L8, solutions to Rubisco's O2 problem Flashcards
when is the stomata closed?
during periods of high heat/drought stress
what occurs during photorespiration?
rubisco oxygenase activates and creates glycolate and downstream intermediates (C2 compounds)
what happens as a result of photorespiration?
more CO2 is released which can feed back into the Calvin cycle (C3 metabolism) - this uses up extra ATP/NADPH from light reactions, protects photosystems from damage, and prevents creation of reactive oxygen species which would then lead to photodamage/photoinhibition
what are the substrates in C4 photosynthesis?
Phosphoenolpyruvate (PEP) and a bicarbonate ion (hydrated to form CO2)
what is the first product in the C4 pathway?
4-carbon oxaloacetate
what is the enzyme that catalyzes the conversion from PEP to oxaloacetate in the C4 pathway?
PEP carboxylase
where does the first stages of C4 photosynthesis occur?
mesophyll cells
what happens to oxaloacetate in the C4 pathway?
it is reduced to either malate or transaminated to aspartate
once reduced to malate/aspartate, where do they go? what happens?
they move from the mesophyll cells bundle-sheath cells where they are decarboxylated to pyruvate (and CO2) which then enters the calvin cycle. pyruvate returns to mesophyll, reacts with ATP and regenerates PEP
what is the hydrated form of CO2 that PEP carboxylase accepts?
HCO3 (bicarbonate)
what are some characteristics of PEP carboxylase?
it has a very high affinity for bicarbonate, is not interested in O2, and is highly efficient even with low concentration of substrates
what does the Kranz anatomy consist of?
bundle-sheath cells surrounding the Xylem and Phloem (vascular bundle), stomata, mesophyll cells, and bundle-sheath cells on their own
what do C4 plants do if they do not have a Kranz anatomy?
C4 pathway occurs within the same cell - C4 carboxylation and the calvin cycle - known as dimorphic chloroplasts
chloroplasts are separated within the same cell which is the equivalent of mesophyll vs bundle-sheath cells
why have C4 plants not taken over the globe?
C3 plants can perform equally well/better in cool climates,
C3 plants are only limited by CO2- so with levels rising they should do better,
fixation of carbon has larger energy cost in C4 plants,
for each molecule of CO2 fixed a molecule of PEP has to be regenerated at cost of 2 phosphate groups (2x ATP)
what is the advantage of CO2 pumped into the bundle-sheath cell?
maintains high CO2/O2 ratio, favors work of rubisco, of photorespiration does occur- fixation of O2 to RuBP- the product, CO2 is released back out to mesophyll where it can be trapped by PEP
carboxylase
CO2 does not escape the leaf which is a big hot climate advantage
