The importance of water

Abstract

All life on Earth needs water to survive, and special strategies are needed to cope with water scarcity, for instance because of extremes of either heat or cold. This situation has promoted the common view that water is a prerequisite for life in the universe as a whole, with important consequences for predictions about the likelihood of habitable environments. But we cannot assess that claim until we have a thorough understanding of the part that water does play in sustaining terrestrial life. In this chapter I will review the case for considering water to be a versatile, adaptive component of the cell that engages in a wide range of biomolecular interactions: for example, mediating protein-protein and receptor-substrate interactions, facilitating proton transport, driving hydrophobic interactions and their sensitivity to small solutes, acting as a reagent in biochemical reactions, and modulating electronic excitation energies. The chapter will aim to provide some basis for assessing water’s often-alleged uniqueness as life’s solvent. I conclude that, while we cannot with any confidence assert that all life must be aqueous, it is hard to identify any other solvent that could match the versatility and in particular the responsiveness of water in mediating the kind of molecular interactions likely to be required in any living system.