Onion epidermal cell Onion epidermal cell The epidermal cells of onions provide a protective layer against viruses and fungi that may harm the sensitive tissues. Because of their simple structure and transparency they are often used to introduce students to plant anatomy or to demonstrate plasmolysis. The clear epidermal cells exist in a single layer and do not contain chloroplasts, because the onion fruiting body (bulb) is used for storing energy, not photosynthesis. Each plant cell has a cell wall, cell membrane, cytoplasm, nucleus, and a large vacuole. The nucleus is present at the periphery of the cytoplasm. The vacuole is prominent and
Evolution of photosynthesis Evolution of photosynthesis The evolution of photosynthesis refers to the origin and subsequent evolution of photosynthesis, the process by which light energy synthesizes sugars from carbon dioxide, releasing oxygen as a waste product. The process of photosynthesis was discovered by Jan Ingenhousz, a Dutch-born British physician and scientist, first publishing about it in 1779. The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or electrons, rather than water. There are three major metabolic pathways by which photosynthesis is carried out: C photosynthesis, C photosynthesis, and CAM photosynthesis.