Final answer:
Agar is the polysaccharide extracted from red algae cell walls and used as a solidifying agent due to its gel-forming abilities, which is essential in biological and culinary applications.
Step-by-step explanation:
The polysaccharide found in red algal cell walls that is a useful solidifying agent is agar. Agar is extracted from the cell walls of red algae and is then purified and dried to form a powder, which can be used to create gels. These gels are suitable for growing a variety of organisms, making agar an essential tool in microbiology for culturing bacteria and other microorganisms.
While other polysaccharides like cellulose and chitin are important structural components of cell walls in plants and arthropods respectively, agar has a unique property of forming gels that can absorb large amounts of water. Such properties make agar distinct from other polysaccharides and an invaluable resource in biological sciences and culinary applications.