Question

A diagram of chloroplast stroma and thylakoid lumen showing chemical energy, ferredoxin, ferredoxin-N A D p reductase, A D P synthase, and oxygen-evolving complex.

What would happen if the electron transport system in the light-dependent reaction of photosynthesis was disabled for some reason?

Answer 1

Short answer: Light reactions would stop (or be greatly reduced). That removes the chloroplast’s supply of ATP and NADPH, stops CO2 fixation, halts growth, and quickly leads to photodamage and failure of photosynthesis.

Details (stepwise):

- Electrons normally flow: water → PSII → plastoquinone → cytochrome b6f → plastocyanin → PSI → ferredoxin → FNR → NADP+ → NADPH. That flow also drives proton pumping into the thylakoid lumen and so builds the proton gradient used by ATP synthase to make ATP.

- If the electron transport chain is disabled:
- Electrons cannot move from PSII to PSI (or beyond), so ferredoxin remains oxidized and FNR cannot reduce NADP+ → NADPH production stops.
- Proton pumping via the PQ/b6f steps ceases (and water oxidation at PSII will be inhibited if P680+ cannot be reduced), so the proton gradient collapses and ATP synthase cannot make ATP.
- Oxygen evolution stops (the oxygen-evolving complex cannot keep oxidizing water if downstream electron acceptors are blocked).
- The Calvin cycle (light-independent reactions) stalls almost immediately because there is no new ATP or NADPH; CO2 fixation and sugar production cease.
- Excess light energy and blocked electron carriers lead to formation of reactive oxygen species and photoinhibition (damage to PSII and other components), accelerating loss of photosynthetic capacity.

- A caveat: if only part of the chain is blocked (for example linear flow inhibited but PSI can still perform cyclic electron flow), some ATP can be made by cyclic flow around PSI. But if the whole ETC is disabled, both ATP and NADPH production fail and the chloroplast cannot sustain photosynthesis.