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REGULATION OF C3
“Synthesis using light.”
Light energy is converted into chemical energy in the form
of sugar and carbohydrate.
Occurs in chloroplasts of the cell.
Involves two reaction which is Light-dependent reaction
and Light-independent reaction.
Occurs on thylakoid which contains antenna pigment.
The main purpose of the light reaction is to generate
organic energy molecules such as “ATP” and “NADPH”
which are needed for the subsequent dark reaction.
It consist of four major protein complexes i.e.
Photosystem2 Cytochrome b6f complex Photosystem1
Chlorophyll pigments are excited and give up their
electrons and to compensate for the loss of electrons,
water is oxidized and protons are released by PS2 in
PS1 reduces NADP to NADPH in stroma
Protons are also transported from cyt b6f complex for proton
These protons then diffuses into ATP synthase enzyme where
their e.p gradient is used to synthesize ATP.
Plastoquinone and plastocyanin are electron carrier which
helps in transfering e- to cyt b6f and PS1.
The Calvin cycle is named after Melvin Calvin, who was an
Calvin was also awarded wit Nobel prize in Chemistry in
the year 1961.
Calvin along with James Benson and Andrew Bassham
worked at the University of California, U.S.A.
They fed Chlorella and Scenedesmus with radioactive 14C
in carbon dioxide and traced their co2 fixation path during
Overview of the Cycle
Cyclical process occurs in 3 phases
In this step 3 mol Co2 reacts with 3 mol of RuBP to obtain
6mol of 3-PGA (stable compound).
Reaction type – synthesis.
Enzyme –Ribulose-1,5-bisphosphate caboxylase.
Energy is absorbed.
REDUCTION OF 3PGA
In this step 6mol of 3-PGA is converted into 6mol of GAP.
It involves usage of NADPH and ATP (light reaction)
ATP is used to phosphorylate each 3-PGA and converts
Reaction type- phosphorylation.
Enzyme – PGAkinase.
Energy is absorbed.
REDUCTION OF 1,3-bisPGA
Electrons from NADPH is used to reduce 1,3-bisPGA.
Results in a carbonyl group which is sugar.
NADPH used synthesizes Glyceraldehyde 3-phosphate (GAP)
Reaction type – redox
Enzyme – Glyceraldehyde 3-phosphate dehydrogenase.
In this phase 1 of the 6 mol of GAP exits the cycle to eventually become
glucose and other type of organic compounds.
The remaining 5 mol of GAP continue in the cycle to regenerate the
10 enzymes are used and is the last and largest set of reactions.
2mol of GAP is converted to 2 mol of Dihydroxyacetone-3
phosphate (DHAP) via triose phosphate isomerase .
Two hydrogen atoms are moved from the center CO group
to the CO group on the end. This moves the double bond
from the CO group on the end to the CO group in the
center of the molecule.
Aldolase catalyzes the aldol condensation of 3rd mol of
GAP with 1 mol of DHAP, yielding a six carbon sugar
FBP is then hydrolyzed to fructose-6-phosphate (F6-P) via
Transketolase transfers a 2-carbon section (C2H3O2)
from F6P to the 4th mol of GAP
yielding Xylulose 5-phosphate (Xu 5P)
and the remaining 4 carbons of F6P
Erythrose 4-phosphate (E4P).
Aldolase combines the E4P and DHAP molecules into a 7-
carbon sedoheptulose 1,7-bisphosphate molecule (SBP).
Sedoheptulose 1,7-bisphospha-tase converts SBP into
sedoheptulose 7-phosphate (S7P), using hydrolysis to de-
Transketolase transfers a
2-carbon section (C2H3O2) from
S7P molecule to the 5th mol of GAP
yielding Xu5P while the
remaining 5 carbon of S7P
Ribose-5-phosphate (R5P) molecule.
Phosphopentose isomerase converts ribose 5-phosphate
(R5P) into Ribulose-5-phosphate (Ru5P).
We now have one Ru5P and two Xu5P molecules.
Phosphopentose epimerase converts each of the two Xu5P
molecules into Ru5P.
3 mol of Ru5P.
Phosphoribulokinase converts 3 ATP molecules into ADP
in order to phosphorylate 3 Ru5P molecules into 3 mol of
This brings us back to where we started in step 1, which
completes the Calvin cycle.
To make 1 molecule of glucose it takes 6 turns.
Each turn uses 3 ATP and 2 NADPH which means 18ATP
and 12 NADPH to produce a single glucose molecule.
Plants uses these sugars for growth and development.
Plants Physiology by Taiz and Zeiger
Introductory Plant Physiology by G. Ray Noggle
and George J. Fritz
Plants Physiology by S.N Pandey and B.K Sinha