  Water shapes and defines the properties of biological systems. Therefore,
understanding the nature of the mutual interaction between water and biological
systems is of primary importance for a proper assessment of biological activity
and the development of new drugs and vaccines. A handy way to characterize the
interactions between biological systems and water is to analyze their impact on
water density and dynamics in the proximity of the interfaces. It is well
established that water bulk density and dynamical properties are recovered at
distances in the order of $\sim1$~nm from the surface of biological systems.
Such evidence led to the definition of \emph{hydration} water as the thin layer
of water covering the surface of biological systems and affecting-defining
their properties and functionality. Here, we review some of our latest
contributions showing that phospholipid membranes affect the structural
properties and the hydrogen bond network of water at greater distances than the
commonly evoked $\sim1$~nm from the membrane surface. Our results imply that
the concept of hydration water should be revised or extended, and pave the way
to a deeper understanding of the mutual interactions between water and
biological systems.
