C4 Pathway

In $C_3$ plants RuBisCO also binds $O_2$, so photorespiration occurs and some fixed carbon is lost.
$C_4$ plants concentrate $CO_2$ at the site of RuBisCO in the bundle sheath cells, so photorespiration does not occur.
This lack of photorespiration is the important difference; it gives $C_4$ plants higher productivity.

Kranz anatomy is the special leaf anatomy of $C_4$ plants, in which large bundle sheath cells are arranged like a wreath around the vascular bundles.
The bundle sheath cells may form several layers; they are characterised by a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.
The thick, gas-tight walls keep the $CO_2$ released from the $C_4$ acid concentrated around RuBisCO, which prevents photorespiration.
Thin walls with intercellular spaces are features of the mesophyll cells.
Hence the bundle sheath cells have thick walls, no intercellular spaces and a large number of chloroplasts.

Kranz anatomy is the special leaf anatomy of $C_4$ plants: the vascular bundles are surrounded by a wreath of large bundle sheath cells that have many chloroplasts, thick walls and no intercellular spaces.
$C_4$ plants are mostly tropical grasses such as sugarcane, maize and sorghum.
Potato, wheat and mustard are $C_3$ plants and do not have Kranz anatomy.
Hence Kranz anatomy is a characteristic of the leaves of sugarcane.

The figure is the Hatch and Slack ($C_4$) pathway, showing a mesophyll cell and a bundle sheath cell.
A (in the mesophyll cell): atmospheric $CO_2$, as $HCO_3^-$, combines with the 3-carbon acceptor phosphoenol pyruvate (PEP) in the presence of PEP carboxylase to form the $C_4$ acid oxaloacetic acid. This is fixation.
The $C_4$ acid (malic or aspartic acid) is transported to the bundle sheath cell.
B (in the bundle sheath cell): the $C_4$ acid is broken down to release $CO_2$ and a 3-carbon acid. This is decarboxylation; the $CO_2$ released enters the Calvin cycle.
C (back in the mesophyll cell): the $C_3$ acid is transported back and converted into PEP again. This is regeneration of the primary acceptor.
Hence A – fixation, B – decarboxylation, C – regeneration.

$C_4$ 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 leaves of $C_4$ plants show Kranz anatomy, with mesophyll cells and large bundle sheath cells around the vascular bundles.
In $C_4$ plants the $C_4$ cycle occurs in the mesophyll cells and the $C_3$ (Calvin) cycle occurs in the bundle sheath cells.
The primary $CO_2$ acceptor, phosphoenol pyruvate (PEP), is present in the mesophyll cells, where the enzyme PEP carboxylase fixes $CO_2$ to form oxaloacetic acid (OAA).
OAA is then reduced to malic acid (or converted to aspartic acid) in the mesophyll cells themselves.
The malic acid is transported to the bundle sheath cells, where it is broken down to release $CO_2$ for the Calvin cycle.
So malic acid formation occurs in the mesophyll cells.

In $C_3$ plants the Calvin cycle needs 3 ATP and 2 NADPH for the fixation of one $CO_2$, so for one hexose (6 $CO_2$) it needs 18 ATP and 12 NADPH.
$C_4$ plants first fix $CO_2$ in the mesophyll cells and then release it in the bundle sheath cells for the Calvin cycle.
To regenerate phosphoenol pyruvate (PEP) from pyruvate in the mesophyll cells, the enzyme pyruvate phosphate dikinase uses 2 high energy bonds of ATP (ATP is converted to AMP) for every $CO_2$ fixed.
So 2 extra ATP are needed for each $CO_2$, i.e. $2 \times 6 = 12$ extra ATP for one hexose.
Total requirement in $C_4$ plants $= 18 + 12 = 30$ ATP per hexose.
So $C_4$ plants need twelve additional ATP molecules compared with $C_3$ plants.

In $C_4$ plants, $CO_2$ is first fixed in the mesophyll cells by PEP carboxylase into a 4-carbon acid.
This acid moves into the bundle sheath cells and is decarboxylated, releasing $CO_2$ there.
This works as a $CO_2$ pump: it raises the concentration of $CO_2$ in the bundle sheath cells, where RuBisCO and the Calvin cycle are located, to a level much higher than in the atmosphere.
Because of this effective pumping, RuBisCO works at near-saturating $CO_2$ even when the atmospheric $CO_2$ is low, and photorespiration is almost eliminated.
So photosynthesis is less limited by atmospheric $CO_2$ levels; the $CO_2$ compensation point of $C_4$ plants is very low.
RuBisCO itself is the same enzyme as in $C_3$ plants, and the fixation by PEP carboxylase or the formation of 4-carbon acids is only a part of this mechanism.
So the reason is the effective pumping of $CO_2$ into the bundle sheath cells.