Green plants are the only plants that produce oxygen and make food, which is called photosynthesis. Photosynthesis means ''putting together with light.'' This takes place in chloroplasts, which have chlorophyll in them. Chlorophyll absorbs the sunlight. From sunlight, green plants combine carbon dioxide and water to make sugar and oxygen. Green plants use sugar to make starch, fats, and proteins. There are tiny pores called stomota. Oxygen and carbon dioxide enter and leave through the stomata , cant rember which was in day and which was in night . Nearly all algae have photosynthetic machinery ultimately derived from the Cyanobacteria, and so produce oxygen as a by-product of photosynthesis