Final answer:
Cellulose differs from starch as it has β-1,4 glycosidic linkages that cannot be digested by human enzymes, making it rigid and indigestible. Starch consists of amylose and amylopectin, which have α-1,4 and α-1,6 linkages that can be easily hydrolyzed by human enzymes to provide energy.
Step-by-step explanation:
The substance that differs from starches in that the chemical links joining individual sugar units cannot be digested by human enzymes in the gastrointestinal (GI) tract is cellulose. Starch is composed of two types of polysaccharides, amylose and amylopectin. Amylose is a linear chain of D-glucose units connected by α-1,4-glycosidic linkages, whereas amylopectin is branched with α-1,6 glycosidic linkages at the branching points. In contrast, cellulose is composed of glucose monomers linked by β-1,4 glycosidic linkages, which are not digestible by human enzymes due to their different three-dimensional structures compared to starch. The rigidity of cellulose is attributed to its β-1,4 glycosidic links and the crosslinks formed between its chains, while starch is less rigid and can be easily broken down by enzymes like salivary amylase to provide energy.