Question

Although both starch and cellulose are polymers of glucose, starch is digestible by humans and cellulose is not. How do the structures of these polysaccharides differ, resulting in their different digestibility?

Short answer

The difference in digestibility of starch and cellulose is due to the type of glycosidic bonds they contain. Starch has α-1,4-glycosidic bonds, which can be broken down by the human enzyme amylase, making it digestible. Cellulose has β-1,4-glycosidic bonds, which humans lack the enzyme (cellulase) to break down, making it indigestible and serving as dietary fiber.

Step-by-step solution

Introduce the Polysaccharides Starch and Cellulose

Starch and cellulose are both polysaccharides, which are long chains of monosaccharide (glucose) units bonded together. However, there is a difference in the structure of these polymers that affects their digestibility.

Describe the Structure of Starch

Starch is a polysaccharide made of α-D-glucose units joined together by glycosidic bonds. The glycosidic bonds found in starch are primarily α-1,4-glycosidic bonds, linking glucose monomers together. Starch can be found in two forms: amylose (a linear polymer) and amylopectin (a branched polymer with additional α-1,6-glycosidic bonds at the branching points).

Describe the Structure of Cellulose

Cellulose is a polysaccharide made of β-D-glucose units joined together by glycosidic bonds. The glycosidic bonds found in cellulose are exclusively β-1,4-glycosidic bonds, which cause the glucose units to be linked in a straight chain with alternating orientations. This arrangement leads to hydrogen bonding between parallel cellulose chains, resulting in a rigid structure.

Explain the Digestibility Difference between Starch and Cellulose

The key difference in the digestibility of starch and cellulose is the type of glycosidic bond they contain. Humans can digest starch because they possess the enzyme called amylase, which is capable of breaking the α-1,4-glycosidic bonds. Since starch contains mostly α-1,4-glycosidic bonds, amylase can efficiently break it down into glucose units, which can then be absorbed by the body. On the other hand, humans lack the enzyme, called cellulase, needed to break down β-1,4-glycosidic bonds in cellulose. As a result, cellulose is not digestible by humans and passes through the digestive system as dietary fiber, providing no nutrients or energy.

Key concepts

Starch Structure

Starch, a staple in many human diets, serves as an essential energy source. Composed of two types of molecules, amylose and amylopectin, starch exhibits unique structural features that influence its digestibility. Amylose forms a linear chain of glucose units connected by α-1,4-glycosidic bonds, contributing to its helical shape. This configuration allows the enzyme amylase to easily access and hydrolyze the bonds, releasing energy-rich glucose.

Amylopectin, on the other hand, is more complex, being highly branched due to additional α-1,6-glycosidic bonds. This branching occurs every 24 to 30 glucose units, enhancing amylase’s enzymatic action as it provides numerous starting points for digestion. This structure makes starch a readily available energy source that the human body can efficiently break down and utilize.

Cellulose Structure

Unlike starch, cellulose presents a challenge to our digestive system due to its distinct molecular organization. Cellulose consists of β-D-glucose units linked by β-1,4-glycosidic bonds that flip each glucose molecule 180 degrees relative to its neighbors. This orientation gives cellulose its straight, rigid structure vital for the cell walls of plants.

These alternating units facilitate hydrogen bonding between cellulose molecules, creating tightly packed, insoluble fibers. This robust architecture resists breakdown by human digestive enzymes. Consequently, cellulose travels through the human gastrointestinal tract undigested, serving as an important dietary fiber that supports digestive health but does not confer caloric energy.

Glycosidic Bonds

Glycosidic bonds are pivotal to the functionality of polysaccharides. These covalent bonds connect monosaccharide units in carbohydrates like starch and cellulose. The type of glycosidic bond determines the three-dimensional structure of the polysaccharide and affects its biological role and digestibility.

The α-1,4-glycosidic bonds in starch align glucose units in a way amenable to amylase, while the β-1,4-glycosidic bonds in cellulose create a conformation inaccessible to our digestive enzymes. These structural differences illustrate how subtle variations at the molecular level can have significant impacts on the properties and functions of biomolecules.

Enzyme Amylase

Amylase is a vital digestive enzyme predominantly found in saliva and pancreatic fluid. It serves as a natural catalyst that speeds up the hydrolysis of starch into smaller sugar units like maltose and glucose. This process is essential for starch digestion, as only monosaccharides can be absorbed efficiently in the small intestine.

Human amylase specifically targets α-1,4-glycosidic bonds present in starch, cleaving these links and rendering the energy within the glucose units accessible. The absence of an enzyme capable of breaking down β-1,4-glycosidic bonds found in cellulose precludes humans from digesting this particular polysaccharide, highlighting the critical function of specific enzymes in digestion.