Photosynthesis can only occur when a plant has access to water, carbon dioxide, sunlight and chlorophyll. Plant cells naturally produce chlorophyll, and they draw carbon dioxide directly from the air. Water and sunlight must be acquired from external sources.
Plants take up carbon dioxide through apertures in their leaves called stomata. These dilate and constrict in response to ambient carbon in the air. They absorb water through their root systems. Sunlight is taken up by the leaves themselves, which are densely packed with chlorophyll-rich organelles called chloroplasts. These chloroplasts use the energy from sunlight to recombine the CO2 with the H2O to produce sugar and O2, which is molecular oxygen that is secreted through the stomata.