C-3

Left (click to enlarge) : The C-3 or reductive pentose phosphate cycle (RPP).

This reaction, in which carbon is fixed, reduced, and utilized, involves the formation of intermediate 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 regenerate 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-Pribulose 5-P + ATP → RuBP (more)

: animation :Table ~ Comparison Photosynthesis Respiration : Table ~ Comparison Plant Bacterial Photosynthesis : Table ~ comparison of C-3, C-4, CAM plants : Table ~ Photosynthesis Overview : Microtextbook Carbon Assimilation :: image_Calvin cycle : image_reductive tricarboxylic acid cycle : image_reductive CoEnzymeA cycle : image_3-hydroxypropionate cycle :: Saccharomyces cerevisiae carbon monoxide dehydrogenase pathway : Arabidopsis thaliana carbon monoxide dehydrogenase pathway : MetaCyc reductive acetyl coenzyme A pathway :