CarbohydratesIntroduction. In the leaf of a plant, the simple compounds carbon dioxide and water are combined to form the sugar (+)-glucose. This process, known as photosynthesis, requires catalysis by the green colouring matter chlorophyll, and requires energy in the form of light. Thousands of ( +)-glucose molecules can then be combined to form the much larger molecules of cellulose, which constitutes the supporting framework of the plant. (+)-Glucose molecules can also be combined, in a somewhat different way, to form the large molecules of starch, which is then stored in the seeds to serve as food for a new, growing plant.When eaten by an animal, the starch - and in the case of certain animals also the cellulose - is broken down into the original (+)-glucose units. These can be carried by the blood stream to the liver to be recombined into glycogen, or animal starch ; when the need arises the glycogen can be broken down once more into (+)-glucose. (+)-Glucose is carried by the blood stream to the tissues, where it is oxidized, ultimately to carbon dioxide and water, with the release of the energy originally supplied as sunlight. Some of the (+)-glucose is converted into fats ; some reacts with nitrogen-containing compounds to form amino acids, which in turn are combined to form the proteins that make up a large part of the animal body.(+)-Glucose, cellulose, starch, and glycogen, all belong to the class of organic compounds known as carbohydrates. Carbohydrates are the ultimate source of most of our food : we eat starch-containing grain, or feed it to animals to be converted into meat and fats which we then eat. We clothe ourselves with cellulose in the form of cotton and linen, rayon and cellulose acetate. We build houses and furniture from cellulose in the form of wood. Thus carbohydrates provide us with the necessities of lifer foodiclothing,and shelter.Basic necessities aside, 6, our present civilization depends to a surprising degree upon cellulose, particularly as paper : the books and newspapers we read, the letters we write, the bills we pay and the money and checks with which we pay them ; marriage licenses, drivers' licenses, birth certificates, mortgages ; paper in the form of bags and boxes, sheets and rolls.The study of carbohydrates is one of the most exciting fields of organic chemistry. It extends from the tremendously complicated problem of understanding the process of photosynthesis to the equally difficult problem of unraveling the tangled steps in the enzyme-catalyzed reconversion of (+)-glucose into carbon dioxide and water. Between these two biochemical problems there lie the more traditional problems of the organic chemist : determination of the structure and properties of the carbohydrates, and the study of their conversion into other organic compounds.Definition and classification. Carbohydrates are polyhydroxy aldehydes, polyhydroxy ketones, or compounds that can be hydrolyzed to them. A carbohydrate that cannot be hydrolyzed to simpler compounds is called a monosaccharide. A carbohydrate that can be