Carbon Fixation in Plants Flashcards
What are primary producers?
Photosynthetic organisms, turning CO2 into biomass using the energy of the sun
The process where photosynthesisers fix CO2 into carbohydrate = the light independent reaction
Also called the Calvin cycle or the reductive pentose phosphate pathway
It is an autocatalytic cycle
How was the Calvin cycle discovered?
Using algae and adding radioactively labelled 14CO2, in order to trace the carbon and determine the products produced
What is the first stage of the Calvin cycle?
Carboxylation:
Ribulose-1,5-bisphosphate (RuBP) is carboxylated with CO2
Using Ribulose bisphosphate carboxylase (Rubisco)
Forms 3-phosphoglycerate (3PG)
3x RuBP + 3x CO2 = 6 x 3PG
Describe Rubisco?
Most abundant protein in the biosphere
8 large subunits and 8 small subunits
L8S8 has D4 symmetry (symmetry of a square prism)
It requires an Mg2+ cofactor to stabilse negative charges
What is the mechanism of Rubisco?
The reaction proceeds via an enediolate intermediate that nucleophilically attacks CO2 to form a beta-keto acid
The intermediate reacts with water to form 2 molecules of 3PG
What is the second stage of the Calvin cycle?
Phosphorylation of 3PG:
3-phosphoglycerate is catalysed by phosphoglycerate kinase and 6x ATP to form 1,3-bisphosphoglycerate (BPG)
6x 3PG = 6x 1,3-BPG
What is the third stage of the Calvin cycle?
1,3-Bisphosphoglycerate is catalysed by glyceraldehyde-3-phosphate dehydrogenase and 6x NADPH to form glyceraldehyde-3-phosphate (GAP)
6x 1,3-BPG = 6x GAP
What happens to the 6x GAP after stage 3 of the Calvin cycle?
1x GAP from 3CO2 goes off for carbohydrate synthesis
5x GAP needs to be resynthesized into 3x RuBP
What is stage 4 of the Calvin cycle?
The start of regeneration of RuBP:
GAP -> DHAP using TIM
DHAP can be converted into fructose-1,6-bisphosphate using aldolase
OR
combining with Erythrose-4-phosphate (E4P) to form Sedoheptulose-1,7-bisphosphate (SBP)
E4P is regenerated later
How are the remaining 5x GAP resynthesised in stage 5 of the Calvin cycle?
The 5x 3C molcules are shuffled around in order to regenerate 3x 5C
DHAP + GAP -> F1,6BP (aldolase)
GAP + F1,6BP -> Erythrose-4-phosphate, E4P + Xu5P (transketolase)
DHAP + E4P -> sedoheptulose-1,7-bisphosphate, S1,7BP (aldolase)
GAP + S1,7BP -> Xu5P + R5P (transketolase)
What is stage 6 of the Calvin cycle?
The final regeneration:
Using Xu5P and R5P from the previous reactions - they are converted back to ribulose-1,5-bisphosphate in order to continue the cycle
Ribose-5-phosphate (R5P) -> Ribulose-5-phosphate (Ru5P) (using ribose phosphate isomerase)
Xylulose-5-phosphate (Xu5P) -> Ribulose-5-phosphate (Ru5P) (using phosphopentose epimerase)
Finally:
Ribulose-5-phosphate (Ru5P) -> RuBP (using Phosphoribulose kinase and ATP hydrolysis)
Give a rundown of the molcules in the Calvin cycle?
Ribulose-1,5-bisphosphate (RuBP)
3-phosphoglycerate (3PG)
1,3-bisphosphoglycerate
Glyceraldehyde-3-phosphate
Intermediates: DHAP, F1,6BP, S1,7BP, E4P
Xylulose-5-phosphate (Xu5P)
Ribose-5-phosphate (R5P)
Ribulose-5-phosphate (Ru5P)
What is the overal stoichiometry of the Calvin cycle?
3 CO2 + 9 ATP + 6 NADPH → GAP + 9 ADP + 8 Pi + 6 NADP+
How is the Cavlin cycle regulated?
The cycle needs ATP and NADPH produced by photosynthetic electron transport and photophosphorylation
Therefore the activity of the cycle is regulated by photosynthetic electron transport rates (the redox potential)
It is controlled indirectly by light
Ferredoxin (which is the electron acceptor from PSI) is the source of the electrons
How can the Calvin cycle be controlled?
Control of Fructose-1,6-bisphosphate
Ferredoxin thioredoxin reductase (intermediate) with a disulphide bond - this reduces thiol groups of thioredoxin
Thioredoxin reduces the target enzyme bisphosphatase i.e. the reduced state with disulphide bonds is active
Many enzymes within the cycle are regulated in this way
How can the Calvin cycle be controlled by pH and Mg2+?
Dark - pH 7 and 1-3 mM Mg2+
Light - pH 8 and 3-6 mM Mg2+
Rubisco has an optimum pH around 8 and Mg2+ increase stimulates it as well
pH increases as protons are being moved into the thylakoid lumen
This allows several Calvin cycle enzymes to be activated
What are some outcomes of the GAP used for synthesis?
Converted into F6P and then G1P (by phosphoglucose isomerase and phosphoglucomutase)
Converted into F-1,6-BP, before being coupled with UDP-glucose/UDP reaction to form sucrose-6-phosphate and then sucrose
What competes with photosynthesis?
Photorespiration
A process where plants consume oxygen and release carbon dioxide in the light (NOT MITOCHONDRIAL RESPIRATION)
Why can photorespiration take place?
Rubisco is an oxygenase as well as carboxylase activity
Ribulose bisphosphate can either fix CO2 or O2
RuBP + CO2 = 2x 3PG
RuBP + O2 = 1x 3PG + 1x 2-phosphoglycolate
When it fixes O2 we don’t get that ‘extra’ molecule of GAP to produce carbohydrates with, therefore we essentially lose this carbon
What is the pathway of converting 2-phosphoglycolate - Choloroplast and Peroxisome?
Chloroplast:
2-PG -> Glycolate (using phosphoglycolare phosphatase)
Glycolate is exported to the peroxisome
Peroxisome:
Glycolate -> Glyoxylate (using glycolate oxidase) - this also forms hydrogen peroxide which is dealt with by catalase
Glyoxylate -> glycine (in a transamination reaction) and is exported to the mitochondrion
What is the pathway of converting 2-phosphoglycolate - Mitochondria, peroxisome and chloroplast?
Mitochondria:
2x glycine -> 1x serine + CO2, serine is transported back to the peroxisome
Peroxisome:
serine -> hydroxypyruvate (transamination)
hydroxypyruvate -> glycerate (using hydroxypyruvate reductase and NADH)
Cytosol:
Glycerate -> 3PG (using glycerate kinase and ATP)
Chloroplast:
3PG is reconverted into RuBP in the Calvin cycle
What are the disadvantages of photorespiration?
Fixed C is lost, also fixed N
Photorespiration can reduce C3 crop photosynthetic efficiency by 20-50%
ATP and NADH are used up recycling
What are the advantages of photorespiration?
It has evolved to protect the plants from excess light energy = exceeding the capacity of the Calvin cycle to use up the ATP and NADPH
This prevents hypereduction and the photosynthetic apparatus from damage = photoinhibition
It uses up ATP and releases CO2
It also detoxifies P-glycolate and allows recycling of C that would otherwise be lost
What are C4 plants?
C4 plants have evolved independently to eliminate photorespiration
Stomata close under conditions of high temperature or low water availability
When stomata are closed CO2 can’t enter
Many tropical plants (sugar cane, maize, sorghum) have solved this problem by physically separating CO2 fixation and the Calvin cycle
This allows them to reduce stomatal opening to limit water loss while ensuring rubisco is saturated for CO2 and effectively eliminates photorespiration
