BIO NOTES: What is the terminal process involved in the fixation of CO2 at the level of carbohydrate?
What is the terminal process involved in the fixation of CO2 at the level of carbohydrate?
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CO2 fixation takes place in Dark Reaction
Dark reaction is otherwise known as Blackman’s reaction
-Dark reaction takes place in the stroma region of the chloroplast
-Dark reaction doesn’t require the presence of light in other wards presence or absence of light doesn’t affect the reaction
-Dark reaction is a thermo chemical reaction
-Dark reaction is totally enzymatic reaction.
-The product of light reaction is necessary to drive dark reaction .In other words without the product of light reaction dark reaction would n’t takes place
-CO2 reduction takes place only in the dark reaction
-The ATP and NADPH2 synthesized in light reaction are essential to reduce CO2 in the dark reaction
-Dark reaction is called Calvin cycle in commemoration of Melvin Calvin who found the sequence of dark reaction
-Dark reaction takes place in three stages 1.Carboxalation 2.Reduction 3. Regeneration of RUBP
Fixation
3 molecules of CO2 took part in each calvin cycle
The cycle has to be repeated twice in order to produce one molecule of ( 6C –sugar) glucose
In Calvin cycle CO2 acceptor molecule is ribulose- 1,5-bisphosphate a 5-Carbon compound
This reaction is Catalyzed by the enzyme RUBP Carboxylase (Rubisco)
As a result of this reaction a highly unstable an intermediate 6-Carbon compound (2-Carboxy-3-ketoarabinital 1,5-bis phosphate) is formed
This unstable 6-Carbon compound splits into two molecules of 3-Carbon PGA (Phosphoglyceric acid )
This process is called carbon fixation,
Reduction
In the reduction stage this 2 molecules of 3-PGA are converted into 2 molecules of G-3-P (Glyceraldehyde -3-phosphate )
Reduction of PGA takes place in two steps
This reaction requires light generated ATP and NADPH,
Step-I
2 molecules of 3-PGA is phosphorylated by 2 molecules of ATP, and become 2-molecules of 1,3-bisphosphoglyceric acid by enzyme PGA kinase and ATP becomes ADP
Step-II
The 2 molecules of 1,3 bisphosphoglyceric acid are reduced into two molecules of Glyceraldehyde -3- phosphate by the enzyme Glyceraldehyde -3- phosphate dehydrogenase with the help of 2 molecules of NADPH2 and NADPH2 becomes NADP
Thus to reduce one molecule of CO2 two ATP and two NADPH2 are required up to the reduction stage
NADP+ and inorganic phosphate Pi are come out as by-products.
Regeneration
Out of six molecule of RuBP Five molecules of Glyceraldehyde -3- phosphate molecules are converted back to RuBP (Ribulose –bisphosphate) RuBP in each cycle
Out of the 6 molecules of Glyceraldehyde -3- phosphate are produced in each calvin cycle 5 molicules are cycled back for the regeneration of RuBP(5C)
1 molecule of Glyceraldehyde -3- phosphate (3C) is the net gain in each calvin cycle from 3molecules CO2
If the cycle repeated twice then 2 molecules of Glyceraldehyde -3- phosphate /DHAP becomes the net gain which inturn combines to form one molecule of glucose
Thus to produce one molicule calvin cycle has to be repeated another time
Regeneration process
Glyceraldehyde -3- phosphate(3C) molecules are converted to RuBP(5C) through series of reactions which generate 4C,6C, and 7C phosphorylated intermediates compounds
1.Some of the molecules of G-3-P are converted to DHAP both are isomers
2.G-3-P (3C)combines with DHAP (3C)and form fructose1,6 –bisphosphate (6C) by the enzyme aldolase
3.Fructose1,6 –bisphosphate (6C)undergoes dephosphorylation to form Fructose 6 –phosphate (6C) by the enzyme phosphatase and 1 inorganic phosphate (Pi) let out
4.Fructose 6 –phosphate(6C) combines with G-3-P (3C) from the fixation of second molecule of CO2 to form Xylulose 5-phosphate (5C) and Erthrose 4-phosphate (4C) by transketolase
5.Erthrose 4-phosphate (4C) combines with DHAP (3C) obtained from second Co2 fixation to form Sedoheptulose 1,7- bisphosphate (7C) by aldolase
6.Sedoheptulose 1,7- bisphosphate (7C) undergoes dephosphorylation to form Sedoheptulose 7- bisphosphate and Pi let out by phosphatase
7.Sedoheptulose 7- bisphosphate(7C) combines with G-3-P(3C) obtained from third CO2 fixation to form Ribose 5- phosphate (5C) and Xylulose 5-phosphate (5C) by transketolase
8.Two molecules of Xylulose 5-phosphate (5C) are transformed into two molecules of Ribulose 5-phosphate (5C)( Ru5P) by Epimerase
9.One molecule of Ribose 5- phosphate (5C) transformed into one molecules of Ribulose 5-phosphate (5C)( Ru5P) by isomerase
10.Three molecules Ribulose 5- phosphate (5C)( Ru5P) thus formed by regeneration are phosphorylated in to Ribulose 1,5- bisphosphate by 3ATP molecules with help of kinase
-For the three molecules of CO2 fixed net gain is only one molecule of DHAP (3C)
-Thus to produce one glucose molecule the cycle has to be repeated twice.
-Two triose phosphate combines to form one molecule of Glucose (6C)
-For every carbon fixation 3ATP and 2NADPH2 molecules are utilized
- thus for the production one molecule of glucose that is for the reduction of 6 carbon -di -oxide molecules the reaction needs 18ATP (12 ATP in fixation and 6ATP in regeneration reactions)and 12NADPH2
In non-cyclic photophosphorylation NADPH+H+ are produced in the ratio of 1:1.The fixation of CO2 into hexose sugar needs NADPH+H+ in the ratio of 1:1:5. The extra requirement of ATP is probably filled by cyclic photophosphorylation
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