Describe the $C_4$ pathway.

(N/A) Plants adapted to dry tropical regions utilize the $C_4$ pathway.
These plants have $C_4$ oxaloacetic acid as the primary $CO_2$ fixation product. They utilize the $C_3$ pathway or the Calvin cycle as the main biosynthetic pathway. Key characteristics of $C_4$ plants include:
$(1)$ $C_4$ plants possess a specialized leaf anatomy and can tolerate higher temperatures.
$(2)$ They exhibit a positive response to high light intensities.
$(3)$ They lack the process of photorespiration.
$(4)$ They demonstrate greater biomass productivity.
$(5)$ In a vertical section of the leaves,mesophyll cells are arranged in a specific pattern.
$(6)$ Large cells surrounding the vascular bundles are called bundle sheath cells,and this arrangement is known as 'Kranz' anatomy.
$(7)$ 'Kranz' means 'wreath',reflecting the circular arrangement of cells.
$(8)$ The bundle sheath may consist of several layers around the vascular bundles.
$(9)$ These cells are characterized by numerous chloroplasts,thick walls impervious to gaseous exchange,and no intercellular spaces.
The pathway is also known as the Hatch and Slack pathway. It is a cyclic process consisting of the following steps:
Step $1$: Atmospheric $CO_2$ enters mesophyll cells where the primary $CO_2$ acceptor is a $3$-carbon molecule,phosphoenolpyruvate $(PEP)$. The enzyme responsible is $PEP$ carboxylase $(PEPcase)$. The $C_4$ acid,oxaloacetic acid $(OAA)$,is formed: $CO_2 + PEP \xrightarrow{PEPcase} OAA$.
Step $2$: $OAA$ is converted into other $4$-carbon compounds like malic acid or aspartic acid in the mesophyll,which are then transported to the bundle sheath cells. Here,these $C_4$ acids are broken down to release $CO_2$ and a $3$-carbon molecule (pyruvic acid).
Step $3$: The $3$-carbon molecule is transported back to the mesophyll,where it is converted back to $PEP$ in the presence of $ATP$,completing the cycle.
Step $4$: The $CO_2$ released in the bundle sheath cells enters the $C_3$ or Calvin pathway. The bundle sheath cells are rich in $RuBisCO$ but lack $PEPcase$. Thus,the Calvin pathway remains the basic mechanism for sugar formation in both $C_3$ and $C_4$ plants.