Final answer:
On a scale from least to most complex, amino acids rank as 1, peptides as 2, polypeptides as 3, and proteins as 4.
Step-by-step explanation:
The complexity of molecules in biology can be ranked according to the size and structure. In the context of protein structure, the complexity ranges from the simplest form, an amino acid, to the most complex structure, a protein. Amino acids are the basic building blocks of proteins. When they join together through peptide bonds, they form a peptide or a polypeptide, depending on the length of the chain.
As more amino acids join, the chain becomes longer and can fold into a complex three-dimensional structure, resulting in a protein which can consist of one or more polypeptide chains and have multiple levels of structural organization.
Using a scale of 1 to 4, with 1 being the least complex and 4 being the most complex, the molecules can be ranked as follows: amino acid (1), peptide (2), polypeptide (3), and protein (4).