Structurally, how does a polysaccharide differ from a polypeptide?

Polysaccharides are , long chains of monosaccharides, made of carbon, hydrogen, and oxygen, often in a 1:2:1 ratio. Polypeptides are , long chains of amino acids, made of carbon, hydrogen, oxygen, nitrogen, and a variety of other element, not in a particular ratio.

Polysaccharides include starch and glycogen and are often used for energy storage in organisms. Below is the structure of part of a starch molecule (the full molecule is too large to show because it can be hundreds of monomers long):

Polypeptides are long, unbranched chains of amino acids and can link together to form proteins like hemoglobin. Below is an image explaining a bit about protein structure: