the chlorophyll molecule, leading to photoionisation.
- S3
Step 3: The high-energy electrons are released and transferred to an electron acceptor.
- S4
Step 4: These electrons then enter the electron transfer chain (ETC).
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.
Common mistakes
- Confusing the roles of ATP and NADPH in the light-dependent reaction. — 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. — 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. — 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. — 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. — 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. — Review common practices like supplemental lighting, CO 2 enrichment, and temperature control, and explain how each addresses a limiting factor.
Where the marks go
- Full worked solution (all marking points)4 marks