What type of polysaccharide from algae is commonly used to solidify growth media?

Agar is a gelatinous substance derived from red algae, specifically from the cell walls of certain species. It has unique properties that make it particularly suitable for solidifying growth media used in microbiology and botany. When dissolved in boiling water and cooled, agar creates a semi-solid gel that provides a stable matrix for the growth of microorganisms and plant tissues. This stability is essential for creating a controlled environment for experiments and cultivations, as it prevents the migration of cells and allows for the establishment of colonies.

The other options, while being polysaccharides, have different functions and aren't typically used for solidifying growth media. Pectin, for instance, is commonly used in food applications, particularly as a gelling agent in jams and jellies. Starch serves as an energy reserve in plants and can be used in some media, but it does not have the setting properties of agar. Cellulose, primarily found in plant cell walls, provides structural support but does not offer the gel-like consistency needed to solidify culture media effectively. This distinct characteristic of agar as a gelatinous solidifying agent makes it the preferred choice in laboratory settings.