write a note on alternative mechanism of co2 fixation
Answers
Alternative Mechanisms of Carbon Fixation
Photorespiration
• On hot, dry and bright days, C3 plants close their
stomata to reduce water loss.
• This causes CO2 levels to drop within the leaves.
• Rubisco catalyzes the oxidation of RuBP (addition of
O2) more often than the carboxylation of RuBP
(addition of CO2).
• This decreases carbohydrate production.
• Instead of producing 2 G3P, 1 G3P (3C) and 1 glycolate
(2C) molecule is produced.
• Glycolate molecules convert into CO2 and G3P.
• Rubisco’s activity is better suited to an early Earth up
with an atmosphere rich in CO2 and poor in O2.
* C4 plants and CAM plants have evolved an alternative
mechanism of carbon fixation to suppress the rate of
photorespiration.C4 Plants
• PEP carboxylase catalyzes the addition of CO2 to PEP
(3C) to form oxaloacetate (4C).
• Oxaloacetate converts to malate and diffuses from
the cytoplasm of the mesophyll cells into the bundle-
sheath cells.
• CO2 is removed (decarboxylation) and malate converts
to pyruvate.
• Pyruvate converts back into PEP as it diffuses back
into the mesophyll cells (requires ATP).
• The CO2 that was removed enters the C3 Calvin cycle
in a second fixation reaction, catalyzed by rubisco.
Photorespiration is
minimized, but it
requires almost twice
as many ATP to
produce 1 glucose.Crassulacean Acid Metabolism (CAM) Plants
• Open stomata at night and close them during the day.
• In the dark, CO2 enters the leaf and is incorporated
into C4 organic acids via PEP carboxylase.
• Organic acids stored in vacuoles until daylight, at
which time they release the CO2 needed for the C3
Calvin cycle.