The Calvin cycle (C3 pathway) occurs in the stroma of the chloroplast and does not directly need light. Its three phases are: (1) Carboxylation - carbon dioxide combines with a 5-carbon acceptor RuBP, catalysed by the enzyme RuBisCO, to form two molecules of 3-carbon 3-phosphoglyceric acid (3-PGA). (2) Reduction - 3-PGA is reduced to G3P (a sugar) using ATP and NADPH from the light reaction. (3) Regeneration - some G3P is used to regenerate the acceptor RuBP so the cycle can continue, using ATP. Glucose is formed from the G3P produced.

(1) In C3 plants the first stable product of carbon fixation is the 3-carbon compound 3-PGA, whereas in C4 plants it is the 4-carbon compound oxaloacetic acid (OAA). (2) C3 plants use only the Calvin cycle, while C4 plants use the Hatch-Slack pathway in addition to the Calvin cycle. (3) C4 plants have a special leaf anatomy called Kranz anatomy with bundle-sheath cells and are more efficient photosynthetically (e.g. maize, sugarcane), while C3 plants lack Kranz anatomy (e.g. rice, wheat).

The main photosynthetic pigments are chlorophyll a (the primary pigment, bluish-green), chlorophyll b (yellowish-green) and the carotenoids (carotenes and xanthophylls, which are yellow to orange). Chlorophyll a is the chief pigment that traps light and drives the light reaction. Chlorophyll b and the carotenoids are accessory pigments: they absorb light at wavelengths that chlorophyll a cannot and pass the energy to chlorophyll a, and carotenoids also protect chlorophyll from harmful photo-oxidation.