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 These cells are part of the characteristic "kranz leaf anatomy", which spatially separates photosynthetic cell-types in a concentric arrangement to accumulate CO near RuBisCO. These chemical and anatomical mechanisms improve the ability of RuBisCO to fix carbon, rather than perform its wasteful oxygenase activity. The RuBisCO oxygenase activity, called photorespiration, causes the RuBP substrate to be lost to oxidation, and consumes energy in doing so. The adaptations of C4 plants provide an advantage over the C3 pathway, which loses efficiency due to photorespiration. The ratio of photorespiration to photosynthesis in a plant varies with environmental conditions, since decreased CO and