Week 8 Flashcards
How much O2 needs to be reduced to stimulate CO2 fixation?
21% to 1-2%
Where do C4 plants primarily live?
Tropical grasslands, inclusing maize and sugar cane
What is the difference C4 and C3?
C4 plants CO2 initially by an auxillary mechanism, not present in C3 plants
Can C4 plants do C3 fixation?
Yes but this process and its biochemical machinery occurs predominately in a distinct tissue location, seperate from the C4 cycle
What is the anatomy of C4 plants leaves?
1-2 concentric cylinders of cells
These are completely enclosed and radiate form bundle-sheath
The bundle-sheath encolses the vascular bundle
Where does the C3 process occur?
Predominately, though not exclusively, in the chloroplasts of the bundle-sheath cells
What is the name of the anatomy of C4 plants?
Kranz anatomy
What connects the the radiate mesophyll and cells of the bundle-sheath?
They are connected by numerous plasmodesmata, these cytoplasmic connections through which metabolites are exchanged
What is the structure of the cell-wall of the bundle-sheath?
They are heavily thickened and often suberinized, consequently they are highly impermeable to gas, carbon dioxide and oxygen
What are the metabolites that are exchanged in C4 cycle?
C4 organic acids from the mesophyll into the bundle-sheath
Pyruvate from the bundle-sheath to the mesophyll
Where and how does the C4 cycle occur?
The chloroplasts of the mesophyll operate a C4 cycle, as they possess the enzyme, PEP carboxylase, that produces a C4 organic acid, oxaloacetate from PEP, bicarbonate (HCO3-)
What is PEP?
Phosphoenolpyruvate
What is the equilibrium of Bicarbonate and dissolved CO2?
Bicarbonate and dissolved CO2 is in equlibrium
How does the plant keep CO2 levels low?
The reaction is catalysed by the enzyme carbonic anhydrase. The action of carbonic anhydrase and PEP carboxylase keeps CO2 at low levels in the mesophyll and provides the diffusion gradient for CO2 entry into the leaf air spaces
What is the difference between Rubisco and PEP carboxylase?
PEP carboxylase does not fix O2 so isnt inhibited by ambient O2
What happens to the carbon fixed by PEP carboxylase?
The CO2 it fixed is passed as a C4 acid (malate) into the bundle-sheath
What happens to the C4 (malate) when in the bundle-sheath?
CO2 is released and the resulting 3C compound, pyruvate, is passed back to the mesophyll
What happens to the C4 concentration of CO2?
The exchange of metabolites is to increase CO2 level of the bundle-sheath to roughly 60 microM, the equivalent of 2000 ppm of atmospheric CO2
What is the consequence of having high CO2 concentration in the bundle-sheath?
The high level of CO2 in a relatively gas tight environment, allows for the Benson-Calvin cycle to occur refixing the CO2 produced. This is because the high CO2 protects against oxygen
What is the cost of C4 carbon fixation?
The extra biochemical machinery of the auxillary C4 process, specifically ATP-dependant phosphorylation of pyrivate to regenerate the PEP substrate of PEP carboxylase in the mesophyll, has an energetic cost
What is the amount of more ATP consumed by C4 carbon fixation?
Additional consumption of 2 ATP per CO2 fixed. Thus the C4 cycle constitutes an ATP-driven Co2 pump
What does this high ATP cost mean for the plants that can use C4 carbon fixation?
Its energetic cost in ATP is one that can only be sustained in high light environments ie those in which ATP generation in the light reactions is high
What plants are considered CAM plants?
CAM plants have a wide taxonomic and ecological distrubution. They include many stem succulents e.g Cacacea, including the Crassulaceae (Kalanchoe, Sedum) in which the phenomenom was first described, and Euphorbiacaea (e.g Mesembryanthemum
How many CAM plants are tropical epiphytes?
Half
What is the similar between CAM plants and C4 plants?
CAM and C4 plants fix CO2 initially by an auxillary C4 mechanism, but this is not an alternative to the C3 process and does not catalyse net assmilation of CO2
What is different between CAM and C4 plants?
C4 plants the intial and refixation of CO2 occur in distinct tissues the mesophyll and bundle sheath
CAM plants the initial fixation by PEP carboxylase and refixation by Rubisco occurs at the same tissues, in the same cells but are processed at different time
How was CAM first described?
The phenomenon was first described as a massive nocturnal accumulatiom of organic acid. As C4 acid malate accmulates in the large vacuoles of these plants
When does Net CO2 fixation occur?
It can only occur when the stomata are open
Why was CAM first described at night?
As stomata are open at night, when temperatures are lower and when the water vapout pressure difference between the leaf air spaces and the atmosphere is low. This means the loss of water to the atmosphere is much reduced at night
What happens to CAM plants during the day?
During the day, the conventional Benson-Calvin cycle fixes CO2 into 3-phosphoglyceric acid, however this Co2 has not diffused in through the stomata, rather it was fixed at night into malate, and released as Co2 and refixed by Rubisco
What happens to the CO2 fixed by PEP carboxylase?
The CO2 it fixes is passed into the vacuole as malate.
What is the key features of malate accumulation?
Malate accumulation is massive, occurs against a concentration gradient - must be energised by a proton pumping ATPase and proton pumping pyrophosphatase
What uptakes Malate2-?
A selective ion channel
What happens to the malate in the vacuole during the day?
The diffusion of malate from the vacuole over takes uptake. This is done through the passive process of diffusion
What are the 2 sets of products that can be formed by malate decarboxylation?
CO2 and pyruvate
CO2 and PEP
