Hemicelluloses are a group of complex carbohydrates found alongside cellulose and lignin in the cell walls of terrestrial plants. As the second most abundant polysaccharide in nature after cellulose, they are a component of plant-based materials we encounter daily, from wood and paper to the vegetables and grains we consume. These polymers are important for the structure and function of the plant itself.
Chemical Makeup and Structure
Unlike cellulose, which is a homopolymer of glucose, hemicelluloses are heteropolymers built from a variety of sugar units. Their composition varies between plant species and even different parts of the same plant. Common building blocks include five-carbon sugars like xylose and arabinose, and six-carbon sugars such as mannose, galactose, and glucose.
Cellulose forms long, unbranched chains that pack into strong, crystalline fibers, but hemicellulose polymers are much shorter and have a branched structure. Their chains consist of 500 to 3,000 sugar units, compared to the 7,000 to 15,000 in a cellulose polymer. This branched nature prevents orderly alignment, giving hemicellulose an amorphous, non-crystalline structure.
The crystalline structure of cellulose makes it highly resistant to breakdown. In contrast, the amorphous structure of hemicellulose makes its chemical bonds more accessible. This allows hemicellulose to be easily hydrolyzed by dilute acids, bases, or various enzymes.
Biological Role in Plants
Hemicelluloses provide structural integration and support within the plant cell wall. They are a component of the matrix that surrounds and interconnects the cellulose microfibrils. Hemicelluloses fill the voids between these fibers, forming a cross-linked network that binds cellulose and lignin together.
This arrangement provides the cell wall with strength and flexibility. The cellulose microfibrils can be thought of as reinforcing bars, while hemicelluloses act as the binding medium holding them in place. This composite structure enables the plant to withstand environmental stresses while allowing for the controlled cell expansion necessary for growth.
Hemicellulose is a dominant polymer in the middle lamella, the layer that cements adjacent plant cells together. It is also a component of the secondary cell walls, which provide most of the plant’s structural rigidity. In some vascular plants, hemicellulose also interacts directly with lignin to provide additional support.
Dietary Significance for Humans
When consumed in plant-based foods, hemicelluloses are a component of dietary fiber. Human digestive systems lack the enzymes to break down their complex structures in the small intestine. As a result, they pass through the upper gastrointestinal tract largely intact.
Once in the large intestine, hemicelluloses serve as a substrate for the resident gut microbiota. These bacteria possess enzymes that ferment the complex carbohydrates. This process produces short-chain fatty acids (SCFAs), which have positive effects on gut health.
Hemicelluloses also contribute to bowel regularity. As a type of bulking fiber, they absorb and hold water, which increases stool weight and softness, promoting more regular bowel movements. Both soluble and insoluble forms are found in foods and contribute to these effects.
Commercial and Industrial Applications
One application for hemicellulose is in biofuel production. It can be extracted from biomass and broken down into its sugars, which are then fermented to produce alcohols like bioethanol. This process is part of the biorefinery concept, which converts plant matter into fuels and other chemicals.
In the food industry, extracted hemicelluloses and their derivatives are used as additives. Their ability to modify the texture and stability of products makes them useful as thickeners, emulsifiers, and gelling agents. Hemicellulose composites are also being developed for use as edible films and food coatings.
Hemicellulose is also a feedstock for producing the sugar substitute xylitol, which involves hydrolyzing xylan and then hydrogenating the resulting xylose. Hemicelluloses also play a role in papermaking, where they are retained in pulp fibers to improve the strength of the final product. The pulp and paper industry is a major source for these applications.