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58 Cards in this Set

  • Front
  • Back
Is photorespiration useful ?
It is a wasteful side reaction catalyzed by the enzyme rubisco which reduces crop yields by competing for the C3 carbon dioxide fixation pathway
What does the C4 pathway in corn plants lead to?
It leads to reduced rates of photorespiration
Why does this lead to reduced rates of photorespiration?
CO2 is concetrated in the chloroplast stroma and out competes O2 for rubisco binding
What are transgenic plants?
They are being developed that contain corn genes encoding multiple enzymes in the C4 pathway
How do plants store energy?
They store light energy in the form of carbohydrates, primarily starch and sucrose
What is the conversion of CO2 to carbohydrate?
It is called the Calvin cycle
What does the calvin cycle require?
It requires the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase
What is this enzyme commonly known as?
What does the calvin cycle generate?
It generates the triose phosphates 3-phosphoglycerate, glyceraldehyde-3P, and dihydroxyacetone phosphate
What are the triose phosphate used for?
They are used to synthesize the hexose phosphates fructose-1,6-bisphosphate and fructose 6-phosphate
What are hexose phosphates produced by the Calvin used for?
They are converted to sucrose for transport to other plant tissues, starch for energy stores within the cell, cellulose for cell wall synthesis, and pentose phosphates for metabolic intermediates
What are the three stages of the Calvin cycle?
What does three turns of the cycle result in?
It results in the fixation of three molecules of CO2 into phosphate 3-phosphoglycerate
What is this catalyzed by?
The rubisco enzyme
What does the rubisco key reaction in stage 1 accomplish?
It combines three molecules of RuBP, a five carbon C5 compound, with three molecules of CO2 to form six molecules of the C3 compound 3-phosphoglycerate
What is the net reaction of three turns of the Calvin cycle?
3 CO2 + 3 RuBP + 6 NADPH + 9 ATP + 6 H2O ---> 1 GAP + 3 RuBP + 6 NADP+ + 9 ADP + 9 Pi
How many C3 compounds are created?
1 C3 compound is created
How many C5 molecules are regenerated?
3 C5 (RuBP) molecules are regenerated
What did Calvin and his colleagues use to follow carbon fixation in photosynthetic algae cells?
They used radioactive labeling with 14CO2
Within a few seconds of adding 14CO2 to the culture, what happened?
The cells accumulated 14C-labeled 3-phosphoglycerate
What does this infer?
It infers that this was the first product of the carboxylation reaction
How many steps can the rubisco reaction be broken down into?
It can be broken down into 4 steps
What is the first step of the rubisco reaction?
The formation of an enediolate intermediate of RuBP
What does this require?
The participation of a carbamoylated lysine residue in the ezyme active site
What happens in step 2?
Carboxylation occurs by nucleophilic attack on the CO2 to form an unstable C6 intermediate
What happens in step 3?
The unstable C6 intermediate is hydrated
What happens in step 4?
Aldol cleavage leads to the formation of two molecules of 3-phosphoglycerate
Is the rubisco reaction exergonic?
Yes, very (-35.1 kj/mol)
What is Rubisco?
It is a multisubunit enzyme consisting of eight identical catalytic subunits at the core surrounded by eight smaller subunits that function to stabilize the complex and presumably enhance enzyme activity
What happens in Stage 2?
Reduction occurs of the 3-phosphoglycerates to form hexose sugars
How does this occur?
3-phosphoglyceraldehyde is converted to glyceraldehyde-3-phosphate by two isozymes of phosphoglycerate kinase and glyceraldehyde-3P-dehydrogenase
What occurs in the final stage of the Calvin cycle?
A series of enzyme reactions convert five C3 molecules (GAP or DHAP) into three C5 molecules (RuBP) to replenish supploes of this CO2 acceptor molecule which is required in the rubisco reaction
What are two of the primary enzymes in this carbon shuffle?
Transketolase and transaldolase
What are they involved in?
They are involved in interconverting C3, C4, C6, and C7 molecules
What happens in the transketolase reaction?
It transfers a C2 group from a ketose to an aldose and requires thiamine pyrophosphate as a coenzyme in the reaction
What does transaldolase do?
It catalyzes the formation of a C6 molecule (fructose-1,6-BP) from two C3 molecules (GAP and DHAP)
The net reaction of the Calvinc cycle can be broken down into what two components?
The synthesis of one glucose molecule from six CO2 using 12 ATP and 12 NADPH, and regeneration of siz RuBP using six ATP
What is the net reaction from siz turns of the Calvin cycle?
6 CO2 + 12 NADPH + 18 ATP + 12 H2O ---> Glucose + 12 NADP+ + 18 ADP + 18 Pi
Why is it crucial that the Calvin cycle only be active in the light?
If glycolysis, the pentose phosphate pathway and the Calvin cycle were all active at the same time, then simultaneous starch degradation and carbohydrate biosynthesis would quickly deplete the ATP and NADPH pools in the stroma
What are calvin cycle enzymes activated by?
They are activated by elevated pH and by increase Mg2+ concentrations in the stroma
What does light activation of the photosynthetic electron transport system cause?
It causes stromal pH to increase from pH 7 to pH 8 as a result of proton pumping into the thylakoid lumen
What does the influx of H+ into the lumen do?
It causes an efflux of Mg2+ to the stroma to balance the charge
How do disulfide bridges interact with Calvin cycle enzymes?
The enzymes are activated thioredoxin-mediated reduction of disulfide bridges
What is thioredoxin?
It is a small protein of 12 kDa that is found throughout nature and functions as a redox protein that can interconvert disulfide bridges and sulfhydrals in cysteine residues of target proteins
What happens to Calvin cycle enzymes when the sun goes down?
When the sun goes down, spontaneous oxidation leads to their inactivation
What is the oxygenase reaction?
It is a reaction catalyzed by Rubisco that combines RuBP with O2 to generate one molecule of 3-phosphoglucerate (C3) and one molecule of 2-phosphoglycolate (C2)
What is the 2-phosphoglycolate metabolized by?
It is metabolized by the glycolate pathway, which generates CO2 and requires a significant amount of cellular energy to salvage the C2 group for a re-incorporation into the Calvin cycle
What is oxygenation of RuBP, and metabolism of 2-phosphoglycolate by the glycolate pathway collectively called?
Photorespiration because O2 is consumed and CO2 is released
Does photorespiration require energy input?
Mesophyll cells are responsible for what?
CO2 capture
What do interior bundle sheath cells use?
They use CO2 released from the C4 intermediate malate to carry out the Calvin cycle reactions
The C4 and C3 pathways are physically separated in the two cell types how?
Malate functions as the CO2 transporter molecule into the bundle sheath cell, and pyruvate servs to recycle carbons back into the mesophyll cell
What does this accomplish?
This separation of labor between the two cell types eliminates the oxygenase reaction in rubisco and thereby blocks photorespiration
What is the price of the C4 pathway?
Two high energy phosphate bonds (AMP + PPi) are required to convert pyruvate to phosphoenolpyruvate in mesophyll cells
What does CAM stand for?
Crassulacean Acid Metabolism
What does CAM accomplish?
It allows photorespiration to be kept to a minimum during the day when the stomata are closed and CO2 levels in the chloroplast ar ehigh due to the decarboxylation of malate
What does the closure of stomatas do?
It inhibits water loss by evaporation
How do sugarcane plants and saguaro cacti avoid the wasteful side effects of photorespiration?
They separate the process of gas exchange through the stroma, from the carbon dioxide fixation reaction mediated by the Calvin cycle enzyme ribulose-1,5 bisphosphate carboxylase/oxygenase (rubisco)