How are protein molecules best described?

Protein molecules are best described as many amino acids linked together. Proteins are complex macromolecules composed of one or more long chains of amino acids, which are the building blocks of proteins. These amino acids are connected by peptide bonds, forming polypeptides. The specific sequence and orientation of amino acids in a protein determine its unique structure and function, which can vary significantly depending on the composition and arrangement of the amino acids.

The other options do not adequately represent proteins. A single amino acid refers to just one unit rather than the complex structure of a protein. Polysaccharides are long chains of sugar molecules, which are entirely different from proteins. Simple lipids consist of fatty acid chains and glycerol, which again do not resemble the structure of proteins. Thus, the option that describes proteins accurately emphasizes the connection of multiple amino acids into complex structures.