OCR A Level Biology: Mastering Photosynthesis - The Calvin Cycle and Limiting Factors (Section 5.2.1 e, f, g)

OCR A Level Biology: Mastering Photosynthesis - The Calvin Cycle and Limiting Factors (Section 5.2.1 e, f, g)

Prior Knowledge to Recap

Before diving into these photosynthesis questions, ensure you understand these foundational concepts:

  • The two stages of photosynthesis: the light-dependent stage (in thylakoid membranes) produces ATP and reduced NADP, whilst the light-independent stage (Calvin cycle in the stroma) uses these products to fix CO₂

  • The Calvin cycle pathway: CO₂ combines with RuBP (catalysed by RuBisCO) to form GP, which is reduced to TP using ATP and reduced NADP, with most TP recycled to regenerate RuBP

  • Limiting factors concept: when a factor is in short supply, it prevents the rate of a process from increasing, even if other factors are optimal

  • Enzyme properties: enzymes are affected by temperature (kinetic energy, denaturation) but not directly by light

  • Products of photosynthesis: oxygen is released from photolysis, glucose/carbohydrates are synthesised from TP

Links to GCSE Content

  • Photosynthesis equation: understanding that plants use CO₂ and water to produce glucose and oxygen using light energy (GCSE Biology)

  • Factors affecting photosynthesis: light intensity, CO₂ concentration, temperature, and chlorophyll (GCSE Biology)

  • Enzymes and temperature: recognising that enzymes work faster at higher temperatures until they denature (GCSE Biology)

Common Question Types and How to Answer Them

Let me walk you through five frequently asked questions from past OCR papers on the Calvin cycle and limiting factors.

Question 1: Interpreting Calvin's Experiment Data

How to Answer:

(i) Explaining Calvin's conclusions:

You need to use specific times from the graph:

Point 1: GP was the only compound seen after 1 second Point 2: TP appears after 5 seconds

This shows that GP must be formed first (as it appears earliest and alone), and then GP is converted into TP (which appears later).

Mark scheme guidance:

  • Award 1 mark for stating GP was the only compound seen after 1 second

  • Award 1 mark for stating TP appears after 5 seconds

  • Allow 'glycerate 3-phosphate' for GP and 'triose phosphate' for TP

(ii) What happens to TP:

Answer: TP is converted into/used to synthesise sugar phosphates, amino acids (e.g. glutamic acid, serine, glycine), or RuBP.

Mark scheme guidance: Award 1 mark. Must be the idea of synthesis/conversion into something, not breaking down.

Common mistake: Don't say TP is "broken down" - it's used as a building block to synthesise other molecules. Also, don't confuse TP with GP.

Question 2: Effects of Light Intensity on Calvin Cycle Intermediates

How to Answer:

The correct answer is B: Only statements 1 and 2 are correct.

Why each statement is correct or incorrect:

Statement 1 is CORRECT: At low light intensity, less GP is converted into TP because there is less ATP and reduced NADP available (products of the light-dependent stage).

Statement 2 is CORRECT: At high light intensity, RuBP concentration is high because more TP is regenerated into RuBP (due to more ATP and reduced NADP being available to convert GP to TP).

Statement 3 is INCORRECT: At high light intensity, RuBP does not accumulate because it cannot be converted to GP. In fact, at high light intensity, RuBP is rapidly converted to GP because there's plenty of CO₂ available (assuming CO₂ isn't limiting).

Mark scheme guidance: Award 1 mark for B only.

Key concept: Light intensity affects the light-dependent stage directly (producing ATP and reduced NADP), which then affects the concentrations of Calvin cycle intermediates. Low light = less ATP/reduced NADP = GP accumulates (can't be reduced to TP). High light = more ATP/reduced NADP = RuBP accumulates (TP is rapidly regenerated to RuBP).

Question 3: Completing a Passage about the Calvin Cycle

How to Answer:

This tests your precise knowledge of the Calvin cycle terminology:

  1. RuBP / ribulose bisphosphate (CO₂ combines with this 5-carbon molecule)

  2. GP / glycerate 3-phosphate (the unstable 6-carbon molecule breaks into two of these)

  3. ATP (used to reduce GP to TP)

  4. NADPH / reduced NADP (also used to reduce GP to TP)

  5. sucrose (hexose phosphates converted to this for transport)

Mark scheme guidance: Award 1 mark for each correct answer (5 marks total). ATP and NADPH can be in either order. Allow abbreviations like GP and RuBP.

Common mistakes:

  • Writing "ribulose biphosphate" instead of "bisphosphate"

  • Confusing GP with "glycerol-phosphate"

  • Writing "NADH" instead of "NADPH" (confusing with respiration)

  • Writing "glucose" instead of "sucrose" for transport (the question specifically mentions transport elsewhere in the plant)

  • Confusing RuBP (the substrate) with RuBisCO (the enzyme)

Top tip: If you find the full chemical names difficult to spell correctly, use the abbreviations RuBP and GP - there's much less opportunity for error!

Question 4: Effects of Reducing CO₂ Concentration on Calvin Cycle

How to Answer:

You need to describe and explain changes in both RuBP and GP:

For GP (glycerate 3-phosphate):

  • Description: GP concentration decreases

  • Explanation: Because less CO₂ is available to react with RuBP to produce GP / less carbon fixation taking place

For RuBP (ribulose bisphosphate):

  • Description: RuBP concentration increases AND then decreases

  • Explanations:

    • RuBP increases because it is not being converted to GP (no CO₂ to react with)

    • RuBP increases because it is still being produced/regenerated from TP

    • RuBP then decreases because less GP is available to regenerate RuBP

Mark scheme guidance: Award up to 3 marks. Maximum 2 marks for RuBP explanations (from MPs 3, 4, 5, and 6).

Understanding the logic: When CO₂ is reduced:

  1. Less CO₂ + RuBP → less GP formed (GP decreases)

  2. RuBP not being used up → RuBP initially increases

  3. But TP is still being used to regenerate RuBP → RuBP continues to increase temporarily

  4. Eventually less GP means less TP, means less regeneration → RuBP then decreases

Common mistake: Students often only describe one molecule (usually GP) and forget to discuss RuBP, or they don't explain the biphasic nature of the RuBP graph (increases then decreases).

Question 5: Why Temperature Affects Light-Independent Stage More

How to Answer:

This requires you to link enzyme action to the Calvin cycle:

Point 1: The light-independent stage is controlled by enzymes (e.g. RuBisCO, and others)

Point 2: Higher temperature increases kinetic energy of enzyme molecules / number of successful collisions / enzyme-substrate complexes formed

Alternative Point 2: High temperatures may denature enzymes (describing denaturation: active site changes shape, substrate no longer complementary/fits)

Mark scheme guidance: Award up to 2 marks maximum.

Full answer example: "The light-independent stage is controlled by enzymes such as RuBisCO, which catalyses the fixation of CO₂ to RuBP. Higher temperatures increase the kinetic energy of enzyme and substrate molecules, leading to more successful collisions and more enzyme-substrate complexes formed per unit time. This increases the rate of reactions in the Calvin cycle. However, at very high temperatures, these enzymes may denature, causing the active site to change shape so substrates can no longer bind, dramatically reducing the rate."

Why the light-dependent stage is affected less: The light-dependent reactions are mainly driven by light energy exciting electrons in photosystems, not by enzyme-catalysed reactions. Whilst some enzymes are involved (e.g. ATP synthase), the rate-limiting steps are physical processes (light absorption, electron transport) rather than enzyme catalysis.

Mark scheme guidance notes:

  • Ignore "no enzymes in light-dependent stage" (this is incorrect but was ignored)

  • Allow "fewer enzymes in light-dependent stage"

  • Don't confuse NADP with NAD (from respiration)

Common mistakes:

  • Stating vaguely that "temperature affects enzymes" without explaining how (kinetic energy, collisions, ESCs)

  • Not mentioning that enzymes control the light-independent stage

  • Confusing the light-independent stage with requiring photons/light energy

  • Not relating high temperatures to denaturation

Additional Exam Technique Tips

  1. For Calvin cycle questions: Always think about the sequence: CO₂ + RuBP → GP → TP → (mostly back to RuBP, some to make other molecules). If one intermediate increases, trace through what must happen to the others.

  2. For limiting factors: Remember that a limiting factor doesn't reduce the rate - it prevents it from increasing further. The rate plateaus when a factor becomes limiting.

  3. For practical investigations: Always consider:

    • Independent variable (what you change)

    • Dependent variable (what you measure)

    • Control variables (what you keep constant)

    • How to improve precision and reduce anomalies

  4. For enzyme-related questions: Link temperature to:

    • Kinetic energy → more successful collisions

    • Enzyme-substrate complex formation

    • But also potential denaturation at high temperatures

  5. Terminology precision: Use the correct names:

    • GP not "glycerol phosphate"

    • RuBP not "RuBisCO" (enzyme vs substrate)

    • NADPH not "NADH" (photosynthesis vs respiration)

    • Sucrose for transport, not glucose

By practising with actual past paper questions and understanding what examiners are looking for, you'll be well-prepared for photosynthesis questions in your OCR A Level Biology exam.