A hemicellulose is any of several heteropolymer

Heteropolymer

A heteropolymer or copolymer is a polymer derived from two monomeric species, as opposed to a homopolymer where only one monomer is used...

s (matrix polysaccharides), such as arabinoxylan

Arabinoxylan

Arabinoxylan is a hemicellulose found in both the primary and secondary cell walls of plants, including woods and cereal grains, consisting of copolymers of two pentose sugars – arabinose and xylose.-Structure:...

s, present along with cellulose

Cellulose

Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 in almost all plant cell wall

Cell wall

The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...

s. While cellulose is crystalline, strong, and resistant to hydrolysis

Hydrolysis

Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

, hemicellulose has a random, amorphous structure with little strength. It is easily hydrolyzed by dilute acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

 or base

Base (chemistry)

For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...

 as well as myriad hemicellulase

Cellulase

400px|thumb|right|alt = Colored dice with checkered background|Ribbon representation of the Streptomyces lividans beta-1,4-endoglucanase catalytic domain - an example from the family 12 glycoside hydrolases...

 enzymes.

Composition

Hemicelluloses include xylan

Xylan

Xylan is a generic term used to describe a wide variety of highly complex polysaccharides that are found in plant cell walls and some algae. Xylans are polysaccharides made from units of xylose ....

, glucuronoxylan, arabinoxylan

Arabinoxylan

Arabinoxylan is a hemicellulose found in both the primary and secondary cell walls of plants, including woods and cereal grains, consisting of copolymers of two pentose sugars – arabinose and xylose.-Structure:...

, glucomannan

Glucomannan

Glucomannan is a water-soluble polysaccharide that is considered a dietary fiber. Glucomannan is a food additive used as an emulsifier and thickener. Products containing glucomannan, marketed under a variety of brand names, are also sold as nutritional supplements for constipation, obesity, high...

, and xyloglucan

Xyloglucan

Xyloglucan is a hemicellulose that occurs in the primary cell wall of all vascular plants, however essential enzymes for xyloglucan metabolism, like XTH and β1→4-Glucan Synthase are found in Charophyceae algae. In many dicotyledonous plants, it is the most abundant hemicellulose in the primary cell...

.

These polysaccharides contain many different sugar monomers. In contrast, cellulose contains only anhydrous glucose. For instance, besides glucose, sugar monomers in hemicellulose can include xylose

Xylose

Xylose is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional group. It is the precursor to hemicellulose, one of the main constituents of biomass...

, mannose

Mannose

Mannose is a sugar monomer of the aldohexose series of carbohydrates. Mannose is a C-2 epimer of glucose. It is not part of human metabolism, but is a component of microbial cell walls, and is therefore a target of the immune system and also of antibiotics....

, galactose

Galactose

Galactose , sometimes abbreviated Gal, is a type of sugar that is less sweet than glucose. It is a C-4 epimer of glucose....

, rhamnose

Rhamnose

Rhamnose is a naturally occurring deoxy sugar. It can be classified as either a methyl-pentose or a 6-deoxy-hexose. Rhamnose occurs in nature in its L-form as L-rhamnose . This is unusual, since most of the naturally occurring sugars are in D-form...

, and arabinose

Arabinose

Arabinose is an aldopentose – a monosaccharide containing five carbon atoms, and including an aldehyde functional group.For biosynthetic reasons, most saccharides are almost always more abundant in nature as the "D"-form, or structurally analogous to D-glyceraldehyde.For sugars, the D/L...

. Hemicelluloses contain most of the D-pentose

Pentose

A pentose is a monosaccharide with five carbon atoms. Pentoses are organized into two groups. Aldopentoses have an aldehyde functional group at position 1...

 sugars, and occasionally small amounts of L-sugars as well. Xylose

Xylose

Xylose is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional group. It is the precursor to hemicellulose, one of the main constituents of biomass...

 is always the sugar monomer present in the largest amount, but mannuronic acid

Mannose

Mannose is a sugar monomer of the aldohexose series of carbohydrates. Mannose is a C-2 epimer of glucose. It is not part of human metabolism, but is a component of microbial cell walls, and is therefore a target of the immune system and also of antibiotics....

 and galacturonic acid also tend to be present.

Structural comparison to cellulose

Unlike cellulose, hemicellulose (also a polysaccharide) consists of shorter chains - 500-3,000 sugar units as opposed to 7,000 - 15,000 glucose molecules per polymer seen in cellulose. In addition, hemicellulose is a branched polymer

Polymer

A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

, while cellulose is unbranched.

Native structure

Hemicelluloses are embedded in the cell walls of plants, sometimes in chains that form a 'ground

Matrix (biology)

In biology, matrix is the material between animal or plant cells, in which more specialized structures are embedded, and a specific part of the mitochondrion that is the site of oxidation of organic molecules. The internal structure of connective tissues is an extracellular matrix...

' - they bind with pectin

Pectin

Pectin is a structural heteropolysaccharide contained in the primary cell walls of terrestrial plants. It was first isolated and described in 1825 by Henri Braconnot...

 to cellulose to form a network of cross-linked fibres.

Biosynthesis

Hemicelluloses are synthesised from sugar nucleotides in the Golgi. Two models explain their synthesis: 1) a ‘2 component model' where modification occurs at two transmembrane proteins, and 2) a '1 component model' where modification occurs only at one transmembrane protein. After synthesis, hemicelluloses are transported to the plasma membrane via golgi vesicles.

Applications

As percent content of hemicellulose increases in animal feed, the voluntary feed intake decreases.

Hemicellulose is represented by the difference between neutral detergent fiber

Neutral Detergent Fiber

Neutral Detergent Fiber is the most common measure of fiber used for animal feed analysis, but it does not represent a unique class of chemical compounds. NDF measures most of the structural components in plant cells , but not pectin...

 (NDF) and acid detergent fiber (ADF).

Functions

Microfibrils are cross-linked together by hemicellulose homopolymers. Lignin

Lignin

Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...

s assist and strengthen the attachment of hemicelluloses to microfibrils.