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.
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. |