Photosynthesis Earth's atmosphere, and supplies all of the organic compounds and most of the energy necessary for life on Earth. Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting
Fractionation of carbon isotopes in oxygenic photosynthesis product 3-phosphoglycerate is depleted in C due to the kinetic isotope effect of the above reaction. The overall C fractionation for C3 photosynthesis ranges between -20 to -37‰. The wide range of variation in delta values expressed in C3 plants is modulated by the stomatal conductance, or the rate of CO entering, or water vapor exiting, the small pores in the epidermis of a leaf. The δC of C3 plants depends on the relationship between stomatal conductance and photosynthetic rate, which is a good proxy of water use efficiency in the leaf. C3 plants with high water-use efficiency tend to