An early definition, still quite appropriate, basically states that fiber is the portion of food derived from plant cell walls that is poorly digested by mammals. Another common definition for fiber is the non-starch polysaccharide component of foodstuffs. The chief components of dietary fiber are cellulose and hemicellulose, both of plant origin. Pectin and pectic acid are other plant polysaccharides often present in diets.
Cellulose is a linear polymer of between and 10, beta-D-glucose molecules in which adjacent glucose molecules are joined covalently through beta glycosidic bonds.
The beta bonds cause the polymer to assume a non-helical, straight structure, which is different from the helical structure imposed on starch molecules by the alpha bonding. The non-helical structure of cellulose also promotes hydrogen bonding between cellulose molecules. Cellulose polymers associate with one another through a huge number of hydrogen bonds to form microfibers. Microfibers interact to form cellulose fibers. A typical fiber contains roughly , cellulose molecules.
The high tensile strength of cellulose fibers reflects the massive number of hydrogen bonds involved in its structure. Hemicellulose is a heteropolymer composed of a variety of sugars, including xylose, arabinose and mannose in a branched structure. In the human body, several enzymes known collectively as amylases degrade starch sequentially into usable glucose units. Glycogen is the energy reserve carbohydrate of animals.
Like starch in plants, glycogen is found as granules in liver and muscle cells. When fasting, animals draw on these glycogen reserves during the first day without food to obtain the glucose needed to maintain metabolic balance. Glycogen is structurally quite similar to amylopectin, although glycogen is more highly branched 8—12 glucose units between branches and the branches are shorter.
When treated with iodine, glycogen gives a reddish brown color. Glycogen can be broken down into its D-glucose subunits by acid hydrolysis or by the same enzymes that catalyze the breakdown of starch. In animals, the enzyme phosphorylase catalyzes the breakdown of glycogen to phosphate esters of glucose. Although the percentage of glycogen by weight is higher in the liver, the much greater mass of skeletal muscle stores a greater total amount of glycogen. Cellulose, a fibrous carbohydrate found in all plants, is the structural component of plant cell walls.
The largest use of cellulose is in the manufacture of paper and paper products. Like amylose, cellulose is a linear polymer of glucose. As a result, cellulose exhibits little interaction with water or any other solvent. Cotton and wood, for example, are completely insoluble in water and have considerable mechanical strength. Because cellulose does not have a helical structure, it does not bind to iodine to form a colored product.
Cellulose yields D-glucose after complete acid hydrolysis, yet humans are unable to metabolize cellulose as a source of glucose. However, certain microorganisms can digest cellulose because they make the enzyme cellulase, which catalyzes the hydrolysis of cellulose. The presence of these microorganisms in the digestive tracts of herbivorous animals such as cows, horses, and sheep allows these animals to degrade the cellulose from plant material into glucose for energy.
Termites also contain cellulase-secreting microorganisms and thus can subsist on a wood diet. This example once again demonstrates the extreme stereospecificity of biochemical processes. Certified diabetes educators come from a variety of health professions, such as nursing and dietetics, and specialize in the education and treatment of patients with diabetes.
Common disaccharides include lactose, maltose, and sucrose Figure 5. Lactose is a disaccharide consisting of the monomers glucose and galactose. It is found naturally in milk.
Maltose, or malt sugar, is a disaccharide formed by a dehydration reaction between two glucose molecules. The most common disaccharide is sucrose, or table sugar, which is composed of the monomers glucose and fructose.
Figure 5. Common disaccharides include maltose grain sugar , lactose milk sugar , and sucrose table sugar. The chain may be branched or unbranched, and it may contain different types of monosaccharides. The molecular weight may be , daltons or more depending on the number of monomers joined. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides.
Starch is the stored form of sugars in plants and is made up of a mixture of amylose and amylopectin both polymers of glucose. The starch in the seeds provides food for the embryo as it germinates and can also act as a source of food for humans and animals.
The starch that is consumed by humans is broken down by enzymes, such as salivary amylases, into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose. The numbers and refer to the carbon number of the two residues that have joined to form the bond. Figure 6. Amylose and amylopectin are two different forms of starch. Because of the way the subunits are joined, the glucose chains have a helical structure. Glycogen not shown is similar in structure to amylopectin but more highly branched.
Glycogen is the storage form of glucose in humans and other vertebrates and is made up of monomers of glucose.
Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis.
Cellulose is the most abundant natural biopolymer. The cell wall of plants is mostly made of cellulose; this provides structural support to the cell. Wood and paper are mostly cellulosic in nature. Figure 7. Because of the way the glucose subunits are joined, every glucose monomer is flipped relative to the next one resulting in a linear, fibrous structure.
As shown in Figure 7, every other glucose monomer in cellulose is flipped over, and the monomers are packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important to plant cells. In these animals, certain species of bacteria and protists reside in the rumen part of the digestive system of herbivores and secrete the enzyme cellulase.
The appendix of grazing animals also contains bacteria that digest cellulose, giving it an important role in the digestive systems of ruminants. Cellulases can break down cellulose into glucose monomers that can be used as an energy source by the animal.
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