  The structure of tree-level open and closed superstring amplitudes is
analyzed. For the open superstring amplitude we find a striking and elegant
form, which allows to disentangle its alpha'-expansion into several
contributions accounting for different classes of multiple zeta values. This
form is bolstered by the decomposition of motivic multiple zeta values, i.e.
the latter encapsulate the alpha'-expansion of the superstring amplitude.
Moreover, a morphism induced by the coproduct maps the alpha'-expansion onto a
non-commutative Hopf algebra. This map represents a generalization of the
symbol of a transcendental function. In terms of elements of this Hopf algebra
the alpha'-expansion assumes a very simple and symmetric form, which carries
all the relevant information. Equipped with these results we can also cast the
closed superstring amplitude into a very elegant form.
