Encircle an arrow in TikZ

Question

I'm preparing some lecture notes and have some trouble creating an encircled arrow like the one in the picture.

What I'd like to do is to create an elliptic shape around an arrow defined in TikZ (like the one in the code example).

\draw[->] (0,0) -- (1,1);

enter image description here

Here's the full code for the picture:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usepackage{mhchem}
\usepackage{textgreek}

\usetikzlibrary{decorations,decorations.pathreplacing,decorations.pathmorphing}

\begin{document}

\begin{tikzpicture}
    \draw (-0.5,-1) node{+}
    (-0.5,5) node{-}
    (-0.5,5.5) node{Redoxpotential}
    (2,-3.7) node{Fotosystem II}
    (6,-2) node{Fotosystem I};
    \path (0.7,-0.7) node(1) {ljus}
    (2,-2) node[rectangle,draw](2) {P680}
    (2,3) node(3) {\ce{e-}}
    (3,2) node(4) {\ce{QH2}}
    (4,1) node(5) {Cyt b$_6$f}
    (3,0) node(6) {\ce{H+} lumen}
    (5,0) node(7) {Plastocyanin}
    (6,-1) node[rectangle,draw](8) {P700}
    (7.2,0.2) node(9) {ljus}
    (6,4) node(10) {\ce{e-}}
    (7.5,2.5) node(11) {\ce{e-}-redoxin (Fd)};
    \draw[decorate, decoration={snake},draw=red,->] (1) -- (2);
    \draw[->] (1.5,-2.74) to[out=60,in=180] (2,-2.24) to[out=0,in=120] (2.5,-2.74);
    \draw (1.3,-2.94) node[]{\ce{2H2O}} (2.98,-2.907)node[]{\ce{O2 + 4H+}};
    \draw[->] (2) -- (3);
    \draw[->] (3) -- node[midway,right]{\ce{e-}-transportkedja} (4);
    \draw[->] (4) -- (5);
    \draw[->] (5) -- (6);
    \draw[->] (5) -- (7);
    \draw[->] (7) -- (8);
    \draw[decorate, decoration={snake},draw=red,->] (9) -- (8);
    \draw[->] (8) -- (10);
    \draw[->] (10) -- node[midway,right]{\ce{e-}-transportkedja} (11);
    \draw[->] (11) -- (8.5,1.5);
    \draw[->] (8.15,0.95) node[below]{NADP} to[out=75,in=215] (8.5,1.5) to[out=35,in=190] (9.1,1.7) node[right]{NADPH};
    \draw (7.6,0) node[right]{\ce{2Fd + NADP -> 2Fd + NADPH}};
    \draw[dotted,->] (11) -- (5);
\end{tikzpicture}

\end{document}

Could you provide some example code of what you've done so far? Thank's in advance! — Thanos, Mar 19 '13 at 07:18
See How to fit ellipse into another object - using intersections and calc library — Jake, Mar 19 '13 at 07:48
Ah, I didn't see that... Well, here it is again. Should probably be closed as duplicate. — nickpapior, Mar 19 '13 at 07:56

score 5 · Accepted Answer · answered Mar 19 '13 at 07:55

You can realize this drawing by in line calculation of the elliptic shape.

That is, you need to calculate:

The radius of the elliptic shape
The angle at which it resides.
The center of the elliptic shape

All of these can be performed using the calc library of TikZ.

Here is an example:

\begin{tikzpicture}
  \coordinate (a) at (0,0); % Save the end-coordinates of the arrow
  \coordinate (b) at (4,7);
  \draw[->] (a) -- (b);
                  % Calculate the vector from a to b:
  \draw let \p1 = ($(b)-(a)$),
                  % radius of the elliptic shape, along the arrow:
            \n1 = {veclen(\x1,\y1)/2},
                  % angle of rotation:
            \n2 = {atan(\y1/\x1)}
             % START DRAWING
            in ($(a)!.5!(b)$) circle[x radius=\n1,
                                     y radius=20pt,
                                     rotate=\n2];
\end{tikzpicture}

Notice that in \n1 = ... you can also add values so that you get an extent of the elliptic shape around the arrow.

Notice also that you can pretty much do whatever you want in this environment, by reusing the coordinates.

enter image description here

Encircle an arrow in TikZ

1 Answers1