Some bacteria species are known to contain in their inner and outer membranes amphipatic lipids based on one or two amino acids linked to a fatty acid through an amide bond and sometimes another through an ester bond. Thus, they have a structural similarity to ceramides or lipopolysaccharides.

The difference is that lipids contain fatty acids and glycerol, while proteins contain amino acids, which have nitrogen.

The 9 essential amino acids are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

An amino acid is an organic compound characterized by having a carboxyl group, amino group, and side-chain attached to a central carbon atom. Amino acids are used as precursors for other molecules in the body. Linking amino acids together forms polypeptides, which may become proteins.

In the structure shown at the top of the page, R represents a side chain specific to each amino acid. The carbon atom next to the carboxyl group is called the α–carbon. Amino acids containing an amino group bonded directly to the alpha carbon are referred to as alpha amino acids.

All proteins, which are functional molecules of living creatures are made by combining 20 kinds of amino acids. Except one amino acid, each amino acid has two forms (isomer) named D (dexer meaning right) and L (meaning left). D-amino acid is a mirror image of L-amino acid.

I think the prevalence of L-amino acids in biological systems is a quirk of our evolutionary history; there’s no reason D-amino acids wouldn’t work chemically, but once early life forms started building proteins out of L-amino acids, L-amino acids became abundant an it became selectively advantageous to use L-amino …

Wrong! In fact, all life forms only use L-amino acids and D-sugars. The presence of only one isomer is called homochirality. Sugars can form rings, and the D-sugar forms a more stable ring than the L-sugar, but L-sugars can still exist.

Thus, D-amino acids have low nutritional value, in part because they are not digested well.