∞ generated and posted on 2016.12.04 ∞
The subunits that combine in linear chains to make up polypeptides.
Amino acids come in 20 naturally translated varieties (this actually is a slight lie and is better stated as "commonly translated" since there are a few additional varieties that a few organisms also use). This idea of commonly or naturally translated, though is differential from that of "essential" as in essential amino acids. The difference is between what amino acids are used to synthesize proteins within your body versus what amino acids are required in your diet so that you can produce those proteins, where a greater variety of amino acids are found in your body than are essential in your diet (the difference being between what your body can and cannot synthesize).
Of the 20 amino acids that are commonly translated, 19 consist of a straightforward chemical form. That is, they consist of a central carbon, or α carbon, that is bonded to four atoms or functional groups. One of those bonds is always to a hydrogen atom. A second is always to a carboxyl group. And a third is always to an amino group (hence, by the way, "amino" "acid"). That leaves one bond which distinguishes among these 19 different amino acids, and that bond is to what is referred to as an R group.
Figure legend: Basic structure of amino acids, ignoring the single covalent bonds between 'boxes'. All amino acids have an amino group, a carboxyl group, and a hydrogen, all bonded to the central or alpha carbon. Since carbon atoms generally will participate in four bonds, the fourth bond is, essentially, to something else. That something else can vary greatly, but in biological systems it typically is limited 20 different structures or R groups. The differences between the chemistries associated with amino acids is a consequence of chemical differences among these R groups.
The 20th amino acid, proline, is a slight variation on this theme, where the R group equivalent instead forms a ring that includes the amino group.
When you get enough "protein" in your diet, what you really mean is that you are getting both enough and the right kinds of amino acids, since, as noted, our bodies can't synthesize all 20 of them.