Calvin cycle (C3 cycle) is divided into three distinct phases :

A. Carbon Fixation: CO2 molecules are added to 5-carbon compound, Ribulose 1,5-bisphosphate (RuBP) to generate an unstable 6-carbon compound that quickly dissipates into two 3-carbon compounds, 3-phosphoglyceric acid (3-PGA).

B. Reduction (glycolytic reversal): ATP and NADPH from the light reactions are used to convert 3-PGA molecules into a carbohydrate precursor, glyceraldehyde-3-phosphate (G3P).

Initially, two ATP are used for the conversion of two molecules of 3-phosphoglyceric acid to 1-3 bisphosphoglyceric acid and two molecules of NADPH are required for the conversion of two molecules of 1-3 bisphosphoglyceric acid to 3 phosphoglyceraldehyde (3-PGAL). 3–phosphoglyceraldehyde is converted into Fructose 1-6 bisphosphate and finally to Glucose.

C. Regeneration of RuBP: Some G3P molecules are used to regenerate RuBP, ensuring the cycle continues. 1 ATP is used to recycle one molecule of RuBP.

So, to fix one carbon dioxide molecule, 3 ATP and 2 NADPH molecules are utilized. Six cycles of Calvin are needed to synthsise one molecule of glucose. So, to fix six carbon dioxide molecules 18 ATP and 12 NADPH are required. The Calvin cycle is also called the C3 cycle as the first stable compound formed is a 3 carbon compound, 3-phosphoglyceric acid.