What happens in Calvin cycle stage?

Answer 1

There are three main phases of the light-independent reactions: fixation, reduction, regeneration of RuBP.

1. During fixation, RuBisCO (ribulose 1,5-bisphosphate carboxylase/oxidase) fixates a carbon to RuBP (ribulose 1,5-bisphosphate) to make 3-phosphoglycerate. There are three RuBP's and each one makes two 3-phosphoglycerates. Ergo, there are six 3-phosphoglycerates.
2. During the reduction phase, 6 ATP and 6 NADPH are used to reduce the 3-phosphoglycerate to glyceraldehyde 3-phosphate (G3P). The ATP goes first and converts the 3-phosphoglycerate into 3-bisphosphoglycerate. Next, the NADPH reduces the 3-bisphosphoglycerate and converts it into G3P. An inorganic phosphate comes off and NADP+ and ADP are released back to the light reactions. There are 6 G3P molecules and one will be used to make the six carbon sugar, glucose.
3. During regeneration, 3 ATP are used to convert the G3P back into 3 RuBP's through a series of reactions. This will restart the cycle.

Two turns of the cycle will produce the six-carbon sugar product.

It is also important to note the amounts of carbon in each step.

1. (5C (RuBP) + 1C (CO2)) x 3 = 18C on six 3-carbon molecules (3-phosphoglycerate)
2. One G3P is used to make a sugar, so 18C - 3C = 15C on five 3-carbon molecules.
3. 5(3C) molecules --> 3 (5C) molecules (RuBP).

Answer 2

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