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Starch and cellulose are both polysaccharides made up of glucose units. Polysaccharides are large carbohydrate molecules formed by the polymerization (combination) of many monosaccharides, such as glucose. In this case, starch and cellulose are polymers of glucose, meaning they are made up of many glucose molecules linked together.

Before diving into the difference between starch and cellulose, it is important to understand the difference between α-glucose and β-glucose since these two forms of glucose are involved in their respective structures. The primary distinction between the two isomers lies in the position of the hydroxyl group (-OH) at carbon 1 (anomeric carbon). In α-glucose, the hydroxyl group is positioned below the plane of the ring, while in β-glucose, it is above the plane of the ring.

Starch is composed of two types of glucose polymers - amylose and amylopectin. In both amylose and amylopectin, the glucose units are connected by α-1,4-glycosidic bonds. Amylose is a linear chain, whereas amylopectin is a branched chain with α-1,6-glycosidic bonds at the branching points. The human body has enzymes, such as amylase and maltase, which can specifically break the α-1,4-glycosidic bonds in starch, thus allowing humans to digest it and release glucose for energy.

Cellulose, on the other hand, contains linear chains of β-glucose units, which are linked together by β-1,4-glycosidic bonds. In cellulose, each glucose unit is inverted relative to its neighbors, forming a straight and rigid structure called a microfibril. These cellulose microfibrils are interconnected by hydrogen bonding, forming a tightly packed arrangement.

The key difference in the digestibility of starch and cellulose lies in the type of glycosidic bond - α-1,4-glycosidic linkage in starch and β-1,4-glycosidic linkage in cellulose. Humans possess enzymes that can hydrolyze the α-1,4-glycosidic bonds in starch, but they lack the necessary enzymes to break the β-1,4-glycosidic bonds in cellulose. This is why humans can digest starch but not cellulose. In conclusion, the difference in the digestibility of starch and cellulose in humans arises from the difference in the glycosidic bonds that connect the glucose units. Starch has α-1,4-glycosidic linkages, which humans possess enzymes to hydrolyze, while cellulose has β-1,4-glycosidic linkages, which humans cannot break down.