Introduction

Cellulose is an organic compound belonging to the category of polysaccharides. It is a polymer made up of glucose subunits. It is found in bacterial and plant cells and is abundantly present in their cell walls. Cellulose plays an important role in the structure and strength of plants. It also finds great importance in the industry. 

In this article, we will study the structure, properties, and synthesis of cellulose. We will also discuss its occurrence and importance in plants. In the end, we will talk about the industrial uses of cellulose. So, keep reading. 

Structure

Cellulose is a made up of thousands of D-glucose subunits. The glucose subunits in cellulose are linked via beta 1-4 glycosidic bonds. 

Contrary to the other polysaccharides, the orientation of glucose molecules in cellulose is reversed. They have beta orientation in which the hydroxyl group of the anomeric carbon or carbon number one is directed above the plane of the glucose ring. The hydroxyl groups of the rest of the carbon atoms are directed below the plane of the ring. 

In order to make beta 1-4 glycosidic bonds, every alternate glucose molecule in cellulose is inverted. The hydroxyl group of carbon 1 is directed upwards, and that of carbon 4 is directed downward. Now, to make a beta 1-4 glycosidic bond, one of these molecules should be inverted so that both the hydroxyl groups come in the same plane. This is the reason for the inversion of every alternate glucose molecule in cellulose. 

Cellulose is an unbranched molecule. The polymeric chains of glucose are arranged in a linear pattern. Unlike starch or glycogen, these chains do not undergo any coiling, helix formation or branching. Rather, these chains are arranged parallel to each other. The hydrogen bonds are formed between these chains due to hydrogen atoms and hydroxyl groups which firmly hold the chains together. This results in the formation of cellulose microfibrils that are firm and strong. 

Cellulose is present in plant cells in the form of cellulose microfibrils. These microfibrils together form polysaccharide or cellulose matrix. Further details of the polysaccharide matrix will be discussed somewhere else in this article. 

Properties

Cellulose differs from the rest of polysaccharides in its properties. The unique properties of cellulose are due to its unique structure. They also depend on the number of glucose subunits present in cellulose. It has the following properties;

• Cellulose is the most abundant carbohydrate present in nature
• It is insoluble in water
• Cellulose is a crystalline solid having a white powdery appearance
• It has high tensile strength due to firm hydrogen bonds between the individual chains in cellulose microfibrils. The tensile strength of cellulose microfibrils is comparable to that of steel
• The alternate arrangement of glucose molecules in cellulose also contributes to the high tensile strength of cellulose
• It is soluble in organic solvents

Synthesis

Cellulose is synthesis does not occur in animals. It is limited to only plants or bacteria. The biosynthesis of cellulose in two organisms follow different steps

Plants

In plants, cellulose synthesis takes place on special complexes present at the cell membrane called rosette terminal complexes. These complexes are the hexameric transmembrane proteins that are capable of free floatation in the plasma membrane. They contain at least three cellulose synthase enzymes. 

These transmembrane rosettes perform two functions; polymerization of glucose residues to form cellulose chain and assembly of cellulose microfibrils. 

Synthesis of Cellulose Chain

The process of cellulose chain synthesis begins on the cytoplasmic end of the rosette terminal complexes. The cellulose synthase enzymes use glucose residues provided by UDP-glucose. 

In the first step, glucose-6-phosphate is converted to glucose-1-phosphate in the cytoplasm of plant cells by phosphoglucomutase enzyme. This step is common in the synthesis of starch, glycogen, and cellulose.

In the next step, UTP and glucose-1-phosphate react to form UDP-glucose and a pyrophosphate molecule is released. The hydrolysis of pyrophosphate makes this step irreversible. It is also the rate-limiting step in cellulose synthesis. 

Cellulase synthase requires a primer for the synthesis of cellulose chains. The steroid molecule sitosterol-beta-glucoside serves the function of primer in the synthesis of cellulose.

The cellulose synthase begins constructing a cellulose chain on primer using glucose residues provided by UDP-glucose molecules. It joins the glucose residues via beta 1-4 glycosidic bonds to form a long chain of cellulose releasing UDP molecules. 

The UDP molecules can then be converted into UTP by certain kinases. 

Assembly of Cellulose Microfibrils

Once a cellulose chain has been elongated to a certain length, the cellulase enzyme present in the cytoplasm cleaves this chain from the primer.

The rosette complexes move this chain across the plasma membrane into the cell wall. 

In the cell wall, different cellulose chains are arranged parallel to each other and hydrogen bonds are formed among them. This results in the formation of cellulose microfibrils with high tensile strength. 

Bacteria

Bacteria use the same family of enzymes for cellulose synthesis as used by plants. However, the bacterial enzymes are encoded by different genes. Another hypothesis is that plants acquired the cellulose synthesis enzymes from bacteria after endosymbiosis. 

Animals

Cellulose is also synthesized by some animals called tunicates. Tunicates are invertebrate animals found in the sea. They have a hard shell that encloses the delicate body of the animal. Cellulose is found in the shell of these animals. 

The process of cellulose synthesis is also somehow same as in the plants and bacteria. The structure of cellulose is essentially the same. 

Cellulose Network in Plants Cell Wall 

Understanding the arrangement of cellulose microfibrils and polysaccharide matrix in the cell wall of plants is also important. 

We have studied earlier that as the cellulose chains are synthesized, they are exported out of the cell into the cell wall. Here the cellulose chains are arranged in parallel fashion forming hydrogen bonds among themselves. This results in the formation of cellulose microfibrils.  

Polysaccharide matrix is formed when other sugar molecules interact with these cellulose microfibrils. In the primary cell wall of plants, glucans and arabinoxylans are the two major components of the polysaccharide matrix. These polysaccharides interact with one another and form a network among the cellulose microfibrils. This network is strengthened by cross-links formation. These cross-links are formed when arabinoxylan residues react with acids like ferulic acid (FA) and diferulic acid (DFA). Due to this reason, it is also said that the polysaccharide matrix is made up of acidic polysaccharides. 

In addition to the cellulose microfibrils and polysaccharide matrix, the primary cell wall also contains cross-linking polysaccharides. These polysaccharides cross-link the cellulose microfibrils to form a complex network. Most important of these cross-linking polysaccharides is hemicellulose. It is a derivative of cellulose and will be discussed briefly towards the end of this article. 

Calcium also plays an important role in network formation. It cross-links the acidic polysaccharides present in the polysaccharide matrix. 

Occurrence

Cellulose is the most abundantly produced biopolymer on earth. It is present in the cell wall of all plant cells. Cellulose is also present in the cell wall of other organisms like bacteria and algae. 

The purest form of cellulose is cotton, that contains around 98% cellulose. Besides, cellulose is also present in wood obtained from the trees. 

Although animal cells do not have cell wall, cellulose is also found in some of the animal species. It is present in the shells of tunicates, the invertebrate animals found in the sea. 

Cellulolysis 

The process of cellulose degradation is called cellulolysis. It can be discussed under three headings; in plants, animals, and upon heat exposure. 

Plants

Cellulose is not normally degraded in plants except in disease conditions. In most of the diseases, the pathogens penetrate the plant cell after degrading the plant cell wall. This degradation of cell wall is carried out by cellulolytic enzymes that disrupt or cleave the cellulose present in the microfibrils. 

The various cellulolytic enzyme are collectively known as cellulase enzymes. These enzymes are produced by various bacteria, fungi, and other parasites of plants. 

Animals

Cellulose degradation takes place in the digestive tract of some of the mammals. It is usually hard to digest cellulose due to extensive cross-linking that exits among its fibers in the plant’s cell wall. However, digestion can be facilitated if it is dissolved in some polar solvents like ionic solutions etc. 

Cellulose digestion is limited to herbivores like cows, goats, sheep, etc. These mammals have bacteria that live in a symbiotic relationship within the digestive tract of these mammals. These include Cellulomonas and Ruminococcus bacterial species. 

These bacteria produce cellulase enzyme that degrades the cellulose present in the diet of these mammals. The breakdown products of cellulose degradation are used by bacteria for their own growth and proliferation. 

The bacteria are later digested by the enzymes of the mammal’s digestive tract. In this way, the cellulose present in bacteria becomes a part of mammals body. 

Two types of enzymes are involved in this process; 

• Cellulases, they act on glucose residues present within the chain and break the beta 1-4 linages
• Glucosidases, they act on the ends of the chain and remove the terminal glucose residues by breaking the glycosidic bonds

Cellulose is not digested in the human digestive system because of lack of the enzymes that break the beta 1-4 glycosidic linkages. 

Thermolysis

Thermolysis means the breakdown of cellulose when it is exposed to high temperature or heat. 

Thermolysis of cellulose occurs at 350 degrees, when decomposes into vapors of carbon dioxide and other aerosols. This temperature is called thermolytic temperature or pyrolytic temperature. 

The melt of cellulose at pyrolytic temperature contains short chains made up of two to seven subunits. 

The aerosols arising at this pyrolytic temperature contain oligomers of cellulose in anhydrous form. These anhydrous molecules are derived from the melt. 

Importance

Cellulose finds profound importance in plants, animals, microorganisms as well as in industry. 

Plants

Cellulose provides rigidity to the plant cells. The high tensile strength of cellulose fibers present in the plant cell wall is responsible for maintaining the shape and rigidity of plant cells. It is due to such strong cellulose fibers in the cell wall that plant cells do not burst like animal cells when placed in a hypotonic solution.

Microorganisms

Cellulose is a component of cell walls of bacteria and algae. It provides rigidity to these cells as well as maintains their shape and structure. 

Animals

It is an important dietary source of carbohydrates in herbivores like goats and sheep. 

In other mammals and humans, it cannot be digested. However, it acts as a bulky fiber required for the health of the gastrointestinal tract. 

Industry

Cellulose is used in different industries for the welfare of mankind. Following are some of its uses:

• Cellulose is used to make paper, paperboards, cardboards, cardstock and other paper products. 
• It is used in the textile industry to make clothes. Different clothes are made using cotton and other plant fibers. 
• It is used to make electrical insulation paper in the electric industry.
• It is used to make bio-fuel.
• Cellulose is used in gunpowder.
• It is used as a stabilizer in different drugs. 
• It is used in biological labs as a stationary phase for chromatography.

Summary

1. Cellulose is the most important structural polysaccharide present in plants. 
2. It is made up of unbranched chains of glucose molecules linked via beta 1-4 glycosidic bonds. 
3. Every alternate glucose molecule in cellulose chains is inverted. These chains are arranged parallel to each other to form microfibrils. 
4. It is synthesized by special rosettes transmembrane complexes present in the plasma membrane of plant cells. 
5. The cellulose microfibrils are cross-linked via hemicellulose molecules. 
6. Polysaccharide matrix with acidic polysaccharide is also present along with cellulose microfibrils in the cell wall of plants. 
7. Cellulose is present in the cell wall of plants, algae, and bacteria, and also in the shell of tunicates. 
8. Cellulose is digested only in herbivores. 
9. In plants, cellulose is degraded by pathogenic enzymes. It also undergoes degradation at 350-degree Celsius temperature. 
10. It provides strength and rigidity to the plant and bacterial cells as well as algae. 
11. It is a source of carbohydrate for herbivores.
12. Cellulose makes the bulk fibers in the human diet. 
13. It is used in industry for the following purposes;
    • To make paper and paper products
    • To make insulation paper
    • As a biofuel
    • As a stationary phase in chromatography
    • To make gunpowder

Frequently Asked Questions

What is cellulose?

Cellulose is a carbohydrate mainly found in plants. It is a polysaccharide made up of glucose molecules. It is insoluble in water. Cellulose is used to make paper and clothes in the industry.

What is the structure of cellulose?

Cellulose is a polysaccharide in which glucose molecules are linked together via 1-4 glycosidic bonds. It is an unbranched molecule. Chains of glucose molecules are arranged in a linear pattern to form cellulose. 

Which food has cellulose?

Cellulose rich foods include green leafy vegetables, roots, and some fruits like apples, pears, etc. 

Why cellulose cannot be digested in the human body?

Cellulose molecules have 1-4 glycosidic bonds. The human digestive system does not have the enzyme needed to break this glycosidic linkage. This is the reason why cellulose cannot be digested in the human body. 

References

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2. Romeo, Tony (2008). Bacterial biofilms. Berlin: Springer. pp. 258–263. ISBN 978-3-540-75418-3.
3. Klemm, Dieter; Heublein, Brigitte; Fink, Hans-Peter; Bohn, Andreas (2005). “Cellulose: Fascinating Biopolymer and Sustainable Raw Material”. Angew. Chem. Int. Ed. 44 (22): 3358–93. doi:10.1002/anie.200460587. PMID 15861454.
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