If photosystem I (PSI) stopped working, the concentrations of ATP, NADPH, and sugars would all decrease.

Quick Scoop: Direct Answer

  • ATP: Decrease
  • NADPH: Decrease
  • Sugars (from Calvin cycle): Decrease

Why this happens (short version)

  1. PSI is needed to make NADPH
    • PSI accepts electrons from the electron transport chain and ultimately reduces NADP⁺ to NADPH.
    • If PSI stops, this final reduction step cannot happen, so NADPH production stops and its concentration falls.
  1. ATP drops as the light reactions stall
    • The normal “Z‑scheme” electron flow depends on electrons moving from water → PSII → ETC → PSI → NADP⁺.
    • When PSI is blocked, electron flow backs up, the proton gradient across the thylakoid membrane is reduced, and photophosphorylation (ATP formation) decreases , so ATP levels go down.
  1. Sugar synthesis slows down
    • The Calvin cycle uses ATP and NADPH to fix CO₂ into sugars.
    • With both ATP and NADPH in short supply, the Calvin cycle cannot run effectively, so sugar production decreases and sugar concentration falls.

Tiny mental picture

You can imagine photosynthesis like a factory line:

  • PSII = the starter station providing electrons and some proton pumping.
  • PSI = the finishing station that turns “charged” electrons into NADPH.
  • Calvin cycle = the assembly floor that uses ATP + NADPH to build sugars.

If the finishing station (PSI) shuts down, the electrons have nowhere meaningful to go, energy flow through the line collapses, and the assembly floor runs out of supplies.
Result: less ATP, less NADPH, fewer sugars.

TL;DR:
If PSI stops working: ATP ↓, NADPH ↓, sugars ↓.

Information gathered from public forums or data available on the internet and portrayed here.