* Water (H₂O): This is the source of electrons and protons used in the light reactions.

* Light energy (photons): This is the energy source that drives the entire process.

Key points to remember:

* Carbon dioxide (CO₂) is NOT a reactant in the light reactions. It is a reactant in the Calvin Cycle, which is the second stage of photosynthesis.

* Oxygen (O₂) is a product of the light reactions, produced when water is split to provide electrons.

In summary: Light reactions use light energy and water to produce ATP, NADPH, and oxygen as byproducts. These products are then used in the Calvin cycle to convert carbon dioxide into sugar.