Question

1. amino acid

20.

1. nucleotide
2. monosaccharide

Explanation:

The structure in question 20 has an amino group ($\ce{-NH2}$), a carboxyl group ($\ce{-COOH}$), and a side - chain ($\ce{-R}$) attached to a central carbon atom, which is the characteristic structure of an amino acid.

Answer:

Amino acid