Hemicellulose or polyose is a complex polysaccharide present along with cellulose in almost all plant cell walls. It is branched and shorter in length than cellulosic molecules and also shows a propensity to crystallize. This molecule can be hydrolyzed by dilute acid or base as well as various enzymes. It contains anhydrous-β-(1-4)-d-xylopyranose, glucopyranose, mannopyranose, and galactopyranose units. It is an important structural component of plant cell walls, as well as a source of energy for certain animals.
Structure of Hemicellulose
It is a complex heteropolymeric molecule that contains a variety of sugar molecules, including xylose, arabinose, mannose, galactose, and glucuronic acid. It is a branched and shorter molecule than cellulose which can be crystallized easily.
It contains 500 to 3,000 sugar units whereas cellulose polymer contains between 7,000 and 15,000 glucose molecules. It is attached with cellulose and pectin to form a network of cross-linked fibers.
This heterogeneous mixture molecule has been divided into four general groups namely xylans, mannans, mixed linkage β-glucans, and xyloglucans. Xylan is considered the most important polysaccharide of this group that is composed of a backbone of β-(1→4)-linked xylose residues. Xylan carbohydrate is found abundantly in nature after cellulose. This matrix polysaccharide molecule is found in practically all terrestrial plant cell walls alongside cellulosic molecules. This polymeric molecule is partially soluble in water or alkali.
Xylan contains β-(1→4)-linked xylose residues and can be further classified as homoxylans or heteroxylans.
Homoxylan consists of D-xylopyranose residues connected by β(1→3) or mixed, β(1→3, 1→4)-glycosidic bonds and provides a good structural shape. Whereas heteroxylan contains glucuronoxylans, glucuronoarabinoxylans provide a D-xylopyranosyl backbone. It provides a short carbohydrate-branched molecule.
Mannan is into two categories, galactomannans, and glucomannans. Galactomannans consist chain of β-(1→4) linked D-mannopyranose residues whereas glucomannans consist of β-(1→4) linked D-mannopyranose and β-(1→4) linked D-glucopyranose residues.
Mixed linkage β-glucans consist of blocks of β-(1→4) D-Glucopyranose which is separated by a single β-(1→3) D-Glucopyranose chain.
Xyloglucans contain D-xylopyranose residues and act like a cellulose-like backbone.
This polymer primarily contains D-pentose sugars, with occasional trace amounts of L-sugars. Then it consists of a variety of sugars, including the five-carbon sugars namely xylose and arabinose, the six-carbon sugars namely glucose, mannose, and galactose, and the six-carbon deoxy sugar rhamnose.
The acidified counterparts, such as glucuronic acid and galacturonic acid are found in this molecule mostly. This molecule provides structural support for plant cell walls as well as strength and stiffness. This molecule can be destroyed by various enzymes than cellulose, making it a significant source of energy for organisms capable of breaking down its complex carbohydrates.
It can be extracted from plant matter and utilized in the manufacture of adhesives, paper, textiles, etc. It has potential medical applications, such as wound healing and drug delivery. Biofuel such as ethanol can be produced by fermentation of this component with various microorganisms. So, it is an important material for using plant biomass as a renewable resource.
Compositions of Hemicellulose
It consists of 25%–35% dry-weight wood material. It contains unique structures and chemical compositions. It consists of a variety of sugars, including the five-carbon sugars namely xylose and arabinose, the six-carbon sugars glucose, mannose, and galactose, and the six-carbon deoxy sugar rhamnose.
The degree of polymerization of glucose units in this polysaccharide ranges from 100–200 units. Pentose which has a xylose subunit in this molecule found in hardwoods. Hexose subunits such as glucose, mannose, and galactose are found in softwood.
Sources of Hemicellulose
This biopolymer can be synthesized from different plant sources and plant locations which have different structures and molecules such as hardwood, softwood, and gramineous plants have different chemical structures.
Properties of Hemicellulose
It is an amorphous Hetero cross-linked polymeric molecule.
It can be hydrolyzed easily with dilute acid and base.
It contains 500 to 3,000 sugar units.
It can be digested easily by humans and ruminants.
The monomers of xylose, galactose, mannose, rhamnose, and arabinose take part in the polymerization reaction to form the hemicelluloses.
It has less strength than cellulosic molecules.
This polymeric molecule can be synthesized from sugar nucleotides in the Golgi apparatus.
This molecule has a small crystalline region.
It is bio-degradable and degraded by a few enzymes of some bacteria and fungi.
It has a lower molecular weight if we compared to it cellulose molecules.
The degree of polymerization of this molecule is ranging from 50 to 300.
It is soft, and hydrolyzed by dilute acids and bases, or even hemicellulase enzymes.
It contains many branches, with acetyl groups.
They are soluble in 17.5%-18.5% NaOH solution.
The thermal degradation of this compound is rapid at lower temperatures (150–180 °C).
It has good moisture absorption properties.
Applications of Hemicellulose
This biopolymer is used for the manufacture of biodegradable bioplastics.
It is used to make films and gels in packaging.
It is used to make biofilms that are hydrophilic and have low mechanical resistance.
It is used as an important compound in papermaking, which facilitates fiber bonding.
It is used to make biofuel as an alternative fuel source.
It can be used as a dietary fiber.
It is used food additive to maintain the texture, taste, and mouth feel.
Frequently Asked Questions (FAQ’s)
What is Hemicellulose?
It is a complex biopolymeric molecule that contains a variety of sugar molecules, including xylose, arabinose, mannose, galactose, and glucuronic acid. It is a branched and shorter molecule than cellulose which can be crystallized easily.
What is the composition of Hemicellulose?
It consists of 25%–35% dry-weight wood material that consists of a variety of sugars, including the five-carbon sugars namely xylose and arabinose, the six-carbon sugars glucose, mannose, and galactose, and the six-carbon deoxy sugar rhamnose.
What types of microorganisms are involved in the degradation of hemicellulose molecules?
The microorganisms namely bacteria, fungi, and some types of archaea are responsible for the degradation of this amorphous molecule.
What are some industrial applications of hemicellulose?
This polymeric compound has many industrial applications such as the production of biofuels, paper pulp, and animal feed.
References
Spiridon, I., & Popa, V. I. (2008). Hemicelluloses: major sources, properties, and applications. In Monomers, polymers, and composites from renewable resources (pp. 289-304). Elsevier.
Ebringerová, A., Hromádková, Z., & Heinze, T. (2005). Hemicellulose. Polysaccharides I: Structure, characterization and use, 1-67.
