to hang out with) other things that have polarity or charge. We call
things that like to hang out with water hydrophilic (Greek hydro =
water, philos = loving). Hydrophilic substances like to be around
water, and water likes to be around them. At the other extreme of
solubility in water, consider the hydrocarbons. These molecules con-
sist only of carbons and hydrogens linked by covalent bonds. When
electrons are shared among carbon and hydrogen atoms, the electrons
are essentially equally likely to be found in proximity to either type
of atom. Thus, there is no significant polarity, or separation of charge,
in a hydrocarbon molecule and so nothing with which a water mole-
cule can form a hydrogen bond. Thus, hydrocarbons don’t dissolve in
water. This is illustrated by the well-known adage that oil and water
don’t mix. Substances such as hydrocarbons that don’t dissolve in
water—that don’t like to hang out with water—are called hydrophobic
(Greek phobos = fearing). Hydrophilic things like to hang out with
other hydrophilic things, and hydrophobic things like to hang out
with other hydrophobic things.
Okay, so where is all this chemistry stuff taking us? Let’s return to the
cell. Cells are the fundamental organizational units for all known liv-
ing organisms. There are some characteristics shared by all cells, from
bacterial cells to cells in the human brain. Among these shared fea-
tures are a boundary membrane (composed of phospholipid bilayer),
genetic material (composed of nucleic acids), ribosomal structures
(for protein synthesis), protein receptors, pumps, and channels
within the cell membrane, and so on. The component materials, the
building blocks of a cell, are molecules, and the machinery of life, that
which characterizes a cell as living rather than nonliving, is under-
stood to be the chemical processes taking place within. Some of the
molecules from which cells are constructed are quite large, consisting