Final answer:
Bacterial cell walls are composed of peptidoglycan, a unique, complex polysaccharide with cross-linking amino acid chains.
Step-by-step explanation:
The complex polysaccharide that composes bacterial cell walls is known as peptidoglycan. This substance is unique to bacterial cell walls and consists of long polysaccharide chains that are cross-linked by short chains of amino acids, forming a mesh-like layer that provides structural support and shape to the bacterial cell.
Gram-positive and Gram-negative bacteria differ in the thickness of their peptidoglycan layer, with Gram-positive bacteria having a thicker layer, while Gram-negative bacteria have a thinner layer covered by an additional outer membrane.