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The organic constituents of plant that give structural topography generally contain six broad groups of constituents as given below:
(i) Cellulose:
Cellulose is the most abundant chemical constituent of a cell. It is a carbohydrate composed of glucose units linked by P-linkages at 1 and 4 carbon atoms of sugars. Total number of glucose units varies from 2000 to 10,000. Total amount of cellulose varies from 15 to 60% in litter.
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(ii) Hemicellulose:
Hemicelluloses are found in close proximity to cellulose in the primary and secondary walls. The simple sugars (or monosaccharides) or uronic acids (derivatives of simple sugars) are bound together to form a large molecule which is called hemicellulose. The common sugars are xylose, arabinose, glucose and galactose, and uronic acids are glucuronic and galacturonic acids. It makes up to 30% of organic matter by weight.
(iii) Lignin:
Certain plants especially woody species contribute large amount of lignin. It is found in the secondary layers of the cell wall and also in middle lamella. The lignin molecule is the aromatic compounds containing C, H and O. The basic unit of lignin is a phenylpropane type structure. It makes up 5 to 30% of plant.
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(iv) Water- Soluble Components:
The water soluble components of plant include simple sugars, amino acids and aliphatic acids. Altogether these contribute to 5 to 30% of plant by weight.
(v) Ether- and Alcohol-Soluble Components:
These components of plant cell wall include fats, oils, waxes, resins and a number of pigments.
(vi) Proteins:
Proteins are the main constituents of nucleic acids and cytoplasm, and found to some extent in cell wall as well. Total amount of proteins vary from 1 to 13% of total tissue weight.
