"In the center is а grаy squаre representing a carbоn atоm, labeled with the letter C, the wоrd ""Carbon,"" the number 6, and the atomic mass 12.0107. A label ""Alpha carbon"" with an arrow points to this central carbon. Extending upward from the central carbon is a single hydrogen atom labeled H. To the left is a cyan box labeled ""Amino functional group"" containing the amino group with nitrogen N bonded to two hydrogen atoms H. To the right is a magenta box labeled ""Carboxylic acid functional group"" containing a carbon C double-bonded to oxygen O and single-bonded to OH. Extending downward from the central carbon is a position labeled ""Side chain functional group (R group)"" with a blue box containing the letter R to represent variable side chains." This monomer comes in at least 22 different varieties in living organisms. Which part varies and is responsible for the different chemical characteristics of these monomers?