The terms associated with $\mathrm{C}_4$ cycle are

Hatch Slack Pathway The process of synthesis of glucose in $\mathrm{C}_4$ plants is different from $\mathrm{C}_3$ plants which was discovered by two scientists MD Hatch and CR Slack (1977). Hence, named Hatch Slack pathway.

PEP Carboxylase It is an enzyme present in mesophyll cells of $\mathrm{C}_4$ plants. It fixes $\mathrm{CO}_2$ to form 4-carbon compound, i.e., oxaloacetate (OAA).

Bundle Sheath Cells These are specialised sclerenchymatous cells present arround the vascular bundle, in the veins of monocot leaves. These have agranal chloroplast. $\mathrm{C}_3$ cycle occurs in these cells to manufacture glucose in $\mathrm{C}_4$ plants.

Calvin Cycle This cycle operates in bundle sheath cells and fixes $\mathrm{CO}_2$ to form glucose molecules.

NADP reductase enzyme is located on the outer side of lamella or thylakoid of the chloroplast. This enzyme causes breakdown of proton gradient to release energy, i.e., NADPH.

ATP synthase enzyme has two parts

(a) $\mathrm{F}_1$-head piece is a peripheral membrane protein complex and contain the site for synthesis of ATP from ADP + pi (inorganic phosphate).

(b) $\mathrm{F}_0$-integral membrane protein complex that forms the channel through which proton cross the inner membrane.

The arrangment of $\mathrm{F}_1$ and $\mathrm{F}_0$ in thylakoid membrane is as follaws

$\mathrm{F}_0$-portion is present within the thylakoid membrane.

$\mathrm{F}_1$-portion of ATP synthase enzyme is present in the stroma if chloroplast. The conformational change occurs in $\mathrm{F}_1$ portion of ATP synthase thus, facilitaling the ATP synthesis.

ATP and NADPH formed during light reaction of photosynthesis are used in dark reaction for fixing $\mathrm{CO}_2$ and to form glucose molecule.

The basis for designating $\mathrm{C}_3$ and $\mathrm{C}_4$ pathways of photosynthesis is as follows

$\mathrm{C}_3$ Pathway	$\mathrm{C}_4$ Pathway
In Calvin cycle or $C_3$ cycle (dark reaction), carbon dioxide is fixed into first stable compound called 3-PGA, (3-phorphoglyceric acid) which is a 3 carbon compund.	In $C_4$ plants, the Calvin cycle $C_3$ cycle occurs in bundle sheath cells. Prior to reaching there, the $\mathrm{CO}_2$ is trapped by mesophyll cells and fixed into a 4-carbon stable product called oxalo acetic acid.