Example: Lipid vesicle

\documentclass{article}
\usepackage{tikz}
\usepackage{tikz}
\usetikzlibrary{decorations,decorations.pathreplacing}

\begin{document}

% Define decoration
\pgfdeclaredecoration{lipidleaflet}{initial}
{
  % Place as many segments as possible along the path to decorate
  % the minimum distance between two segments is set to 7 pt.
  \state{initial}[width=\pgfdecoratedpathlength/floor(\pgfdecoratedpathlength/7pt)]
  {
    % Draw the two acyl chains
    \pgfpathmoveto{\pgfpoint{-1pt}{0pt}}
    \pgfpathlineto{\pgfpoint{-1pt}{-10pt}}
    \pgfpathmoveto{\pgfpoint{1pt}{0pt}}
    \pgfpathlineto{\pgfpoint{1pt}{-10pt}}
    % Draw the head group
    \pgfpathmoveto{\pgfpoint{1pt}{0pt}}
    \pgfpathcircle{\pgfpoint{0pt}{2pt}}{2.5pt}
  }
  \state{final}
  {
    \pgfpathmoveto{\pgfpointdecoratedpathlast}
  }
}

% Draw a vesicle composed of two lipid layers
\begin{tikzpicture}
% Micelle
\draw[decorate, decoration={lipidleaflet, mirror}] (0, 3) circle (0.6cm);
\draw (0, 2) node {Micelle};

% Inverted micelle
\draw[decorate, decoration={lipidleaflet}] (0, 0) circle (0.45cm);
\draw (0, -1) node {Inverted micelle};

% Lipid bilayer
\draw[decorate, decoration={lipidleaflet, mirror}]
  (-1, -2.8) -- (2, -2.8);
\draw[decorate, decoration={lipidleaflet}]
  (-1, -2) -- (2, -2);
\draw (0, -3.5) node {Lipid bilayer};

% Vesicle
\draw[decorate, decoration={lipidleaflet}] (5, 0.5) circle (2.5cm);
\draw[decorate, decoration={lipidleaflet, mirror}] (5, 0.5) circle (3.3cm);
\draw (5, -3.5) node {Vesicle};

\end{tikzpicture}

\end{document}