Question

How many $ATP$ are required in $C4$ cycle?

Answer 1

$C4$ cycle is also called Hatch- Slack pathway which takes place in the bundle sheath of the plant . Oxaloacetic acid is the first $C{O_2}$ fixation product in this cycle. This process takes place in the xerophytes.

Complete answer:
Plants that are adaptive to dry tropical regions have the $C4$ pathway. Though these plants have the $C4$ pathway or calvin cycle as the main biosynthetic pathway. $C4$ Plants are special: They have a special type of leaf anatomy,they tolerate high temperatures,they respond to high light intensities,they lack a process called photorespiration and have a greater productivity of biomass .Let us understand these one by one .

The particularly large cells around the vascular bundle of the $C4$ pathway plants are called bundle sheath cells,and the leaves which have such anatomy are said to have "kranz"anatomy.Kranz means wreath and is a reflection of the arrangement of cells .The bundle sheath cells may from several layers around the vascular bundles;they are characterised by having a large number of chloroplasts ,thick walls impervious to gaseous exchange and intercellular spaces.

This pathway that has been named the Hatch and Slack pathway,is again a cyclic process .
The primary $C{O_2}$ acceptor is a $3 - {\text{ }}carbon$molecule phosphoenolpyruvate and is present in the mesophyll cells .The enzyme responsible for this fixation is pep carboxylase or pepcase . It is important to note that the mesophyll cells lack RuBisCo enzyme .The $C4$ acid $OAA$is formed in the mesophyll cells. It then other $4 - {\text{ }}carbon$compounds like malic acid or aspartic acid in the mesophyll itself ,which are transported to the bundle sheath cells.

The $3 - {\text{ }}carbon$ molecules is transported back to the mesophyll where it is converted to pep again ,thus ,completing the cycle.The $C{O_2}$ released in the bundle sheath cells entres the $C3$ or the calvin pathway , a pathway common to all plants .The bundle sheath cells are rich in an enzyme Ribulose bisphosphate carboxylase-oxygenase,but lack pepcase .Thus , the basic pathway that result in formation of the sugar .The calvin pathway is common to the $C3$ and $C4$ plants.

Total thirty $ATP$s are required for the fixation of six $C{O_2}$ molecules. It occurs in $C4$ pathways also in the bundle sheath cells. The $C3$ cycle required eighteen $ATP$s for six $C{O_2}$. Addition to it $C{O_2}$ initially fixed in $C4$ needs two $ATP$s to fix each $C{O_2}$ molecule. In total 12 $ATP$s are required.

Note: $C4$ cycle owes the name to the discovery by Marshall Davidson Hatch and Charles Roger Slack that some plants, when supplied with $C{O_2}$, incorporate the $C - 14$ label into the four- carbon molecules first. It is ancestral and more common to $C3$ carbon fixation.