Chapter 2 – Orientation for the Bio-Curious  27

KEY POINT 2.7

Phospholipid bilayers are ubiquitous self-​assembled structures in cells that represent

an enormous barrier for water molecules.

2.3.3  AMINO ACIDS, PEPTIDES, AND PROTEINS

Amino acids are the building blocks of larger important biological polymers called “peptides”

or, if more than 50 amino acids are linked together, they are called “polypeptides” or, more

commonly, “proteins.” Amino acids consist of a central carbon atom from which is linked an

amino (chemical base) group, −NH2, a carboxyl (chemical acid) group, −COOH, a hydrogen

atom −H, and one of 23 different side groups, denoted usually as −R in diagrams of their

structures (Figure 2.5a), which defines the specific type of amino acid. These 23 constitute the

natural or proteinogenic amino acids, though it is possible to engineer artificial side groups

to form unnatural amino acids, with a variety of different chemical groups, which have been

utilized, for example, in bioengineering (see Chapter 9). Three of the natural amino acids

are usually classed as nonstandard, on the basis of either being made only in bacteria and

archaea, or appearing only in mitochondria and chloroplasts, or not directly being coded by

the DNA, and so many biologists often refer to just 20 natural amino acids, and from these

the mean number of atoms per amino acid is 19.

It should be noted that theα-​carbon atom is described as chiral, indicating that the amino

acid is optically active (this is an historical definition referring to the phenomenon that a

solution of that substance will result in the rotation of the plane of polarization of incident

light). The α-​carbon atom is linked in general to four different chemical groups (barring the

simplest amino acid glycine for which R is a hydrogen atom), which means that it is possible

for the amino acid to exist in two different optical isomers, as mirror images of each other—​

a left-​handed (L) and right-​handed (D) isomers—​with chemists often referring to optical

isomers with the phrase enantiomers. This isomerism is important since the ability for other

molecules to interact with any particular amino acid depends on its 3D structure and thus is

specific to the optical isomer in question. By far, the majority of natural amino acids exist as

l-​isomers for reasons not currently resolved.

The natural amino acids can be subdivided into different categories depending upon a

variety of physical and chemical properties. For example, a common categorization is basic

FIGURE 2.4  Structures formed from lipid bilayers. (a) Liposome, light and dark showing

different phases of phospholipids from molecular dynamics simulation (see Chapter 8). (b) The

cell membrane and nuclear membranes, from a human cheek cell taken using phase contrast

microscopy (Chapter 3).

(a: Courtesy of M. Sansom; b: Courtesy of G. Wright, CIL:12594.)