Cellulose is a type of carbohydrate present in the cell walls of plants. It gives flexibility to the plant. It is found in many foods, like lettuce, celery and bran. Cellulose provides roughage in human diet. However, it cannot be digested by humans. Only herbivores such as horses and cows can digest it. Raw cotton is composed of cellulose. Wood also has high percentage of cellulose. Cellulose cannot be manufactured chemically in a factory. Natural cellulose can be chemically treated to produce products like photographic films, rayon thread, cellophane, plastic, etc.