Week 8 Flashcards
(35 cards)
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
