Polysaccharides: Starch, Cellulose, and Glycogen

Polysaccharides, a major class of carbohydrates, are complex macromolecules that play vital roles as energy storage molecules and structural components in living organisms. This guide provides a detailed look at the three most significant polysaccharides: Starch, Cellulose, and Glycogen. Understanding their structure, properties, and functions is a fundamental requirement for JEE-level chemistry and biology.

1.0What are Polysaccharides?

Polysaccharides are long-chain polymers made up of monosaccharide units joined together by glycosidic bonds. They are often referred to as glycans. The process of forming a polysaccharide from monosaccharides is called polymerization or condensation, as it involves the removal of a water molecule for each bond formed. Polysaccharides are generally not sweet, are insoluble in water, and are not reducing sugars. They can be classified into two main types:

• Homopolysaccharides: Composed of only one type of monosaccharide unit (e.g., starch, cellulose, glycogen, all made of glucose).
• Heteropolysaccharides: Composed of two or more different types of monosaccharide units.

Starch, cellulose, and glycogen are all homopolysaccharides made from glucose units, but their different structures give them distinct functions.

2.0Starch: The Plant's Energy Reservoir

Starch is the primary energy storage polysaccharide in plants. It is a polymer of α-D-glucose and is found in high concentrations in seeds, roots, and tubers. The body breaks down starch into glucose, which is then used as fuel.

Structure of Starch

Starch is not a single compound but a mixture of two components: Amylose and Amylopectin.

a) Amylose:

• Structure: Amylose is an unbranched polymer of α-D-glucose units. The glucose units are linked by α-1,4-glycosidic bonds.
• Shape: The unbranched structure causes it to coil into a helix, similar to a spring. This helical shape is responsible for the characteristic blue-black color it gives with iodine solution, as iodine molecules get trapped inside the helix.
• Properties: Amylose is soluble in hot water but has a low solubility in cold water. It accounts for about 15-20% of the total starch.

`b) Amylopectin:

• Structure: Amylopectin is a branched polymer of α-D-glucose units. The main chain is formed by α-1,4-glycosidic bonds, while the branches are attached by α-1,6-glycosidic bonds. A branch occurs approximately every 20-30 glucose units.
• Shape: The branching gives it a tree-like structure.
• Properties: Amylopectin is insoluble in water and accounts for about 80-85% of total starch. It gives a red-violet color with iodine solution.

Properties and Functions of Starch

• Energy Storage: Starch serves as a readily accessible energy reserve for plants. When plants need energy (e.g., during germination), they can hydrolyze starch into glucose.
• Digestion: Enzymes like amylase in saliva and pancreatic fluids break down starch into maltose and then into glucose, which is absorbed by the body.

3.0Cellulose: The Structural Backbone of Plants

Cellulose is the most abundant organic polymer on Earth. It is a structural polysaccharide that provides rigidity and strength to the cell walls of plants.

Structure of Cellulose

• Monomer: Cellulose is a linear polymer of β-D-glucose units.
• Bonding: The glucose units are joined by β-1,4-glycosidic bonds. This is a crucial difference from starch.
• Shape: Due to the β-linkage, the glucose units are rotated 180° with respect to their neighbors. This "alternating" arrangement allows cellulose chains to lie side-by-side in long, straight fibers.
• Intermolecular Forces: These parallel chains are held together by extensive intermolecular hydrogen bonds. This creates strong, stable microfibrils and fibers, which are responsible for the high tensile strength of cellulose.

Properties and Functions of Cellulose

• Structural Support: The primary function of cellulose is to provide structural support to plant cell walls, making them rigid and strong.
• Insolubility: Due to its high degree of hydrogen bonding and crystalline structure, cellulose is insoluble in water and most organic solvents.
• Indigestibility: Humans lack the enzyme cellulase needed to hydrolyze the β-1,4-glycosidic bonds. Therefore, cellulose is not digestible by humans and is an important component of dietary fiber, which aids in digestion. Herbivores like cows and termites can digest cellulose because their digestive systems contain microorganisms that produce cellulase.

4.0Glycogen: The Animal's Energy Reserve

Glycogen is the animal equivalent of starch. It is the principal energy storage form in animals and fungi. It is stored primarily in the liver and muscles.

Structure of Glycogen

• Monomer: Glycogen is a highly branched polymer of α-D-glucose units.
• Bonding: The main chain is linked by α-1,4-glycosidic bonds, and the branches are attached by α-1,6-glycosidic bonds.
• Branching: Glycogen is more highly branched than amylopectin, with branches occurring every 8-12 glucose units. This dense branching provides numerous ends from which glucose can be quickly released for a burst of energy.

Properties and Functions of Glycogen

• Energy Storage: Glycogen serves as a short-term energy reserve. When the body needs glucose, it can rapidly break down glycogen in the liver to release glucose into the bloodstream. In muscles, glycogen serves as a fuel source for muscle contraction.
• Solubility: Glycogen is readily soluble in water.
• Rapid Mobilization: The high degree of branching allows for rapid hydrolysis of glucose from multiple ends simultaneously, which is crucial for meeting the body's immediate energy needs.

5.0Difference Between Starch, Cellulose, and Glycogen

The key to distinguishing these polysaccharides lies in their structure, specifically the type of glycosidic linkage and the degree of branching.