Despite these common properties, and the fundamental unity of life they reveal, it is difficult to make generalizations about living organisms. - Albert L. Lehninger

The chemistry of living organisms is organized around carbon, which accounts for more than half the dry weight of cells.

All living organisms fall into one of three large groups (domains) [Bacteria, Archeara, Eukarya] that define three branches of evolution from a common progenitor.

Water is a polar solvent. It readily dissolves most bio­ molecules, which are generally charged or polar com­ pounds (Table 2-2); compounds that dissolve easily in water are hydrophilic (Greek, "water-loving"). In contrast, nonpolar solvents such as chloroform and benzene are poor solvents for polar biomolecules but easily dis­ solve those that are hydrophobic-nonpolar molecules such as lipids and waxes.