Water is the solvent of life. Human beings are 65% water, tomatoes are 90% water, and a typical cell is about 70% water. Indeed, most organisms are mostly water, be they bacteria, cacti, whales, or elephants. Many of the organic molecules requiredfor the biochemistry of living systems dissolve in water. In essence, water rendersmolecules mobile and permits Brownian-motion-powered interactions betweenmolecules. What is the chemical basis of water’s ability to dissolve so manybiomolecules?

Water is a simple molecule, composed of two hydrogen atoms linked by covalent bonds to a single atom of oxygen. The important properties of water are due to the fact that oxygen is an electronegative atom. That is, although the bonds joining the hydrogen atoms to the oxygen atom are covalent, the electrons of the bond spend more time near the oxygen atom. Because the charge distribution is not uniform, the water molecule is said to be polar. The oxygen atom is slightly negatively charged (designated δ⁻), and the hydrogen atoms are correspondingly slightly positively charged (δ⁺).

This polarity has important chemical ramifications. The partially positively charged hydrogen atoms of one molecule of water can interact with the partially negatively charged oxygen atoms of another molecule of water. This interaction is called a hydrogen bond (Figure 2.1). As we will see, hydrogen bonds are not unique to water molecules and in fact are common weak bonds in biomolecules. Liquid water has a partly ordered structure in which hydrogen-bonded clusters of molecules are continually forming and breaking apart, with each molecule of water hydrogen-bonding to an average of 3.4 neighboring molecules. Hence, water is cohesive. The polarity of water and its ability to form hydrogen bonds renders it a solvent for any charged or polar molecule.

The giant trees of the redwood forests—trees that can be hundreds of feet tall—are a remarkable demonstration of the cohesive power of water imparted by hydrogen bonds. Water rises to the treetops by transpiration, the evaporation of water from the topmost leaves. Transpiration pulls the water up from the roots. This column of water is maintained by hydrogen bonds between water molecules. Indeed, the strength of the hydrogen bond may play a limiting role in the ultimate height attained by a redwood tree. When the bonds break, an embolism (air bubble) forms, preventing further water flow to the top of the tree (Figure 2.2).

Although water’s ability to dissolve many biochemicals is vitally important, the fact that water cannot dissolve certain compounds is equally important. A certain class of molecules termed nonpolar, or hydrophobic, cannot dissolve in water. These molecules, in the presence of water, behave exactly as the oil in an oil-and-vinegar salad dressing does: they sequester themselves away from the water, a process termed the hydrophobic effect. However, living systems take great advantage of this chemical animosity to power the creation of many elaborate structures required for their continued existence. Indeed, cell and organelle membranes form because of the hydrophobic effect.