C4 pathway Photosynthesis and photorespiration - This lecture explains about the C4 pathway of photosynthesis also known as Hatch Slack pathway. This is a process to prevent photorespiration by plants that helps in minimizing the energy loss for the plant.
Plants that adapted to these warmer environments have
evolved two principal ways that use the C4 pathway to
deal with this problem. In one approach, plants conduct
C4 photosynthesis in the mesophyll cells and the Calvin
cycle in the bundle sheath cells. This creates high local
levels of CO2 to favor the carboxylation reaction of rubisco.
These plants are called C4 plants and include corn,
sugarcane, sorghum, and a number of other grasses. In the
C4 pathway, the three-carbon metabolite phosphoenolpyruvate
is carboxylated to form the four-carbon
molecule oxaloacetate, which is the first product of CO2
fixation (figure 10.21). In C4 plants, oxaloacetate is in turn
converted into the intermediate malate, which is transported
to an adjacent bundle-sheath cell. Inside the bundlesheath
cell, malate is decarboxylated to produce pyruvate,
releasing CO2. Because bundle-sheath cells are impermeable
to CO2, the CO2 is retained within them in high concentrations.
Pyruvate returns to the mesophyll cell, where
two of the high-energy bonds in an ATP molecule are
split to convert the pyruvate back into phosphoenolpyruvate,
thus completing the cycle.
The enzymes that carry out the Calvin cycle in a C4
plant are located within the bundle-sheath cells, where the
increased CO2 concentration decreases photorespiration.
Because each CO2 molecule is transported into the bundlesheath
cells at a cost of two high-energy ATP bonds, and
because six carbons must be fixed to form a molecule of
glucose, 12 additional molecules of ATP are required to
form a molecule of glucose. In C4 photosynthesis, the energetic
cost of forming glucose is almost twice that of C3
photosynthesis: 30 molecules of ATP versus 18. Nevertheless,
C4 photosynthesis is advantageous in a hot climate:
photorespiration would otherwise remove more than half
of the carbon fixed.
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