File:Visual cycle.svg

\documentclass[12pt,border=0pt,tikz,class=scrartcl]{standalone}
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\usepackage[latin1]{inputenc}
\usepackage{lmodern}
\usepackage{mathtools,amssymb}
\usepackage{pgfplots}
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\usetikzlibrary{positioning,calc,arrows,fit,backgrounds,decorations.text,shapes}

\usepackage{chemfig,chemmacros}
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\pagestyle{empty}
\begin{document}
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		\chemfig{*6(--(-)=(
		-=_[::-60]-[::60](-[2])
		=_[::-60]-[::60]=_[::-60]-[::60](-[2])
		=_[::-60]-[::60]=^[::-60]N(-[6,.6,,1,draw=none]H|^{\scriptstyle+})-[::60,,,1]R|hodopsin
		)-(-[::-30])(-[::-90])--)}
	};
	\node at ($(n1)-(25em,0)$) (n2) {
		\chemfig{*6(--(-)=(
		-=_[::-60]-[::60](-[2])
		=_[::-60]-[::60]=_[::-60]-[::60](-[2])
		=_[::-60]-[::60]=_[::-60]O
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	};
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		\chemfig{*6(--(-)=(
		-=_[::-60]-[::60](-[2])
		=_[::-60]-[::60]=_[::-60]-[::60](-[2])
		=_[::-60]-[::60]-[::-60]OH
		)-(-[::-30])(-[::-90])--)}
	};
	\node at ($(n3)-(0,3.8)$) (n4) {
		\chemfig{*6(--(-)=(
		-=_[::-60]-[::60](-[2])
		=_[::-60]-[::60]=_[::-60]-[::60](-[2])
		=_[::-60]-[::60]-[::-60]OH
		)-(-[:120])(-[:60])--)}
	};
	\node at ($(n4.west)-(0,4)$) (n5) [anchor=west] {
		\chemfig{*6(--(-)=(
		-=_[::-60]-[::60](-[2])
		=_[::-60]-[::60]=_[::-60]-[::60](-[2])
		=_[::-60]-[::60]-[::-60]O-[::60](=[2]O)-[::-60]R
		)-(-[:120])(-[:60])--)}
	};
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		\chemfig{*6(--(-)=(
		-=_[::-60]-[::60](-[2])
		=_[::-60]-[::60]=_[::-60]-[6](-[::-60])=^[::60]-[::-60]-[::60]OH
		)-(-[:120])(-[:60])--)}
	};
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		\chemfig{*6(--(-)=(
		-=_[::-60]-[::60](-[2])
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		\chemfig{*6(--(-)=(
			-=_[::-60]-[::60](-[2])
			=_[::-60]-[::60]=_[::-60]-[6](-[::-60])=^[::60]-[::-60]=^[::60]
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			)-(-[:120])(-[:60])--)}
	};
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		\chemfig{*6(--(-)=(
			-=_[::-60]-[::60](-[2])
			=_[::-60]-[::60]=_[::-60]-[6](-[::-60])=^[::60]-[::-60]-[::60]OH
			)-(-[:120])(-[:60])--)}
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		\chemfig{*6(--(-)=(
			-=_[::-60]-[::60](-[2])
			=_[::-60]-[::60]=_[::-60]-[6](-[::-60])=^[::60]-[::-60]-[::60]O-[::-60] (-[::60]R)=[::-60]O
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	\end{pgfonlayer}
	\draw [-latex,ultra thick] 
		(n1.west) -- (n2.east) 
		node [pos=.5,above] {\large\chemfig[green!50!black]{H_2O}};
	\draw [-latex,ultra thick] 
		(n2.south) node [above right] {\large all-trans retinal} 
		-- (n3.north) 
		node [pos=.5,left] {\large\color{green!50!black}all-trans RDH};
	\draw [-latex,ultra thick] 
		(n3.south) node [above right] {\large all-trans retinol} 
		-- (n4.north);
	\draw [-latex,ultra thick] 
		(n4.south) node [above right] {\large all-trans retinol} 
		-- (n4.north |- n5.north) 
		node [pos=.5,right] {\large\color{green!50!black}LRAT};
	\draw [-latex,ultra thick] 
		node at ($(n5)+(1,-1)$) {\large all-trans retinyl ester} 
		(n5.east) -- (n6.west) 
		node [pos=.5,above] {\large\color{green!50!black}RPE65};
	\draw [very thick] 
		(n9.north) .. controls ++(0,1.5) and ++(-1.5,0) .. ($(n5.east)!.5!(n6.west)$);
	\draw [-latex,very thick] 
		($(n5.east)!.5!(n6.west)$) .. controls ++(1.5,0) and ++(0,1.5) .. (n10.north);
	\draw [-latex,ultra thick] 
		node at ($(n6)+(0,-1)$) {\large 11-cis retinol} 
		(n6.north) -- ($(n7.south)+(0,.45)$) 
		node [pos=.5,left] {\large\color{green!50!black}11-cis RDH} 
		node [pos=1,above] {\large 11-cis retinal};
	\draw [-latex,ultra thick]
		(n7.north) -- (n7.north |- n8.south)
		node [pos=1,left] {\large 11-cis retinilidene};
	\draw [-latex,ultra thick]
		(n8.north) -- ($(n1.south)-(0,.1)$)
		node [pos=1,above] {\large all-trans retinilidene}
		node [pos=.5,right,inner xsep=8pt] {\large\color{green!50!black}h$\nu$};
	\draw [decorate, decoration={brace,amplitude=2em}, ultra thick]
		(big1.south west) -- (big1.north west)
		node [pos=.5,anchor=south,xshift=-20pt,rotate=90] {\Large Rod outer segment};
	\draw [decorate, decoration={brace,amplitude=2em}, ultra thick]
		(big2.south west) -- (big2.north west)
		node [pos=.5,anchor=south,xshift=-20pt,rotate=90] {\Large Retinal pigment epithelium};
\end{tikzpicture}
\end{document}