Plant Cell Walls Are Important Substrates for Fermentative Digestion and Significant Nutrient Sources for Many Species
Forages, or the foliage of plants, are both the major feedstuff of large herbivores and an important substrate for fermentative digestion. Some appreciation of the physical and chemical nature of plants is important to an understanding of the fermentative digestion of forages.
This understanding may be aided by a brief comparison of plant and animal tissue structure.At the cellular level, a major difference between plants and animals is the existence of a cell wall in plants. The cell wall is a complex of various carbohydrate molecules. The structural parts of plants, the leaves and stems, contain a large portion of cell-wall material. This material gives the plants their rigid framework and protects them from weather and other elements during growth. The cell-wall structure of plants can be roughly compared to the connective tissue structure of animals. Long, fiberlike molecules of cellulose have a strengthgiving role similar to that of collagen, whereas hemicellulose, pectin, and lignin cement the cellulose together, much as hyaluronic acid and chondroitin sulfate do in animal connective tissue. With the exception of lignin, all these cell-wall molecules are carbohydrate.
Cellulose is composed of nonbranching chains of glucose monomers joined by β∣ 1-4] glycosidic linkages, in contrast to the α[l-4] linkages in starch. Pectin and hemicellulose are chemically more heterogeneous than cellulose, being composed of various proportions of several sugars and sugar acids. None of the cell-wall materials is subject to hydrolytic digestion by mammalian glandular digestive enzymes. However, cellulose, hemicellulose, and pectin are subject to the hydrolytic action of a complex of microbial enzymes known as cellulase. This enzyme system releases monosaccharides and oligosaccharides from the complex carbohydrates of cell walls, but the released saccharides are not directly available for absorption by the animal. Rather, they are further metabolized by the microbes, as discussed later.
Lignin, a heterogeneous group of phenolic chemicals, is resistant to the action of either mammalian or microbial enzymes, and only a small portion of lignin is digested by either process. Lignin is important not only because it is indigestible itself, but also because it tends to encase the cell- wall carbohydrates, reducing their digestibility by protecting them from the action of bacterial cellulase. The lignin concentration of plants increases with age and ambient temperature; thus young, cool-season plants are more digestible than mature plants grown in hot weather.