The word macromolecule refers to a small molecule of low molecular weight and is often referred to as a monomer

Amino acids
The fundamental units of protein polymers are α-amino acids. They are composed of an amino group, a carboxyl group, a hydrogen atom, and a distinctive side chain, all bonded to a carbon atom (the α -carbon). Table 9-1 lists the 20 amino acids aсcording to their side chains.
One of the 20 amino acids, proline, is an amino acid, not an α -amino acid as are the other 19. A few other amino acids are found in a number of proteins but are not coded for in DNA; they are derived from one or another of the 20 fundamental amino acids after these have been incorporated into the protein chain (post-translational modification). The derived amino acids are 4-hydroxyproline, 5-hydroxylysine, e-N-methyl-lysine, 3-methyl-histidine, γ -carboxyglutamate, desmosine, and isodesmosine.
With the exception of glycine, all amino acids contain at least one asymmetric carbon atom and are, therefore, optically active. Irrespective of the direction of rotation of plane polarized light, which can be levo- or dextro-, the only optically active amino acids that are incorporated into proteins are of the L-configuration. D-Amino acids are found in bacterial products (e.g., in cell walls) but are not incorporated in­to proteins via the ribosomal protein synthesizing system.
Amino acids are amphoteric molecules—that is, they have both basic and acidic groups. Monoamino-monocarboxylic acids exist in aqueous solution as dipolar molecules (zwit-terions). The α -carboxyl group is dissociated and negatively charged. The α -amino group is protonated and positively charged. Thus, the overall molecule is electrically neutral.
At high concentrations of hydrogen ion (low pH), the carboxyl group accepts a proton and becomes uncharged, so that the overall charge on the molecule is positive. At low concentrations of hydrogen ion (high pH), the amino group loses its proton and becomes uncharged; thus, the overall charge on the molecule is negative.
Some amino acids have side chains containing dissociating groups. Those of Aspartic and Glutamic are acidic; those of Histidine, Lysine, and Arginine are basic. Two others, Cysteine and Tyrosine, have a negative charge on the side chain when dissociated. Whether or not these groups are dissociated depends upon the prevailing pH and the pK_a of the dissociating groups. These dissociating amino acids also exist in solution as zwitterions.
12. Make questions to each paragraph.
13. Match the following English words and word combinations from the article with Russian equivalents.

1. amino acids d	a) амфотерные молекулы
2. a hydrogen atom f	b) заряд
3. chain g	c) диссоциирующие группы
4. amphoteric molecules a	d) амино кислоты
5. uncharged j	e) низкие концентрации
6. low concentrations e	f) атом водорода
7. charge b	g) цепь
8. dissociating groups c	h) раствор
9. solution h	i) ион водорода
10. hydrogen ion i	j) не имеющий заряда

14.Watch the video and answer the questions.
https://www.youtube.com/watch?v=IMC9hiRwzI4

1. 
   What are the functions of proteins?
   
2. 
   What are amino acids?
   
3. 
   What are essential and non-essential amino acids?
   
4. 
   What are the sources of amino acids?