Page 160 - PG 101-Course notes مذكرة النظري 24-25 with spec

Medicinal plants (PG 101) Level 1 Clinical Pharmacy-PharmD

Steps of Calvin cycle

(1) Fixation of CO2:

1- One ATP molecule is used to make ribulose-1,5-bisphosphate (RuBP) from ribulose-
5-phosphate (Ru5P). This energises the sugar molecule

2- CO2 is added to RuBP via the enzyme rubisco (Ribulose Bisphosphate

Carboxylase/Oxidase) to form unstable six-carbon compound immediately breaks
down into two molecules of 3-phosphoglycerate (PGA) {the first stable product of
calvin cycle}.

(2) Reduction of 3-PGA

3- Two molecules of ATP add phosphates to the three-carbon acids to form 1,3-
bisphosphoglycerate (BPG) which are energised.

4- Two molecules of NADPH add electrons to (BPG) reducing them into
glyceraldehyde-3-phosphate (G3P).

(3) Regeneration of RuBP:

5- After three turns of Calvin cycle enough carbon has been fixed to allow one molecule
of (glyceraldehyde-3-phosphate) (G3P) to leave the cycle and be available to form
other types of sugars and leaving enough carbon to regenerate Ru5P (ribulose-5-
phosphate) to complete the cycle.

The Problem?

➢ Air contains 21% O 2 and 0.036% CO 2 (ratio 600:1)

➢ RuBISCO works fine under this condition.

➢ Some physiological or environmental conditions tend to change this ratio

promoting the oxygenase reaction (photorespiration)

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