Complex polysaccharides from plants are divided into several distinct groups based on chemical structure. The most common structural component of the cell wall is cellulose. Cellulose is composed of long linear chains of glucose molecules (approx. 10,000) with beta 1-4 links, which cannot be broken down by digestive enzymes. The hemicelluloses found in plants are a heterogeneous group of compounds that contain a variety of different sugars arranged in a primary chain or backbone, with numerous side chains. The specific names of the hemicelluloses are based on their sugar content, such as xyloglucans (xylose and glucose), glucomannans (glucose and mannose), and arabinoxylans (arabinose and xylose). Solubility in water is dependent on the sugar components and the physical structure of the individual hemicellulose, but most compounds in this group are soluble in dilute alkali. Pectin from plants refers to a complex mixture of substances that is composed of a galacturonic acid core esterfied with methyl groups on the uronic acid residues. Rhamnose is also found in the backbone of pectins. Pectin is water soluble and used to produce highly viscous gels, whereas protopectins are insoluble. The only nonpolysaccharide included in the definition of dietary fiber is lignin. The lignins consist of chains of phenyl propane residues that cement and anchor the cell wall matrix and stiffen the plant tissue. As plants grow and age they undergo a process of lignification, and the dietary fiber makeup changes over time.

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