  Living cells are soft bodies of a characteristic form, but endowed with a
capacity for a steady turnover of their structures. Both of these material
properties, i.e. recovery of the shape after an external stress has been
imposed and dynamic structural reorganization, are essential for many cellular
phenomena. Examples are mechanical properties of tissue, cell motility, cell
growth and division, and active intracellular transport. Numerous experiments
in vivo and in vitro have shown that the structural element responsible for the
extraordinary mechanical and dynamical properties of eukaryotic cells is the
cytoskeleton, a three-dimensional assembly of protein fibers such as actin
filaments and microtubules. In addition to those biopolymers various proteins
with structural and regulatory functions have a major influence on the
mechanical properties. At the relevant length-scales (a few microns at most)
the building blocks of these biomaterials are very different from conventional
polymeric material. In contrast to flexible polymers the persistence length is
of the same order of magnitude as their total contour length or even larger.
This implies that the physics of such a system is determined by a subtle
interplay between energetic and entropic contributions. We review our present
understanding of the physics of biopolymers using concepts from macromolecular
and statistical physics complemented by computer simulation. These systems open
up a new field of soft condensed matter research, which to date is only poorly
understood but has a great potential for interesting new physical phenomena.
