PS I:
Ferredoxin is a constituent of Photosystem I (PS I) in the process of photosynthesis.

Explanation:
Ferredoxin is a small iron-sulfur protein that plays a crucial role in the electron transport chain during photosynthesis. It acts as an electron carrier, transferring electrons from the light-dependent reactions to the Calvin cycle.

Photosystem I (PS I):
Photosystem I is one of the two photosystems in the thylakoid membrane of chloroplasts. It is responsible for absorbing light energy and transferring electrons to ferredoxin.

Function of Ferredoxin in PS I:
- In PS I, ferredoxin receives electrons from the primary electron donor, P700.
- Ferredoxin then transfers these electrons to the enzyme ferredoxin-NADP+ reductase (FNR).
- FNR uses these electrons to reduce NADP+ to NADPH, which is essential for the Calvin cycle to produce sugars.

Relationship to Hill Reaction and P680:
- The Hill reaction is a part of the light-dependent reactions of photosynthesis where water is split to release oxygen and produce ATP and NADPH.
- P680 is the reaction center chlorophyll in Photosystem II (PS II), which is involved in the initial step of the light-dependent reactions.
Therefore, Ferredoxin's role in photosynthesis is specifically associated with Photosystem I, where it functions as an electron carrier in the generation of reducing power for the Calvin cycle.