Геміцелюлози: структура, склад та класифікація полісахаридів рослинних клітинних стінок.

Class of plant cell wall polysaccharides
A hemicellulose (also known as polyose) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all terrestrial plant cell walls. Cellulose is crystalline, strong, and resistant to hydrolysis. Hemicelluloses are branched, shorter in length than cellulose, and also show a propensity to crystallize. They can be hydrolyzed by dilute acid or base as well as a myriad of hemicellulase enzymes.

Diverse kinds of hemicelluloses are known. Important examples include xylan, glucuronoxylan, arabinoxylan, glucomannan, and xyloglucan. Hemicelluloses are polysaccharides often associated with cellulose, but with distinct compositions and structures. Whereas cellulose is derived exclusively from glucose, hemicelluloses are composed of diverse sugars, and can include the five carbon sugars xylose and arabinose, the six carbon sugars glucose, mannose and galactose, and the six carbon deoxy sugar rhamnose. Hemicelluloses contain most of the D pentose sugars, and occasionally small amounts of L sugars as well. Xylose is in most cases the sugar monomer present in the largest amount, although in softwoods mannose can be the most abundant sugar. Not only regular sugars can be found in hemicellulose, but also their acidified forms, for instance glucuronic acid and galacturonic acid can be present. In addition, hemicelluloses may be branched polymers, while cellulose is unbranched. Hemicelluloses are embedded in the cell walls of plants, sometimes in chains that form a 'ground' – they bind with pectin to cellulose to form a network of cross linked fibres. Based on the structural difference, like backbone linkages and side groups, as well as other factors, like abundance and distributions in plants, hemicelluloses can be categorized into four groups as following: 1) xylans, 2) mannans; 3) mixed linkage β glucans; 4) xyloglucans.

Xylans usually consist of a backbone of β (1→4) linked xylose residues and can be further divided into homoxylans and heteroxylans. Homoxylans have a backbone of D xylopyranose residues linked by β(1→4) glycosidic linkages. Homoxylans mainly have structural functions. Heteroxylans such as glucuronoxylans, glucuronoarabinoxylans, and complex heteroxylans, have a backbone of D xylopyranose and short carbohydrate branches. For example, glucuronoxylan has a substitution with α (1→2) linked glucuronosyl and 4 O methyl glucuronosyl residues. Arabinoxylans and glucuronoarabinoxylans contain arabinose residues attached to the backbone

As a polysaccharide compound in plant cell walls similar to cellulose, hemicellulose helps cellulose in the strengthening of plant cell walls.

There are many ways to obtain hemicellulose; all of these rely on extraction methods through hardwood or softwood trees milled into smaller samples. In hardwoods the main hemicellulose extract is glucuronoxlyan (acetylated xylans), while galactoglucomannan is found in softwoods. Prior to extraction the wood typically must be milled into wood chips of various sizes depending on the reactor used. Following this, a hot water extraction process, also known as autohydrolysis or hydrothermal treatment, is utilized with the addition of acids and bases to change the yield size and properties. The goal of hot water treatment is to remove as much hemicellulose from the wood as possible. This is done through the hydrolysis of the hemicellulose to achieve smaller oligomers and xylose. Xylose when dehydrated becomes furfural. When xylose and furfural are the goal, acid catalysts, such as formic acid, are added to increase the transition of polysaccharide to monosaccharides. This catalyst also has been shown to also utilize a solvent effect to be aid the reaction. This is due to the increase of cellulose. The hot water process is operated at a temperature range of 160 to 240 degrees Celsius in order to maintain the liquid phase. This is done above the normal boiling point of water to increase the solubilization of the hemicellulose and the depolymerization of polysaccharides. This process can take several minutes to several hours depending on the temperature and pH of the system. Higher temperatures paired with higher extraction times lead to higher yields. A maximum yield is obtained at a pH of 3.5. If below, the extraction yield exponentially decreases. In order to control pH, sodium bicarbonate is generally added. The sodium bicarbonate inhibits the autolysis of acetyl groups as well as inhibiting glycosyl bonds. Depending on the temperature and time the hemicellulose can be further converted into oligomers, monomers and lignin.