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Purposes of CO2 concentration
Make rubisco run at Vmax, minimize photorespiration, increase WUE and NUE
Where do plants concentrate CO2?
In the stroma of the chloroplast
What is the C4 cycle?
PEP Carboxylase produces a four-carbon compound to elevate CO2 concentration at the site of Rubisco
What are the most productive crops?
C4 grasses (maize, sorghum, and sugar cane)
How much of our caloric intake is C4 plants?
25%
How much of the world’s plants are C4 crops?
8/10
Difference in C3 and C4 plant structure?
Kranz anatomy says that there’s a big vascular tissue surrounded by bundle sheath and mesophyll cells
What does PEP carboxylase do better than Rubisco?
Fix CO2 faster
How is Rubisco running in C4?
At Vmax and with no photorespiration
How does C4 photosynthesis work?
Atmospheric CO2 enters mesophyll cells and is converted to bicarbonate which is then carboxylated to PEP and produces OAA, which is then transported to the bundle sheath cell and decarboxylated, releasing CO2 at Rubisco, which initiates the Calvin-Benson cycle. A three-carbon compound returns to the mesophyll cell.
Where does carboxylase saturation and oxygenase-inhibiting CO2 occur?
The bundle sheath
What step requires energy in C4 photosynthesis?
Regenerating PEP requires 2ATP per CO2 fixed
Total energy per CO2 fixed by Rubisco in C4
5ATP + 2NADPH
Why did C4 photosynthesis evolve?
When atmospheric CO2 decreased
What water advantage do C4 plants have over C3 plants?
PEPC fixes CO2 effectively even when CO2 is low, reducing stomata opening and losing water
Ratio of water loss to carbon gain during photosynthesis C4
C4: 300 H2O / 1 CO2
What type of plants do CAM photosynthesis?
Desert plants, big plants in rainforests
Economically important CAM plants?
Vanilla, pineapple, aloe vera, agave
How does CAM cycle work?
CO2 uptake occurs at night (less water loss), CO2 is fixed by PEP Carboxylase, CO2 is stored as C4 acids in the vacoule, CO2 is released in the daytime, Rubisco fixes CO2 during the day even though stomata are closed, bc there is light energy
CAM leaf anatomy?
CAM leaves are very thick and densely packed, creates succulent leaf due to vacuoles
CAM Cycle simplified
Night - open & PEPC, Day - closed & Rubisco
WUE in CAM Plants
100 H2O / 1 CO2
Origins of CAM
Declines in CO2 (origins are older, but expansion related to more recent declines in CO2)
Which carbon is radioactive
C14
What is PDB standard from?
Carbon-13 work sampling shells of a squid, had a high C13:C12 ratio and was established as the standard value of zero
What does Rubisco favor?
C12
Why is the fixation of 12CO2 more rapid?
Because C12 is lighter and processed faster
Why has C13 decreased since start of Industrial Revolution?
Fossil fuels increase 12CO2 faster than 13CO2, driving C13 downward in “The Suess Effect”
PEPC discriminates blank
Less strongly against C13 than Rubisco
What does the amount of C13:C12 mean in a plant
Indicates contribution of photosynthetic PEPC carboxylation
Discrimination against 13C order
C3<CAM<C4
When was the global expansion of C4 ecosystems?
6-8 Ma years ago
What drove emergence of genus Homo?
C4 grasslands expanded, woods decreases, couldn’t live in trees anymore
How much has atmospheric CO2 concentration increased since the start of the industrial revolution?
50%
What are the four representative concentration pathways?
2.6 - peak between 2010 and 2020, 4.5 - peak between 2040, 6 - peak around 2080, 8.5 - rise throughout 21st century
What happens to stomata with more CO2?
Less stomata
What plants will benefit more from elevated CO2?
C3 plants
What plants will benefit more from rising temperatures?
C4 plants because they do not do photorespiration
What do plants do overtime with elevated CO2?
Make biochemical changes in response to elevated CO2 that reduces the fertilization effect.
What ends up decreasing with elevated CO2?
N (protein) and macro and micro nutrients
what is principal photoreceptor in plants?
chlorophyll
What is the primary function of the photosynthetic reaction center and its specialized chlorophyll pair?
To emit an electron for photosynthetic electron transport.
What happens to the energy of a blue photon compared to a red photon when it is absorbed by chlorophyll?
It is lost as heat.
What term refers to light damage of PSII
Photoinhibition
How many substrate carbons are involved in the carboxylation reaction of Rubisco?
6
What is water potential expressed in?
Pascals (Greek letter psi)
Water potential equation
Sum of pressure potential and osmotic potential
How much water is absorbed by roots?
97%
How much water supplies growth?
2%
How much water is consumed in the biochemical reactions of photosynthesis and other metabolic processes
1%
Note
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