page_title: Hemicellulose
section_title: Structural comparison to cellulose
hierarchical_section_title: Hemicellulose / Structural comparison to cellulose
context_page_description: A hemicellulose is one of a number of heteropolymer, such as arabinoxylans, present along with cellulose in almost all terrestrial plant cell walls. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicelluloses have random, amorphous structure with little strength. They are easily hydrolyzed by dilute acid or base as well as a myriad of hemicellulase enzymes.
context_section_description: Unlike cellulose, hemicelluloses consist of shorter chains – 500–3,000 sugar units.  In contrast, 7,000–15,000 glucose molecules comprise each polymer of cellulose. In addition, hemicellulose 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, hemicellulose could be characterized into four groups as following: 1) Xylans, 2) Mannans; 3) Mixed linkage β-glucans; 4) Xyloglucans
  Xylans
  
  Xylans usually consist of backbone of β-(1→4)-linked xylose residues. And it could be further divided into homoxylans and heteroxylans. Homoxylans has a backbone of D-xylopyranose residues linked by β(1→3) or mixed ,β(1→3, 1→4)-glycosidic linkages. Homoxylans mainly carry structural functions. Heteroxylans such as glucuronoxylans, glucuronoarabinoxylans, and complex heteroxylans, have a backbone of D-xylopyranose and short carbohydrate branches. For examples, glucuronoxylan has substitution with α-(1→2)-linked glucuronosyl and 4-O-methyl glucuronosyl residues. And arabinoxylans and glucuronoarabinoxylans contain arabinose residues attached to the backbone
  Mannans
  The mannan-type hemicellulose can be classified into two types based on their main chain difference, galactomannans and glucomannans. Galactomannans have only β-(1→4) linked D-mannopyranose residues in linear chains. Glucomannans consist of both β-(1→4) linked D-mannopyranose and β-(1→4) linked D-glucopyranose residues in the main chains. As for the side chains, D-galactopyranose residues tend to be 6-linked to both types as the single side chains with various amount.
  Mixed linkage β-glucans
  
  The conformation of the mixed linkage glucan chains usually contains blocks of β-(1→4) D-Glucopyranose separated by single β-(1→3) D-Glucopyranose. The population of β-(1→4) and β-(1→3) are about 70% and 30%. These glucans primarily consist of cellotriosly (C₁₈H₃₂O₁₆) and cellotraosyl (C₂₄H₄₂O₂₁)segments in random order. There are some study show the molar ratio of cellotriosly/cellotraosyl for oat (2.1-2.4), barley (2.8-3.3), and wheat (4.2-4.5).
  Xyloglucans
  Xyloglucans have a backbone similar to cellulose with α-D-Xylopyranose residues at position 6. To better describe different side chains, a single letter code notation is used for each side chain type. G -- unbranched Glc residue; X -- α-d-Xyl-(1→6)-Glc. L -- β-Gal , S -- α-l-Araf, F-- α-l-Fuc. These are the most common side chains.
  The two most common types of xyloglucans in plant cell walls are identified as XXXG and XXGG.
caption: Beta-D-glucopyranose with carbon positions.
exif: {}