How does the plant get all of the reactants needed for photosynthesis?

The reactants of photosynthesis are CO2 or Carbon Dioxide and H2O or Water.

Water is gains from the air or from the ground or from droplets on leaves from rain. If from the ground then the plants roots suck up the water and it travels up the plant via the capillary action to the leaves where the chloroplasts are. Other wise water enters via osmosis.

CO2 enters the plant via simple diffusion into the cells of the plant leaves and eventually into the chloroplasts.