Tikzpicture: Drawing a graph with curved and straight parts

I am trying to produce a graph displaying concentration vs. time for a chemical equilibrium. These graphs look something like this (though note that this is just an example, not what I specifically want to produce):

I’m using the following code for my own graph:

...
pgfplotsset{compat=1.11}
...

begin{document}

begin{tikzpicture}[scale=1]
    begin{axis}[x post scale=1, y post scale=1, xmin=0, xmax=9, ymin=0, ymax=7, axis lines=left,
    restrict x to domain=0:8, enlargelimits=false, legend style={anchor=north, at={(0.5,1)}}, legend columns={2},
    ylabel={Concentration}, xlabel={Time (min)}, xtick={0, 1, 2, 3, 4, 5, 6, 7, 8}, ytick=empty]
        draw plot [smooth, domain=0:8, samples=50] coordinates {
            (0,0)
            (3,3)
            (5,3)
            (7,1)
            (8,1)
        };
    end{axis}
end{tikzpicture}

end{document}

This is what it produces:

In contrast, this is what I want (without the parts in red):

Is there a way to force the curve to flatten itself into a straight line between two coordinates, so that I can produce something like what is depicted in the second image above?

begin{tikzpicture}

draw (0,0) .. controls (1.5,1.5) and (2.3,3) .. (3,3) .. controls (4,3) and (4,3) .. (5,3) .. controls (5.5,3) and (6.6,1) .. (7,1) .. controls (7.5,1) and (7.5,1) .. (8,1);
end{tikzpicture}