Final answer:
The alpha-helix and pleated sheet are characteristic shapes of the secondary protein structure, which results from hydrogen bonds in the peptide backbone.
Step-by-step explanation:
The alpha-helix and pleated sheet are examples of secondary protein structure. This level of protein structure is characterized by the shape that the polypeptide chain takes on due to hydrogen bonds between the carbonyl and amino groups in the backbone of the peptide chain. The alpha-helix and beta-pleated sheet are two common types of secondary structure. The primary structure simply refers to the sequence of amino acids in the polypeptide chain, while the secondary structure forms as a result of local folding of the primary structure into these characteristic shapes. Further folding and interactions lead to the tertiary structure, and when multiple polypeptide chains come together, the quaternary structure is formed.