3. Polar Molecules—What’s So Great About Water?

With our discussion of hydrogen bonding, we’ve stumbled onto concepts that permit us to discuss the structure of water and its importance in biological systems.

Water, as all terrestrial living things know, is great stuff. It’s a unique molecule—it occupies three physical phases (gas, liquid, solid) at terrestrial temperatures and pressures. It’s our most abundant greenhouse gas, and therefore important for climactic regulation. It exhibits surface tension and capillary action, which allow for the efficient sequestration and mobilization of water by ecosystems and organisms. It’s less dense as a solid than as a liquid, which is very important ecologically—if ice sank, rivers, lakes and ponds would become uninhabitable in winter –in fact, they’d freeze completely. But for the cell, the most important features of water have to do with its solvent properties, and these in turn depend on its nature as a polar molecule.