6. even though lipids and carbohydrates are made from the same elements (carbon, hydrogen, and oxygen), why is it easy to tell them apart when viewing a model of each?

Lipids doesn't have monomers while carbohydrates does and can be categorised into three: monosaccharide, disaccharide, polysaccharide.

For a polymer to be considered a polymer it must be comprised of monomers like proteins and other macromolecules.

There are 20 different type of monomers present in proteins. Amino acids are in reality the monomers of proteins. Out of the 20 amino acids, only 9 are considered essential. The other 11 are of little use to the humans. The nine of the essential amino acids required by humans are Tryptophan, lysine, Histidine, Isoleucine, Valine, Threonine, Phenylalanine, Methionine and Leucine. The amino acids combine with one another to form different types of proteins. These proteins satisfy a number of our physical functional needs.