* Carbon dioxide (CO2): This is the source of carbon for building sugars. It enters the plant through tiny pores called stomata on the leaves.

* Water (H2O): This is a source of electrons and hydrogen ions for the light-dependent reactions. It is absorbed from the soil through the roots.

* Light energy: This is the energy source for photosynthesis, captured by chlorophyll in the chloroplasts. It comes from the sun.

These three substances are the inputs for photosynthesis.