Chloroplasts An examination of leaves, stems, and other types of plant tissue reveals the presence of tiny green, spherical structures called chloroplasts, visible here in the cells of an onion. Chloroplasts are essential to the process of photosynthesis, in which captured sunlight is combined with water and carbon dioxide in the presence of the chlorophyll molecule to produce oxygen and sugars that can be used by animals. Without the process of photosynthesis, the atmosphere would not contain enough oxygen to support animal life.