The Calvin Cycle: Light Independent Reactions: 2nd stage of Photosynthesis
What to know: The Calvin Cycle is anabolic, meaning it builds up molecules and sythesizes them using chemical energy.
Purpose of the Calvin Cycle... Why do we need to understand this? (I know you're thinking when is this ever going to matter to me?) Plants are autotrophs which are organisms that convert light energy into chemical energy, in order to make food and what they need to grow and reproduce. Plants are a vital part of our ecosystem because they produce oxygen, which humans and other animals need to survive. Moreover, organisms present on earth are carbon based. The carbon atoms present in our bodies (in the form of sugars or other molecules) and in other life forms, come from The Calvin Cycle.
What the cycle uses to function: ATP and NADPH *** ATP: Adenosine triphosphate is a form of chemical energy, which is used as a reducting agent and for the regeneration of RuBP ***NADPH is a cofactor used in anabolic reactions, and also as a reducing agent (sounds confusing?...Keep reading, it all comes together) The Calvin Cycle uses these products from the light reactions as input to generate new products.
Key Phrases:
Reduce: refers to the addition of a hydrogen molecule or the removal of an oxygen from a molecule, which causes it to lose an electron. This goes hand and hand with oxidation.
Oxidative Phosphorylation: metabolic pathway that uses enzymes to oxidize molecules in order to release energy, which is then used to reform ATP. This often takes place in the mitochondira of eukaryotic organisms and especially in aerobic organisms.
Redox Reactions: oxidative-reduction reactions used in the calvin cycle. The basic steps/phases: 1) Carbon Fixation 2) Reduction 3) Regeneration
Carbon Fixation: The process which is in charge of fixing a carbon into place; taking an inorganic CO2 molecule and attaching it to an organic molecule like a carbohydrate. The enzyme that speeds up this reaction and fixes the carbon is called Ribulose-1,5-bisphosphate carboxylase aka Rubisco. So, CO2 combines with a 5 carbon sugar ribulose biphosphate with the help of Rubisco. This results in a 6 carbon compound which will break down into 2 molecules of 3- phosphoglyceric acid (PGA).
Reduction: The 3-PGA molecules are then reduced by NADPH and ATP to form molecules of a 3 carbon sugar referred to as glyceraldehyde-3-phosphate (G3P) AKA (PGAL). This reaction is catalyzed by the enzyme phosphoglycerate kinase.
Regeneration: Some G3P is recycled to regernerate RuBp to keep the calvin cycle going and some is used to produce glucose and other sugars. G3P is rearranged to dihydroxyaceton phoshpate (DHAP-3 carbon sugar) which is then catalyzed by various enzymes and used to regenerate 3 RuBP, which then enables the system for more CO2 molecules to be fixed. Ultimately, there are 15 carbons present in teh RuBP and only one G3P sugar is available for conversion to a sugar and is sent away to be used by the plant.