Lecture 14: Water, Carbon, and Energy Balance of Crops - 10/30 Flashcards
What is photorespiration?
When the Rubisco enzyme oxygenates RuBP
Why is photorespiration bad?
It wastes some of the energy produced by photosynthesis, decreasing the efficiency of conversion of light energy to chemical energy
C4 mechanism
CO2 fixation and the Calvin cycle are spatially separated and a new enzyme is used instead of Rubisco
CAM mechanism
CO2 fixation and the Calvin cycle are temporally separated and a new enzyme is used instead of Rubisco
What is the new enzyme used by C4 and CAM plants?
Phosphoenolpyruvate (PEP) carboxylase
Why is PEP carboxylase a better alternative than Rubisco?
It has a much higher CO2 affinity than Rubisco
What does PEP carboxylase produce?
PEP carboxylase produces oxaloacetate (OAA, a 4-carbon molecule and the first stable product of CO2 fixation) from PEP and CO2. It is then converted to malate.
Oxaloacetate doesn’t have Pi (a phosphate group)
How is carbon fixed in C4 plants?
Carbon is fixed in mesophyll cells via PEP carboxylase, then transported to bundle sheath cells as malate (C4, 4-carbon).
Then, the malate is broken back down into PEP and CO2, Rubisco fixing the latter into sugars; Calvin cycle (C3) reactions occur without having to deal with oxygen
Into which cells is carbon fixation divided into for C4 plants?
Carbon moves from mesophyll cells to bundle-sheath cells
How much extra ATP do C4 plants need to regenerate PEP?
2 extra ATP. They also need more quanta of light compared to C3 plants
How do CAM plants separate CO2 fixation?
Stomata open at night, letting in CO2 that will be fixed by PEP carboxylase and eventually converted to malate. During the day, that malate is converted back to CO2 to use in the Calvin cycle (fixed by Rubisco)
What is the difference between C4 and CAM plants for malate?
C4 plants don’t store malate whereas CAM plants do
How do C3, C4, and CAM plants compare in terms of energy efficiency?
In order from least to most energy required is C3, C4, and CAM plants.
C3 plants require the least energy to fix CO2, whereas CAM plants require the most
How do C3, C4, and CAM plants compare in terms of transpiration ratio?
In order from least to most water efficient, C3, C4, and CAM plants.
CAM plants are the most water efficient, and C3 plants are the least water efficient
Around when did C4 and CAM plants evolve?
24-35 million years ago
This happened multiple times (convergent evolution)
