|
The light-independent reactions of photosynthesis are chemical reactions that convert carbon dioxide and other compounds into glucose. These reactions occur in the stroma, the fluid-filled area of a chloroplast outside of the thylakoid membranes. These reactions take the products (ATP and NADPH) of light-dependent reactions and perform further chemical processes on them. There are three phases to the light-independent reactions, collectively called the Calvin cycle: carbon fixation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration. Despite its name, this process occurs only when light is available. Plants do not carry out the Calvin cycle during nightime. They instead release sucrose into the phloem from their starch reserves. This process happens when light is available independent of the kind of photosynthesis (C3 carbon fixation, C4 carbon fixation, and Crassulacean acid metabolism); CAM plants store malic acid in their vacuoles every night and release it by day in order to make this process work. ==Coupling to other metabolic pathways == These dark reactions are closely coupled to the thylakoid electron transport chain as reducing power provided by NADPH produced in the photosystem I is actively needed. The process of photorespiration, also known as C2 cycle, is also coupled to the dark reactions, as it results from an alternative reaction of the Rubisco enzyme, and its final byproduct is also another glyceraldehyde-3-P. The Calvin cycle, Calvin–Benson–Bassham (CBB) cycle, reductive pentose phosphate cycle or C3 cycle is a series of biochemical redox reactions that take place in the stroma of chloroplasts in photosynthetic organisms. It is also known as the light-independent reactions. The cycle was discovered by Melvin Calvin, James Bassham, and Andrew Benson at the University of California, Berkeley by using the radioactive isotope carbon-14. It is one of the light-independent reactions used for carbon fixation. Photosynthesis occurs in two stages in a cell. In the first stage, light-dependent reactions capture the energy of light and use it to make the energy-storage and transport molecules ATP and NADPH. The light-independent Calvin cycle uses the energy from short-lived electronically excited carriers to convert carbon dioxide and water into organic compounds that can be used by the organism (and by animals that feed on it). This set of reactions is also called ''carbon fixation''. The key enzyme of the cycle is called RuBisCO. In the following biochemical equations, the chemical species (phosphates and carboxylic acids) exist in equilibria among their various ionized states as governed by the pH. The enzymes in the Calvin cycle are functionally equivalent to most enzymes used in other metabolic pathways such as gluconeogenesis and the pentose phosphate pathway, but they are to be found in the chloroplast stroma instead of the cell cytosol, separating the reactions. They are activated in the light (which is why the name "dark reaction" is misleading), and also by products of the light-dependent reaction. These regulatory functions prevent the Calvin cycle from being respired to carbon dioxide. Energy (in the form of ATP) would be wasted in carrying out these reactions that have no net productivity. The sum of reactions in the Calvin cycle is the following: :3 + 6 NADPH + 5 + 9 ATP → glyceraldehyde-3-phosphate (G3P) + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi (Pi = inorganic phosphate) Hexose (six-carbon) sugars are not a product of the Calvin cycle. Although many texts list a product of photosynthesis as , this is mainly a convenience to counter the equation of respiration, where six-carbon sugars are oxidized in mitochondria. The carbohydrate products of the Calvin cycle are three-carbon sugar phosphate molecules, or "triose phosphates," namely, glyceraldehyde-3-phosphate (G3P). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Light-independent reactions」の詳細全文を読む スポンサード リンク
|