Illustration of photosynthesis in a leaf. Photosynthesis is the process by which most plants convert sunlight into chemical energy. The chemical energy is stored as sugars. It occurs in organelles called chloroplasts. Photosynthesis requires water, which is delivered via the vascular bundle at centre, and carbon dioxide, which diffuses through pores in the leaf surface known as stomata. Oxygen is a byproduct of the process and diffuses out of the leaf via the stomata.