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In leaves of $C_4$ plants malic acid synthesis during $CO_2$ fixation occurs in:
A
bundle sheath
B
guard cells
C
epidermal cells
D
mesophyll cells
Detailed Solution
$C_4$ plants show Kranz anatomy, with two types of photosynthetic cells: mesophyll cells and bundle sheath cells.
The primary $CO_2$ acceptor is the 3-carbon compound phosphoenol pyruvate (PEP), present in the mesophyll cells.
The enzyme PEP carboxylase (PEPcase) of the mesophyll cells fixes $CO_2$ to form the 4-carbon oxaloacetic acid (OAA).
OAA is then converted into other 4-carbon acids, malic acid or aspartic acid, in the mesophyll cells themselves.
Malic acid is then transported to the bundle sheath cells, where it is decarboxylated to release $CO_2$ for the Calvin cycle.
So malic acid synthesis occurs in the mesophyll cells; the bundle sheath is the site of its breakdown, not its synthesis.
The primary $CO_2$ acceptor is the 3-carbon compound phosphoenol pyruvate (PEP), present in the mesophyll cells.
The enzyme PEP carboxylase (PEPcase) of the mesophyll cells fixes $CO_2$ to form the 4-carbon oxaloacetic acid (OAA).
OAA is then converted into other 4-carbon acids, malic acid or aspartic acid, in the mesophyll cells themselves.
Malic acid is then transported to the bundle sheath cells, where it is decarboxylated to release $CO_2$ for the Calvin cycle.
So malic acid synthesis occurs in the mesophyll cells; the bundle sheath is the site of its breakdown, not its synthesis.