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The Light-independent Reactions (Calvin Cycle)
The molecules of the Calvin Cycle:
Carbon dioxide (CO2) Enters the leaf through the stomata from the air, then enters the
stroma of the chloroplast. It is fixed to RuBP.
Ribulose bisphosphate (RuBP) a 5-carbon sugar. It is the CO2 acceptor in the carbon
Ribulose bisphosphate carboxylase-oxygenase (RUBISCO) The enzyme that catalyses
carbon fixation to RuBP. The most abundant protein on earth.
3-Phosophoglyceric acid (3-PG) the result of splitting the unstable 6-carbon product of
the carbon fixation stage. It is a 3-carbon sugar, and the first intermediate of the reduction
Adenosine triphosphate (ATP) the main source of energy for the Calvin Cycle. It is a
product of the light-dependant reactions, and phosphorylates other molecules (adds an
inorganic phosphate group to them), and becomes Adenosine diphosphate (ADP).
1,3-Biphosphoglycerate - a 3-carbon sugar, it is the second intermediate of the reduction
phase and has a phosphate group on both ends.
Nicotinamide adenine dinucleotide phosphate (reduced NADP) provides energy by
reducing 1,3-biphosphoglycerate, and it then becomes NADP. It is a product of the
Gyceraldehyde-3-phosphate (G3P) the end product of the Calvin Cycle. It is the simplest
sugar known. For every six G3P produced, five are recycled to regenerate RuBP and one
exits the cycle and is used to make sugars, fatty acids, amino acids etc.
Cycle diagram and explanation on the following page....
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The Calvin Cycle Process:
CO2 enters the stroma of the chloroplast via the stomata in the leaves. Rubisco fixes CO2 to
RuBP to create an unstable 6-carbon molecule that instantly splits into two 3-carbon
molecules of 3-PG.
ATP phosphorylates each 3-PG molecule, creating 1,3-bisphosphoglycerate. This results in
ATP (adenosine triphosphate) losing a phosphate group and so becoming ADP (adenosine
NADPH reduces 1,3-bisphosphoglycerate, causing the phosphate group to split off again.
The molecule then picks up a proton (H+) and becomes glyceraldehyde-3-phosphate.…read more
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For every six molecules of G3P created five molecules continue on to phase 3 while one
leaves to be used for organic compounds. ATP is once again needed for regeneration of
RuBP by phosphorylation of G3P.…read more