Cellulose is one of the most abundant natural biopolymers. The cell walls of plants are mostly made of cellulose, which provides structural support to the cell. Wood and paper are mostly cellulosic in nature. Cellulose is made up of glucose monomers that are linked by bonds between particular carbon atoms in the glucose molecule.
Every other glucose monomer in cellulose is flipped over and packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important to plant cells. Cellulose passing through our digestive system is called dietary fiber. While the glucose-glucose bonds in cellulose cannot be broken down by human digestive enzymes, herbivores such as cows, buffalos, and horses are able to digest grass that is rich in cellulose and use it as a food source. In these animals, certain species of bacteria reside in the rumen (part of the digestive system of herbivores) and secrete the enzyme cellulase. The appendix also contains bacteria that break down 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.
Directional selection is a type of natural selection that favors one extreme phenotype over the mean or another extreme. Examples of directional selection include giraffes that have long necks and the darkening of London's peppered moths after the Industrial Revolution.
Stabilizing selection, also known as purifying selection, is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value. An example of stabilizing selection is birth weigh in humans.
Disruptive selection is a type of natural selection that shows a preference toward the reproduction of genetic material at the extremes within a population.One of the most studied examples of disruptive selection is the case of London's peppered moths.
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