Alternative Mechanisms of Carbon Fixation

Report
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Allow for the entry of
CO2 and exit of water
vapor (transpiration).
On sunny, hot, dry
days, guard cells close
to preserve water, but
this poses a problem for
photosynthesis.
As O2 accumulates, it
competes with CO2 in
binding with rubisco.
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When oxygen more plentiful than CO2, RuBP
is oxidized.
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Recap: in C3:
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In Photorespiration:
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3 CO2 + 3 RuBP (5C)  6
PGA (3C)
3 O2 + 3 RuBP (5C)  3
PGA (3C) + 3 glycolate (2C)
Some carbons of glycolate
returned to the Calvin
cycle as Glyceraldehyde 3phosphate (G3P)
Some carbons converted
to CO2.
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Decreases the production of carbs due to removal
of PGA from the Calvin cycle.
¼ - ½ of carbon fixed in C3 plants are lost by
photorespiration
Optimal temperature for photorespiration (30-47
degrees Celsius) is much higher than
photosynthesis (15 – 25 degrees Celsius)
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Relate back to Enzymes: O2 more likely to be the
substrate of the enzyme Rubisco when temperatures are
_______________.
Hot, dry, bright days facilitate _____________.
Global warming?
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Rubisco worked fine in early Earth, when oxygen
levels weren’t very high and CO2 was plentiful.
Photosynthesis increased amount of O2 in
atmosphere.
As oxygen levels increased, rubisco did not adapt
to rid itself of its ability to oxidate.
Solution: evolution of alternate mechanisms of
carbon fixation where CO2 is concentrated at the
site of Rubisco  suppress rate of
photorespiration.
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C4 photosynthesis
Crussalacean Acid Metabolism (CAM)
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Several thousand species of plants undergo C4
Phosphenolpyruvate carboxylase (PEP
carboxylase) first catalyzes addition of CO2
molecule to PEP (3C)  oxaloacetate (OAA)
(4C)
C3 vs. C4?
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Leaf anatomy and function of C4 plants: two types
of photosynthetic cells:
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Mesophyl cells located around bundle-sheath cells.
 IN THE CYTOPLASM, NOT THE CHLOROPLAST:
 PEP carboxylase: CO2 + PEP (3C)  OAA (4C)
 OAA  malate
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Bundle-sheath cells surrounding a vein
 Malate diffuses from mesophyll cells into bundle-sheath cells
through cell-cell connections called plasmodesmata.
 CO2 removed from malate (decarboxylation)  pyruvate
(3C)
 This CO2 enters the Calvin Cycle: catalyzed by rubisco
 Pyruvate  mesophyll cell  converted to PEP
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Why all the hassel?
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Reduces the amount of
photorespiration by
continually pumping
CO2 towards rubisco.
CO2 outcompetes O2
Sugar production
maximized
Costs plant 2 ATP
molecules per CO2
produced.
C3 plants: 18 molecules
of ATP used/glucose
C4 plants: 30 molecules
of ATP used/glucose
When is it worth it?
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Occur in succulents: water-storing plants
Cacti
 Pineapples
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Open stomata at night and close them during the
day.
Closing stomata during the day helps
___________________ but prevents _____ from
entering the leaves.
When stomata open, CO2 taken in, incorporated
into C4 organic acids (using PEP carboxylase).
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Organic acids stored in vacuoles until morning  CO2
molecules enter the C3 Calvin cycle  ______________.
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C4: first part of carbon fixation and Calvin cycle occur in
separate compartments of the leaf (spacial separation)
CAM: two steps occur in the same compartments, but at
different times of the day (temporal separation)
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Both represent evolutionary solutions to the
problem of maintaining photosynthesis when
stomata are closed.
Initially produce organic acids that eventually
transfer CO2 to the Calvin cycle.
Pg. 172
#1-6
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