Final answer:
Cellulose is the polysaccharide that humans cannot digest but microorganisms in termite guts can, allowing termites to utilize it for energy.
Step-by-step explanation:
The polysaccharide that is indigestible by humans but digestible by microorganisms living in the digestive tracts of termites is cellulose.
Humans lack the enzyme cellulase, necessary for breaking down the ß-glycosidic linkages in cellulose, therefore we cannot obtain glucose from it. However, the microorganisms in the guts of termites produce cellulase, enabling termites to digest cellulose from wood, which serves as a glucose source for energy.