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
Fractionation of carbon isotopes in oxygenic photosynthesis elevated O concentrations would increase the efficiency of photorespiration. Atmospheric CO on Earth decreased abruptly at a point between 32-25 million years ago. This gave a selective advantage to the evolution of the C4 pathway, which can limit photorespiration rate despite the reduced ambient CO. Today, C4 plants represent roughly 5% of plant biomass on Earth, but about 23% of terrestrial carbon fixation. Types of plants which use C4 photosynthesis include grasses and economically important crops, such as maize, sugar cane, millet, and sorghum. Isotopic fractionation is differs between C4 carbon fixation and C3, due to the spatial separation in