Agar

Agar is a gel-forming mixture of polysaccharides extracted mainly from red algae. It forms firm gels that remain solid at temperatures where gelatin melts.

Agarose: Agarose is the neutral polysaccharide fraction of agar, widely used to form gels for laboratory separations. This fraction supplies most of agar’s gel strength and is purified for laboratory use.
Agar plate: An agar plate is a shallow dish containing solidified nutrient medium for growing microorganisms. Agar’s firmness and low melting temperature make it practical for culturing microbes.
Red algae: Red algae are a diverse group of photosynthetic organisms, mostly aquatic, containing the pigment phycoerythrin. Agar is extracted from the cell walls of several red-algal groups.
Gelatin: Gelatin is a mixture of proteins produced by partial hydrolysis of collagen and used as a gelling agent. Unlike animal-derived gelatin, agar is plant-like in origin and remains firm at warmer temperatures.
Agaropectin: Agaropectin is the charged, chemically variable polysaccharide fraction of agar. Its sulfate and other charged groups distinguish it from agarose and affect gel properties.
Microbiological culture: Microbiological culture is the growth of microorganisms in controlled laboratory conditions. Solid agar media make it possible to isolate and observe microbial colonies.
Gelidium: Gelidium is a genus of red algae that has long been an important commercial source of agar. Its species yield agar valued for forming strong gels.
Carrageenan: Carrageenan is a family of sulfated polysaccharides extracted from red algae and used as a thickener and gelling agent. It shares an algal source with agar but has distinct structures and gel behavior.
Thermoreversible gel: A thermoreversible gel liquefies when heated and reforms upon cooling. Agar sets on cooling and melts at a substantially higher temperature than its setting point.
Agar well diffusion test: The agar well diffusion test assesses antimicrobial activity by measuring inhibition zones around substances diffusing through agar. The gel provides a matrix through which tested antimicrobial compounds spread.

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