Starch and cellulose are both polysaccharides, but the bond between saccharide units is slightly different. Consequently, humans can digest starch and use it for energy, whereas cellulose cannot be digested and passes directly through humans.

Starch and cellulose are two distinct types of polysaccharides integral to the plant structure and energy storage systems. Polysaccharides are large, complex carbohydrates made up of many monosaccharide units. In the case of plants, starch serves as a storage form of energy, which can be broken down and utilized when the plant requires it. Humans consume starch in many common foods like potatoes, rice, and bread.

On the other hand, cellulose is a key component of plant cell walls and provides structural support. While both starch and cellulose are made from the same monosaccharide, glucose, they differ significantly in their function and digestibility in humans. Starches are readily broken down by human digestive enzymes into glucose that the body uses for energy. Cellulose, despite also being composed of glucose, is not digested by humans. Instead, it acts as dietary fiber, aiding in digestion by helping to move food through the gut.

The digestibility of polysaccharides by humans is primarily determined by the specific types of glycosidic bonds linking the monosaccharide units. Glycosidic bonds are covalent chemical bonds that join a carbohydrate molecule to another group, which can be another carbohydrate. In starch, glucose units are linked together mostly by alpha-1,4 glycosidic bonds and at branching points by alpha-1,6 glycosidic bonds.

These types of bonds create a shape that is accessible to human digestive enzymes. In cellulose, the glucose units are linked by beta-1,4 glycosidic bonds. This bond type results in a straight, rigid structure that human digestive enzymes cannot break down. The orientation of the glycosidic bonds effectively determines the three-dimensional shape of the polysaccharide, which then influences its functionality and digestibility.

Human digestive enzymes are specialized proteins designed to break down our food into nutrients that can be absorbed and used by the body. The enzyme responsible for breaking down starch into sugars is called amylase, which is present in saliva and secreted by the pancreas. Amylase cleaves the alpha glycosidic bonds in starch, simplifying it into smaller sugars, and ultimately into glucose. However, humans lack the enzyme cellulase, which is necessary to break down beta glycosidic bonds found in cellulose. As a result, cellulose travels through the human digestive tract unchanged, contributing to the bulk of stools and playing a key role in maintaining bowel health.

The structural composition of polysaccharides is a key factor in their diverse roles and properties. These complex carbohydrates are more than just energy sources; they can also provide structure and protection. The physical properties of starch and cellulose, such as solubility in water and the ability to form gels, arise from their molecular structures.

Starch typically exists in two forms: amylose, which is largely unbranched and composed of long chains of alpha-linked glucose; and amylopectin, which is highly branched and also contains alpha-linkages. This branching allows starches to be more easily digested as it increases the number of ends from which enzymes can attack. Cellulose's beta-1,4 glycosidic linkages, conversely, lead to a tightly packed, linear structure that resists enzymatic breakdown and is thus indigestible by humans. These structural differences are crucial for students to comprehend when considering the functionality and nutritional value of these polysaccharides.