Why is water biologically important? ‘Water as a biomolecule’ considers how water influences biological processes and what properties of the water molecule enable it to do so by focusing on the role of water in the structure and operation of enzymes. Water molecules are critical in maintaining the active structures of other biomolecules such as stabilizing proteins. It also asks: how can we be sure that water is essential to life? Could other molecules fill a similar role or roles? Could water be replaced by some other medium in some other life form elsewhere in the universe that can still metabolize and reproduce?