In photosynthesis, redox reactions ultimately transfer electrons from water to carbon dioxide.

Here's the breakdown:

* Water (H₂O) is the electron donor in photosynthesis. The water molecule is split, releasing electrons, protons (H+), and oxygen (O₂).

* Carbon dioxide (CO₂) is the electron acceptor. These electrons are used to reduce carbon dioxide into glucose (C₆H₁₂O₆), which is the energy source for the plant.

So, the overall flow of electrons is:

Water → Photosystem II → Photosystem I → NADPH → Carbon Dioxide

This entire process is powered by sunlight, which is absorbed by chlorophyll and used to energize the electrons.