Water

Water (H₂O) is vital for all living organisms, and it is no exaggeration to say that life could not occur without it. The central feature of the water molecule is the bond between the strong electron attractor oxygen and the weak attractor hydrogen. This creates a polar covalent bond, with a weak positive charge on each hydrogen and a weak negative charge on the oxy­gen. The polar water molecule dissolves ions (such as sodium, essential for membrane transport) and polar molecules (including sugars) while exclud­ing large nonpolar molecules such as fats. This selective solvation forms the basis of cell structure and function, in which large insoluble membranes enclose aqueous solutions of nutrients and other small molecules.

Water is liquid between 0 and 100 degrees Celsius (32 to 212 degrees Fahrenheit), a temperature range that is high enough to promote random mixing in aqueous solutions (necessary for biochemical reactions) but low enough to prevent random breaking of most covalent bonds, which would make stable life forms impossible. Most organisms must live in the low end of this range. Finally, its high heat capacity moderates temperature changes, especially in organisms with large bodies.