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Dark reactions

Interactive diagram The Calvin cycle incorporates carbon dioxide into sugar molecules.

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The Calvin cycle, also known as the dark reactions, is a series of biochemical reactions that fixes CO₂ into G3P sugar molecules and uses the energy and electrons from the ATP and NADPH made in the light reactions. The Calvin cycle takes place in the stroma of the chloroplast.^[1]

Carbon fixation and G3P synthesis

The Calvin cycle starts by using the enzyme Rubisco to fix CO₂ into five-carbon Ribulose bisphosphate (RuBP) molecules. The result is unstable six-carbon molecules that immediately break down into three-carbon molecules called 3-phosphoglyceric acid, or 3-PGA. The ATP and NADPH made in the light reactions is used to convert the 3-PGA into glyceraldehyde-3-phosphate, or G3P sugar molecules. Most of the G3P molecules are recycled back into RuBP using energy from more ATP, but one out of every six produced leaves the cycle—the end product of the dark reactions.^[1]

Sugar synthesis

Glyceraldehyde-3-phosphate can double up and form glucose-1-phosphate, glucose-6-phosphate or fructose-6-phosphate molecules which each include a phosphate group, or be transported into the cytoplasm to double up and form glucose.^[2]

Photorespiration

Photorespiration can occur when the oxygen concentration is too high. Rubisco cannot distinguish between oxygen and carbon dioxide very well, so it can accidentally add O₂ instead of CO₂ to RuBP. This process reduces the efficiency of photosynthesis—it consumes ATP and oxygen, releases CO₂, and produces no sugar. It can waste up to half the carbon fixed by the Calvin cycle.^[3] Several mechanisms have evolved in different lineages that raise the carbon dioxide concentration relative to oxygen within the chloroplast, increasing the efficiency of photosynthesis. These mechanisms are called carbon dioxide concentrating mechanisms, or CCMs. These include Crassulacean acid metabolism, C₄ carbon fixation,^[3] and pyrenoids.

^ ^a ^b Cite error: The named reference Biology-Campbell&Reece-200-201 was invoked but never defined (see the help page).
^ Clarke, Jeremy M. Berg; John L. Tymoczko; Lubert Stryer. Web content by Neil D. (2002). Biochemistry (5. ed., 4. print. ed.). New York, NY [u.a.]: W. H. Freeman. pp. Section 20.1. ISBN 0-7167-3051-0.{{cite book}}: CS1 maint: multiple names: authors list (link)
^ ^a ^b Cite error: The named reference CampbellBiology-Photosynthesis was invoked but never defined (see the help page).

[Biology-Campbell&Reece-200-201-1] Cite error: The named reference Biology-Campbell&Reece-200-201 was invoked but never defined (see the help page).

[Biochemistry—Calvin_cycle-2] Clarke, Jeremy M. Berg; John L. Tymoczko; Lubert Stryer. Web content by Neil D. (2002). Biochemistry (5. ed., 4. print. ed.). New York, NY [u.a.]: W. H. Freeman. pp. Section 20.1. ISBN 0-7167-3051-0.{{cite book}}: CS1 maint: multiple names: authors list (link)

[CampbellBiology-Photosynthesis-3] Cite error: The named reference CampbellBiology-Photosynthesis was invoked but never defined (see the help page).

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