Transcript C4 Photosynthesis - Madison County Schools

C4 Photosynthesis
Improving upon photosynthetic
efficiency
Evolved a special add on feature
• When CO2 enters the leaf, it is absorbed by
the usual photosynthesizing cells, the
mesophyll cells.
• Instead of being fixed by rubisco into PGA, the
CO2 combines with PEP
(phosphoenolpyruvate) to form OAA
(oxaloacetate)
• The fixing enzyme is PEP carboxylase
C4
• The first product of this pathway, has 4 carbon
atoms, thus the name C4.
• OAA is converted to malate, and the malate is
shuttled to specialized cells within the leaf, the
bundle sheath cells.
• Here malate is converted to pyruvate and CO2
• The pyruvate is then shuttled back to the
mesophyll cells were one ATP is required to
convert the pyruvate back to PEP.
C4
• Then the process repeats.
• The overall effect is to move CO2 from
mesophyll cells to the bundle sheath cells.
• The purpose for moving the CO2 to bundle
sheath cells is to increase the efficiency of
photosynthesis
C4
• The bundle sheath cells surround the leaf veins and are
themselves surrounded by densely packed mesophyll
cells.
• Since bundle cells rarely make contact with an
intercellular space, very little oxygen reaches them.
When malate delivers CO2 to a bundle sheath cell,
rubisco begins the Calvin cycle (C3 photosynthesis).
• Because little oxygen is present, rubisco can fix CO2
without competition for O2. thus, little
photorespiration takes place, and photosynthesis is
more efficient.
C4
• Higher rate of photosynthesis among C4
plants allow them to reduce the time that the
stomata are open, thereby, reducing H2O loss.
• C4 plants are found in hot, dry climates,
where they possess an adaptive advantage
over C3 plants.
C4 plants—sugarcane and crab grasses are two
examples
CAM photosynthesis
• Another “add on” feature to photosynthesis is
crassulacean acid metabolism (CAM)
• The physiology of this pathway is almost identical
to C4 photosynthesis.
• PEP carboxylase still fixes CO2 to OAA, as in C4
• Instead of malate, however, OAA is converted to
malic acid.
• Malic acid is shuttled to the vacuole of the
cell(not moved out of the cell to bundle sheath
cell as in C4)
CAM
• Stomata are open at night.
• During the night, PEP carboxylase is active and
malic acid accumulates in the cell’s vacuole.
• Stomata are closed during the day (reverse of
other plants).
• At this time, malic acid is shuttled out of the
vacuole and converted back to OAA, releasing
CO2.
• The CO2 is now fixed by rubisco, and the Calvin
cycle proceeds.
CAM
• The advantage of CAM is that photosynthesis can
proceed during the day while the stomata are
closed, greatly reducing H2O loss.
• As a result, CAM provides an adaptation for
plants that grow in hot, dry environments with
cool nights (such as deserts).
• The name crassulacean acid metabolism comes
from the early discovery of CAM in the succulent
plants of the family Crassulaceae.
• Found in many families, including cacti