Cellulose

• Cellulose is an example of a linear polysaccharide composed of repeating glucose monomers linked by β-1,4-glycosidic bonds, unlike starch, which is made of α-1,4-glycosidic bonds. The polymerization of glucose units forms long chains, which combine to create the cellulose microfibrils, a critical structural component in plant cell walls. The hydroxyl groups (-OH) on the glucose molecules form hydrogen bonds with oxygen atoms.
• The linear structure allows cellulose molecules to form strong hydrogen bonds with neighboring molecules, creating a highly stable network.
• Each glucose molecule is flipped 180 degrees during joining, compared to its adjacent glucose molecule in the chain. This alternating orientation of glucose units results in a straight, linear chain.
• Pure cellulose is odorless and flavorless. Due to numerous hydroxyl groups, cellulose is hydrophilic but insoluble in water.
• Enzymatic hydrolysis of cellulose forms glucose.
• Cellulose has both crystalline and amorphous regions in its microfibrils. The highly ordered crystalline parts provide strength and resistance to deformation, while the amorphous regions offer flexibility and allow for enzymatic degradation.

Four different polymorphs of cellulose exist. They are cellulose I, II, III, and IV. Cellulose I is naturally occurring which exists as I_α and I_β. Cellulose found in bacteria is enriched in I_α, while in higher plants, it exists as I_β.

In contrast, cellulose II exists in regenerated cellulose fibers. The conversion of cellulose I to cellulose II is irreversible. Cellulose III is obtained by treatment of cellulose I or II with amines. Similarly, cellulose IV forms after treatment of cellulose III with glycerol at very high temperatures.

Cellulose serves many functions in living organisms.

1. Provides Rigidity to Plants: Cellulose primarily offers structural support to plant cells. It forms a robust and rigid cell wall, giving plant cells their characteristic shape and preventing them from collapsing under pressure.
2. Regulates Water Uptake and Transportation: It also helps regulate water uptake in plants and transportation. It helps maintain the osmotic pressure in plant cells, ensuring proper water balance and turgidity.
3. Provides Growth and Development: Cellulose is involved in cell growth and expansion during plant development. As the plant grows, new cellulose microfibrils develop, increasing the size of the cell wall and enabling cell elongation.
4. Protects from Pathogens: The cellulose cell wall is a physical barrier, protecting plant cells from external threats, such as pathogens and herbivores.
5. Produces Biofilms: Some bacteria secrete cellulose to form biofilms that help microorganisms to attach and organize into colonies.

One of Earth’s most abundant and renewable biomaterials, cellulose, finds various commercial applications. Its unique properties, such as biodegradability, strength, and chemical reactivity, make it an attractive choice for sustainable and eco-friendly products.

• The most significant commercial application of cellulose is paper and paperboard production. Wood pulp, which contains a high percentage of cellulose, is processed to extract the cellulose fibers. These fibers are then converted into pulp to manufacture various paper products, including newspapers, books, packaging materials, and cardboard.
• Cellulose-based fibers are used in the textile industry to produce fabrics like rayon, modal, and lyocell. These fibers are derived from cellulose obtained from plant sources such as wood, bamboo, or cotton.
• Derivatives, such as cellulose acetate and methylcellulose, are used to produce biodegradable films and coatings, which are used to pack food items.
• Derivatives, like microcrystalline cellulose, are widely used as excipients in the pharmaceutical industry, serving as bulking agents in tablets and capsules.
• Cellulosic biomass, including agricultural residues, wood chips, and energy crops, is a potential feedstock for biofuel production.
• Many cellulose-derived polymers are biodegradable and serve as renewable resources. They tend to be non-toxic. Some common cellulose derivatives are celluloid, cellulose acetate, cellulose triacetate, nitrocellulose, methylcellulose, cellulose sulfate, and rayon.
• It is used in the construction industry to produce cellulose insulation.

Starch and cellulose are structurally similar yet functionally distinct polysaccharides found abundantly in the plant kingdom. The critical differences between starch and cellulose are listed in the table below.

The critical differences between amylose and cellulose are listed in the table below.