If Photosystem I (PSI) stopped working, the concentrations of ATP, NADPH, and sugars would be affected as follows:
- NADPH concentration would decrease significantly. PSI is responsible for the final step in the light reactions of photosynthesis where electrons are transferred to NADP+ to form NADPH. Without PSI activity, NADP+ cannot be reduced to NADPH, leading to a drop in NADPH production
- ATP concentration would also decrease, but less drastically and depending on cyclic electron flow. Normally, electrons flow from Photosystem II (PSII) through the electron transport chain to PSI, driving proton pumping and ATP synthesis. If PSI is nonfunctional, linear electron flow stops, reducing proton gradient formation and ATP synthesis. However, PSI can also operate in cyclic photophosphorylation mode, producing ATP without NADPH. If PSI is inactive, cyclic photophosphorylation cannot occur, further reducing ATP production
- Sugar synthesis would decrease due to lack of NADPH and ATP. The Calvin cycle requires both ATP and NADPH to fix carbon dioxide into sugars. With PSI inactive, NADPH is not produced and ATP synthesis is impaired, limiting the Calvin cycle's ability to generate sugars
In summary, PSI inactivity leads to a sharp decline in NADPH, a reduction in ATP (especially from cyclic photophosphorylation), and consequently, a decrease in sugar production in the chloroplast
