C-3

Left (click to enlarge):
The C-3, Calvin, photosynthetic carbon reduction (PCR), reductive pentose phosphate cycle (RPP), carbon fixation.

This reaction takes place in the leaves of most plants. During C-3, carbon is fixed, reduced, and utilized via the formation of intermediate 3-carbon sugar phosphates in a cyclic sequence. One complete RPP cycle incorporates three molecules of carbon dioxide and produces one molecule of the three-carbon compound glyceraldehyde 3-phosphate.

The C-3 pathway proceeds in three stages:
1. CO2 fixation (carboxylation) by Rubisco,
2. Carbon reduction to (CH2O)
12 PGA + 12 ATP -> 12 bisPGA + 12 NADPH + 12 H+ -> 12 GAP + 12 NADP+ + 12 Pi, and
3. Regeneration of the CO2 acceptor moleucle (ribulose 1,5-bisphosphate).

Some of the glyceraldehyde 3-phosphate generated in the reductive stage undergoes gluconeogenesis to form glucose. In plants, glucose is converted to sucrose or starch for later use.

The C-3 reactions are sometimes called the "dark reactions" of photosynthesis because photon energy is not used directly – the reactions are light-independent. However, ATP and NADPH generated by light-dependent photophosphorylation reactions are required.

Table ~ comparison of C-3, C-4, CAM plants :

The reaction cycle employs combinations of different length sugar-phosphates and eventually regenerates RuBP in addition to sugar-phosphate for sucrose/starch synthesis.
3-GAP (3 C) → DHAP (3 C)
DHAP (3C) + GAP (3 C) → fructose-1,6-bisphosphate (6C)
fructose-1,6-bisP (6C) + H2O → fructose-6-P (6C) + Pi
fructose-6-P (6 C) + GAP (3 C) → Xylulose-5-phosphate (5 C) + erythrose-4-phosphate (4 C)
erythrose 4-P (4 C) + DHAP (3 C) → sedoheptulose 1,7-bisphosphate (7 C)
sedoheptulose 1,7-bisP + H2O → sedoheptulose 7-P + Pi
sedoheptulose 7-P (7 C) + GAP (3 C) → xylulose 5-phosphate (5 C) + ribose 5-phosphate (5 C)
xylulose 5-P or ribose 5-P → ribulose 5-P
ribulose 5-P + ATP → RuBP (more)

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