Which of the following are correctly associated?
(A) primary protein structureindividual amino acids
(B) secondary protein structurealpha helices
(C) tertiary protein structurebeta-pleated sheets
(D) quaternary protein structurethe completed protein complex
(B) and (D) are associated correctly. You may have been tempted to pick (A), but primary protein structure is the polypeptide chain, not the individual amino acids. Tertiary protein structure is the completed folding of the polypeptide, but not necessarily the completed protein (C), which may have multiple polypeptides. Beta-sheets are another secondary protein structure.