A: Order of reaction is evaluated from the mechanism of a reaction
R: Order of reaction can be zero

Explanation

1) The reactions of photosynthesis which do not require light energy are called dark reactions. These reactions occur both in presence and absence of light.
2) Dark reactions occur in the stroma of the chloroplast and these were discovered by American scientist Melvin Calvin.
3) In the first reaction carbon dioxide is accepted by Ribulose 1- 5 biphosphate, a five carbon sugar to form hexose sugar.
4) It is an unstable compound so it breaks down into two molecules of three carbon compounds called phospho glyceric acid (PGA).
`CO_2` + Ribulose biphosphate `to` Unstable compound Unstable hexose sugar `to` 2 PGA
6) Phospho glyceric acid undergoes a series of reactions and is converted to glyceraldehyde - 3 phosphate. NADPH and ATP produced in light reactions and used up at this stage.
`2PGA underset("NADPH")overset("ATP")to` Glyceraldehyde 3 - phorphase
7) Two molecules of glyceraldehyde -3 phosphate combine together to form glucose molecule.
8) In the end glucose is converted into starch.
Summary reaction :
`CO_2` + Ribulose 1 -5 biphosphate `to` Hexose sugar
Hexose sugar (6c) `to` 2 PGA (3c)
PGA `to` glyceralde -3 phosphate
glyceraldehyde -3 phosphate `to` glucose
glucose `to` starch