Figure 1.

The plant cell wall is composed of polysaccharides such as cellulose, hemicellulose, and pectin. (A). Cellulose is a polymer of d-glucose units linked by β1-4 bonds, also known as glucosidic bonds. (B). Xylans and glucomannans are two components of hemicellulose. The xylan backbone, which is composed of d-xylose linked together by β1-4 bonds, also contains varying amounts of acetyl, methylglucuronyl, and arabinofuranosyl side chains. The arabinofuranosyl side chains can also be linked to aromatic acids (e.g., p-coumaric acid). Glucomannans are composed of d-glucose and D-mannose at a ratio of 1.6:1 (linked by β1-4 bonds), and with branches of d-mannose residues at the C3 position (β1-3 bonds), on both glucose and mannose. (C). Pectin is the methylated ester of polygalacturonic acid, consisting of chains of 300 to 1000 galacturonic acid units joined with 1α→4 bonds. The structure shows three methyl ester forms (−COOCH3) for every two carboxyl groups (−COOH), having a 60% degree of esterification. The substituted residues at C-4 with neutral and acidic oligosaccharide side chain composing of arbinose, galactose, fructose, and glucuronic acid. The length of the chains and the attached residues and their placement varies among plant species [11,12]. The chemical structures have been drawn with ACD/ChemSketch (Freeware Software, version 2020.2.1, www.acdlabs.com, accessed on 20 August 2021).