Final answer:
Cellulose is the most abundant carbohydrate on Earth, forming unbranched chains of β-glucose units and is a key structural component in plant cell walls. It is indigestible by humans due to the absence of cellulase but aids in digestion by contributing to dietary bulk.
Step-by-step explanation:
The most abundant carbohydrate on Earth, which is composed of unbranched chains of β-glucose subunits and serves as the primary structural fiber in plant cell walls, is cellulose. Cellulose is made up of β-D-glucose units linked by β-1→4 glycosidic linkages. It forms significant structural materials such as wood and paper. Humans cannot digest cellulose because we lack the enzyme cellulase, but it still contributes to dietary bulk and aids in gastrointestinal movement.