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The $C_4$ plants are photosynthetically more efficient than $C_3$ plants because:
A
the $CO_2$ efflux is not prevented
B
they have more chloroplasts
C
the $CO_2$ compensation point is more
D
$CO_2$ generated during photorespiration is trapped and recycled through PEP carboxylase
Detailed Solution
In $C_3$ plants the enzyme RuBisCO also binds $O_2$ and carries out photorespiration, in which fixed carbon is lost as $CO_2$ without the production of ATP or sugar. This lowers the efficiency of photosynthesis.
$C_4$ plants have Kranz anatomy. In the mesophyll cells $CO_2$ is first fixed by PEP carboxylase, an enzyme that has a very high affinity for $CO_2$ and does not react with $O_2$.
The $C_4$ acid formed is transported to the bundle sheath cells and decarboxylated there, so the $CO_2$ concentration at the site of RuBisCO becomes very high.
This makes RuBisCO work as a carboxylase and minimises its oxygenase activity, so photorespiration practically does not occur.
Any $CO_2$ that is released in the bundle sheath cells is trapped and refixed by PEP carboxylase in the surrounding mesophyll cells, so no $CO_2$ escapes from the leaf.
For the same reason the $CO_2$ compensation point of $C_4$ plants is very low (not more), and the $CO_2$ efflux is prevented (not allowed).
So $C_4$ plants are more efficient because the $CO_2$ generated during photorespiration is trapped and recycled through PEP carboxylase.
$C_4$ plants have Kranz anatomy. In the mesophyll cells $CO_2$ is first fixed by PEP carboxylase, an enzyme that has a very high affinity for $CO_2$ and does not react with $O_2$.
The $C_4$ acid formed is transported to the bundle sheath cells and decarboxylated there, so the $CO_2$ concentration at the site of RuBisCO becomes very high.
This makes RuBisCO work as a carboxylase and minimises its oxygenase activity, so photorespiration practically does not occur.
Any $CO_2$ that is released in the bundle sheath cells is trapped and refixed by PEP carboxylase in the surrounding mesophyll cells, so no $CO_2$ escapes from the leaf.
For the same reason the $CO_2$ compensation point of $C_4$ plants is very low (not more), and the $CO_2$ efflux is prevented (not allowed).
So $C_4$ plants are more efficient because the $CO_2$ generated during photorespiration is trapped and recycled through PEP carboxylase.