Calvin cycle: Difference between revisions
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<StructureSection load='1rcx' size='350' side='right' caption='Spinach RuBisCO 8 large and 8 small chains complex with substrate ribulose-1,5- bisphosphate, [[1rcx]]'> | <StructureSection load='1rcx' size='350' side='right' caption='Spinach RuBisCO 8 large and 8 small chains complex with substrate ribulose-1,5- bisphosphate, [[1rcx]]'> | ||
See also [https://en.wikipedia.org/wiki/Calvin_cycle]. | See also [https://en.wikipedia.org/wiki/Calvin_cycle] and [[Carbon Fixation]]. | ||
3 CO<sub>2</sub> + 6 NADPH + 6 H<sup>+</sup> + 9 ATP + 5 H<sub>2</sub>O → glyceraldehyde-3-phosphate (G3P) + 6 NADP<sup>+</sup> + 9 ADP + 8 P<sub>i</sub> (P<sub>i</sub> = inorganic phosphate) (See also [https://en.wikipedia.org/wiki/Calvin_cycle]) | 3 CO<sub>2</sub> + 6 NADPH + 6 H<sup>+</sup> + 9 ATP + 5 H<sub>2</sub>O → glyceraldehyde-3-phosphate (G3P) + 6 NADP<sup>+</sup> + 9 ADP + 8 P<sub>i</sub> (P<sub>i</sub> = inorganic phosphate) (See also [https://en.wikipedia.org/wiki/Calvin_cycle]) | ||
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''Step 3'' | ''Step 3'' | ||
The enzyme [[ | The enzyme [[Glyceraldehyde-3-Phosphate Dehydrogenase|glyceraldehyde-3-phosphate dehydrogenase]] catalyzes the reduction of <scene name='39/392339/Cv1/5'>1,3-bisphosphoglycerate</scene> by NADPH (which is another product of the light-dependent stage). <scene name='39/392339/Cv1/4'>Glyceraldehyde-3-phosphate</scene> (also called G3P, GP, TP, PGAL, GAP) is produced, and the NADPH itself is oxidized and becomes NADP+. Again, two NADPH are utilized per CO2 fixed. This reaction is reverse to that in the [[glycolysis]]. | ||
'''The next stage in the Calvin cycle is to regenerate RuBP'''. Five G3P molecules produce three RuBP molecules, using up three molecules of ATP. Since each CO2 molecule produces two G3P molecules, three CO2 molecules produce six G3P molecules, of which five are used to regenerate RuBP, leaving a net gain of one G3P molecule per three CO2 molecules (as would be expected from the number of carbon atoms involved). | '''The next stage in the Calvin cycle is to regenerate RuBP'''. Five G3P molecules produce three RuBP molecules, using up three molecules of ATP. Since each CO2 molecule produces two G3P molecules, three CO2 molecules produce six G3P molecules, of which five are used to regenerate RuBP, leaving a net gain of one G3P molecule per three CO2 molecules (as would be expected from the number of carbon atoms involved). | ||
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'''Light-dependent regulation''' | '''Light-dependent regulation''' | ||
These reactions do not occur in the dark or at night. There is a light-dependent regulation of the cycle enzymes, as the third step requires NADPH. There are two regulation systems at work when the cycle must be turned on or off: the thioredoxin/ferredoxin activation system, which activates some of the cycle enzymes; and the [[ | These reactions do not occur in the dark or at night. There is a light-dependent regulation of the cycle enzymes, as the third step requires NADPH. There are two regulation systems at work when the cycle must be turned on or off: the [[Thioredoxin|thioredoxin]]/[[Ferredoxin|ferredoxin]] activation system, which activates some of the cycle enzymes; and the [[RuBisCO]] enzyme activation, active in the Calvin cycle, which involves its own activase. | ||
The thioredoxin/ferredoxin system activates the enzymes [[Glyceraldehyde-3-Phosphate Dehydrogenase|glyceraldehyde-3-phosphate dehydrogenase]], glyceraldehyde-3-P phosphatase, [[fructose-1,6-bisphosphatase]], sedoheptulose-1,7-bisphosphatase, and ribulose-5-phosphatase kinase, which are key points of the process. This happens when light is available, as the ferredoxin protein is reduced in the [[Photosystem I]] complex of the thylakoid electron chain when electrons are circulating through it. | |||
</StructureSection> | </StructureSection> | ||
== References == | == References == | ||
<references/> | <references/> | ||