Protein   Macromolecules that are polymers of individual amino acids arranged in a chain and joined together by peptide bonds (and so also referred to as polypeptides). A minimum polymer length of approximately 40 amino acid units appears to be a functional size limit, and polymers shorter than this limit are commonly referred to as peptides. Proteins comprise approximately 50% of the dry weight of cells and fill a number of purposes, both functional and structural.

Used in the following modules: Absorption, Distribution and Storage of Chemicals, Carbohydrates, Cells, DNA I, DNA II, Fats and Proteins, The Nitrogen Cycle