brane. In many ways, these membranes are more like fluids in their
properties than they are like solids.
This brings us to the next category of biological molecule: proteins.
Proteins are large molecules built from amino acids linked into long
chains by covalent chemical bonds, called peptide bonds. So, first, what
is an amino acid? In organic chemistry, an amino acid is a molecule
that contains both an amine group (-NH2) and a carboxylic acid group
(-COOH). The amino acids used as protein building blocks by all life
on Earth are characterized by having amine and acid groups linked
to the same carbon atom; these are termed alpha-amino acids. Here R
represents a portion of the molecule containing other atoms. Differ-
ent amino acids are characterized by having different R groups. The
simplest amino acid has R = H. This amino acid is called glycine. The
next simplest has R = -CH3. This is alanine. Another amino acid, called
phenylalanine, has an R group consisting of a carbon attached toa
benzene ring. Life on Earth uses twenty different amino acids (charac-
terized by twenty different R groups) as the molecular building blocks
of proteins.
NH>
R-C-COOH
H
Alpha-amino
acid