page_title: Fractionation of carbon isotopes in oxygenic photosynthesis
section_title: Photosynthesis reactions
hierarchical_section_title: Fractionation of carbon isotopes in oxygenic photosynthesis / Photosynthesis reactions
context_page_description: Photosynthesis converts carbon dioxide to carbohydrates via several metabolic pathways that provide energy to an organism and preferentially react with certain stable isotopes of carbon. The selective enrichment of one stable isotope over another creates distinct isotopic fractionations that can be measured and correlated with among oxygenic phototrophs. The degree of carbon isotope fractionation is influenced by several factors, including the metabolism, anatomy, growth rate, and environmental conditions of the organism. Understanding these variations in carbon fractionation across species is useful for biogeochemical studies, including the reconstruction of paleoecology, plant evolution, and the characterization of food chains.
  Oxygenic photosynthesis is a metabolic pathway facilitated by autotrophs, including plants, algae, and cyanobacteria. This pathway converts inorganic carbon dioxide from the atmosphere or aquatic environment into carbohydrates, using water and energy from light, then releases molecular oxygen as a product.
context_section_description: The chemical pathway of oxygenic photosynthesis fixes carbon in two stages: the light-dependent reactions and the light-independent reactions.
  
  The light-dependent reactions capture light energy to transfer electrons from water and convert NADP⁺, ADP, and inorganic phosphate into the energy-storage molecules NADPH and ATP. The overall equation for the light-dependent reactions is generally:
  2 H₂O + 2 NADP⁺ + 3 ADP + 3 Pᵢ + light → 2 NADPH + 2 H⁺ + 3 ATP + O₂
  The light-independent reactions undergo the Calvin-Benson cycle, in which the energy from NADPH and ATP is used to convert carbon dioxide and water into organic compounds via the enzyme RuBisCO.
  The overall general equation for the light-independent reactions is the following:
  3 CO₂ + 9 ATP + 6 NADPH + 6 H⁺ → C₃H₆O₃-phosphate + 9 ADP + 8 Pᵢ + 6 NADP⁺ + 3 H₂O
  The 3-carbon products (C₃H₆O₃-phosphate) of the Calvin cycle are later converted to glucose or other carbohydrates such as starch, sucrose, and cellulose.
caption: Overview of the Calvin cycle and carbon fixation C3 Pathway