Here's why:

* Tertiary structure refers to the three-dimensional shape of a single polypeptide chain. This shape arises from interactions between the amino acid side chains, including:

* Hydrogen bonds: Form between polar side chains.

* Ionic bonds: Form between oppositely charged side chains.

* Hydrophobic interactions: Non-polar side chains cluster together, excluding water.

* Disulfide bridges: Covalent bonds formed between cysteine residues.

* Secondary structure (alpha-helices and beta-sheets) is the local folding of the polypeptide chain, which then folds further to form the tertiary structure.

* Quaternary structure describes the arrangement of multiple polypeptide chains (subunits) within a protein. While not all proteins have quaternary structure, almost all proteins have tertiary structure.

Therefore, the tertiary structure is essential for the protein's function, and it's the result of the interactions between the amino acids within the polypeptide chain.