IB Biology HLPhotosynthesisPaper 1 & 2~12 min read
Interdependent Photosynthetic Reactions
Switch off the light and, within seconds, one Calvin cycle intermediate piles up while another vanishes. Which one does which tells you exactly where the pathway broke — and this is the single most reliable exam question in the topic.
📚 What you need to know
The two stages are mutually dependent: neither can run for long without the other.
The light-dependent reactions supply ATP and reduced NADP to the Calvin cycle.
The Calvin cycle returns ADP, Pi and NADP to the thylakoid, which the light reactions need in order to keep working.
If light is removed: GP rises and RuBP falls.
If CO2 is removed: GP falls and RuBP rises.
The reasoning is always the same — work out which reaction stops, then decide what accumulates and what runs out.
Chloroplast structure supports both stages: thylakoid membranes for the light reactions and the stroma for the Calvin cycle, side by side.
These transient experiments were the original evidence for the Calvin cycle pathway.
The two-way exchange
It is easy to think of photosynthesis as a one-way conveyor: light reactions make things, Calvin cycle uses them. The return traffic matters just as much.
The light-dependent reactions cannot make ATP unless there is ADP and phosphate available to make it from, and they cannot reduce NADP unless there is oxidised NADP arriving. Those are supplied by the Calvin cycle. Stop the Calvin cycle and the light reactions eventually stall too, because their raw materials stop coming back.
The two stages sit millionths of a metre apart inside one organelle. That proximity is the point — ATP and reduced NADP are short-lived and have almost no distance to travel.
Removing the light
This is the classic experiment. A photosynthesising culture is at steady state, then the light is switched off and the concentrations of GP and RuBP are followed over the next minute.
Both lines are flat before the switch because the cycle is at steady state: every intermediate is being made as fast as it is used.
The reasoning, step by step
No light means no ATP and no reduced NADP are produced.
The reduction of GP to TP requires both, so that step stops.
GP is still being made by fixation for a short while, but is no longer being used, so GP accumulates.
TP is no longer being made, so RuBP cannot be regenerated — and regeneration also needs ATP.
The RuBP still present continues to be used up in fixation, so RuBP falls.
There is a general rule hiding in that reasoning, and it works for every version of this question. An intermediate accumulates when the reaction that uses it stops, and falls when the reaction that makes it stops. Identify the blocked reaction first, then read off which side of it each molecule sits on.
Removing carbon dioxide
Now block the other end of the cycle and everything reverses.
No CO2 means fixation stops, so no new GP is made.
GP already present is still being reduced to TP, because ATP and reduced NADP are still arriving from the light reactions, so GP falls.
TP continues to be converted back into RuBP.
RuBP is being made but is no longer being used up by fixation, so RuBP rises.
Change
Which reaction stops
GP
RuBP
Light removed
Reduction of GP to TP
Rises — made but not used
Falls — used but not regenerated
CO2 removed
Fixation of CO2
Falls — used but not made
Rises — made but not used
🧠
Which one goes up?
Whatever sits immediately before the blocked step piles up behind it, like traffic at a closed junction. Whatever sits after it drains away.
Why the concentrations level off. Neither curve keeps changing forever. Once the substrate feeding the surviving reactions is exhausted, everything stops and the concentrations settle. That is why the graph shows GP climbing to a new plateau rather than rising without limit.
Worked examples
WE 1
Explain the effect of darkness
Explain why the concentration of GP increases and the concentration of RuBP decreases when a plant is placed in darkness. (4 marks)
Point 1: what stops
Without light, no ATP or reduced NADP is produced by the light-dependent reactions.
Point 2: why GP rises
The reduction of GP to TP requires both, so it stops. GP continues to be formed by fixation briefly but is not used, so it accumulates.
Point 3: why RuBP falls
TP is no longer produced and ATP is unavailable, so RuBP cannot be regenerated.
Point 4: completing the reasoning
The RuBP already present continues to be used up in fixation, so its concentration falls.
GP is made but not used; RuBP is used but not madesay what stops FIRST — the rest of the answer follows from it
WE 2
Predict the opposite experiment
Predict and explain what happens to GP and RuBP if a brightly illuminated plant is deprived of carbon dioxide. (3 marks)
Point 1: the predictionGP falls and RuBP rises.Point 2: why GP falls
Without CO2, fixation stops so no new GP is formed, but existing GP is still reduced to TP because ATP and reduced NADP are still being supplied.
Point 3: why RuBP rises
TP continues to regenerate RuBP, but RuBP is no longer consumed by fixation, so it accumulates.
The mirror image of the darkness experimentit is only the mirror image because the light reactions are still running — say so
WE 3
Explain the return traffic
Explain why the light-dependent reactions cannot continue indefinitely if the Calvin cycle is inhibited. (3 marks)
Point 1: what the light reactions need
They require ADP, phosphate and oxidised NADP as raw materials.
Point 2: where those come from
These are supplied by the Calvin cycle as it uses ATP and reduced NADP.
Point 3: the consequence
If the cycle is inhibited, the supply stops, so there is nothing left to phosphorylate or reduce and the light-dependent reactions slow and stop.
The cycle is not just a customer — it is also the supplierthis is the point of the red return arrow on the summary diagram
💡 Exam tips
Identify the blocked reaction first, then work out what rises and what falls.
Memorise the pair: no light → GP up, RuBP down. The CO2 case is the reverse.
Say “made but not used” and “used but not made” — the phrasing itself scores.
Mention the return of ADP, Pi and NADP when asked about interdependence.
Note that ATP is needed for both the reduction and the regeneration steps.
Explain the plateau: the change stops once the remaining substrate runs out.
⚠ Common mistakes
Getting the pair the wrong way round. In darkness GP rises.
Saying the Calvin cycle stops instantly. It runs briefly on the ATP and reduced NADP still present.
Forgetting the return arrow. The light reactions need ADP, Pi and NADP back.
Saying RuBP falls because it is broken down. It is consumed by fixation and not regenerated.
Predicting a curve that rises forever. Concentrations settle at a new level.
Explaining only one molecule. These questions always want both GP and RuBP.
That completes Photosynthesis. Read the eleven pages in order and one thread runs through them: pigments capture light, light drives electrons off water, those electrons make ATP and reduced NADP, and the Calvin cycle spends both to turn carbon dioxide into sugar.
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