Talk about CO2 fixation step.

The Rubisco ( ribulose - bisphosphate - carboxylase ), an enzyme representing more than 40 % of soluble protein in most leaves, plays a key role in the so-called C3 plants (most green plants such as trees, potato and beet) . This enzyme fixes each of the 3 CO2 molecules (3x1C) to 1 molecule of ribulose 1,5-diphosphate (RUBP 3x5C), a 5-carbon sugar. 6 molecules of a 3 carbons product, the phosphoglyceric acid (PGA 6x3C), are formed after passing through an unstable intermediate. The first product of the cycle contains 3 carbon atoms, which is why the Calvin cycle is also known as the C3 pathway.