A nutritionist is comparing the biological molecules found in a chicken breast and a stick of butter. Explain how the structures and functions of proteins and lipids differ, and state one element found in proteins that is not found in lipids.

Proteins are polymers of amino acids, which contain an amino group, a carboxyl group and a variable side chain that can include nitrogen.
Lipids are composed of glycerol esterified with fatty acids; they lack the amino and carboxyl groups and are largely non‑polar.
Because of their varied side chains, proteins can fold into complex three‑dimensional shapes and act as enzymes, antibodies, hormones and structural components.
Lipids, being hydrophobic, are stored as energy reserves, provide insulation and waterproofing, and form cell membranes.
Nitrogen (or sulfur) is present in the side chains of amino acids but is absent from lipid molecules.