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The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle [1] of photosynthesis is a series of chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. The Calvin cycle is present in all photosynthetic eukaryotes and also many ...
The free energy created is then used, via a chain of nearby electron acceptors, for a transfer of hydrogen atoms (as protons and electrons) from H 2 O or hydrogen sulfide towards carbon dioxide, eventually producing glucose. These electron transfer steps ultimately result in the conversion of the energy of photons to chemical energy.
C 3 carbon fixation is the most common of three metabolic pathways for carbon fixation in photosynthesis, the other two being C 4 and CAM. This process converts carbon dioxide and ribulose bisphosphate (RuBP, a 5-carbon sugar) into two molecules of 3-phosphoglycerate through the following reaction: CO 2 + H 2 O + RuBP → (2) 3-phosphoglycerate
There are also many varieties of anoxygenic photosynthesis, used mostly by bacteria, which consume carbon dioxide but do not release oxygen. [13] [14] Carbon dioxide is converted into sugars in a process called carbon fixation; photosynthesis captures energy from sunlight to convert carbon dioxide into carbohydrates.
Glucose (blood sugar) is distributed to cells in the tissues, where it is broken down via cellular respiration, or stored as glycogen. [3] [4] In cellular (aerobic) respiration, glucose and oxygen are metabolized to release energy, with carbon dioxide and water as endproducts. [2] [4]
Ultimately almost all biomolecules come from the assimilation of carbon dioxide in plants and microbes during photosynthesis. [67]: 359 The free energy of formation of α-d-glucose is 917.2 kilojoules per mole. [67]: 59 In humans, gluconeogenesis occurs in the liver and kidney, [88] but also in other cell types.
The reaction for the aerobic respiration is essentially the reverse of photosynthesis, except that now there is a large release of chemical energy which is stored in ATP molecules (up to 38 ATP molecules are formed from one molecule of glucose and 6 O 2 molecules). The simplified version of this reaction is: C 6 H 12 O 6 + 6 O 2 → 6 CO 2 + 6 H
Conversely, plants undergo the Calvin Cycle to photosynthesize glucose-3-phosphate from CO 2 and H 2 O in the presence of light; the phosphate is quickly hydrolyzed into glucose. [12] Glucose to Glycogen Pathway. Digestion of complex carbohydrates allows glucose molecules to be re-polymerized into a form that is recognized by enzymes. [12]