<P> Agar consists of a mixture of two polysaccharides: agarose and agaropectin, with agarose making up about 70% of the mixture . Agarose is a linear polymer, made up of repeating units of agarobiose, a disaccharide made up of D - galactose and 3, 6 - anhydro - L - galactopyranose . Agaropectin is a heterogeneous mixture of smaller molecules that occur in lesser amounts, and is made up of alternating units of D - galactose and L - galactose heavily modified with acidic side - groups, such as sulfate and pyruvate . </P> <P> Agar exhibits hysteresis, melting at 85 ° C (358 K, 185 ° F) and solidifying from 32--40 ° C (305--313 K, 90--104 ° F). This property lends a suitable balance between easy melting and good gel stability at relatively high temperatures . Since many scientific applications require incubation at temperatures close to human body temperature (37 ° C), agar is more appropriate than other solidifying agents that melt at this temperature, such as gelatin . </P> <P> An agar plate or Petri dish is used to provide a growth medium using a mix of agar and other nutrients in which microorganisms, including bacteria and fungi, can be cultured and observed under the microscope . Agar is indigestible for many organisms so that microbial growth does not affect the gel used and it remains stable . Agar is typically sold commercially as a powder that can be mixed with water and prepared similarly to gelatin before use as a growth medium . Other ingredients are added to the agar to meet the nutritional needs of the microbes . Many specific formulations are available, because some microbes prefer certain environmental conditions over others . Agar is often dispensed using a sterile media dispenser . </P> <P> As a gel, an agar or agarose medium is porous and therefore can be used to measure microorganism motility and mobility . The gel's porosity is directly related to the concentration of agarose in the medium, so various levels of effective viscosity (from the cell's "point of view") can be selected, depending on the experimental objectives . </P>

What makes agar a good support for microbial growth
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