Carbohydrates in depth

Carbohydrates are a large class of natural organic substances that includes sugars, starches and cellulose and are made exclusively of the atoms carbon, hydrogen, and oxygen.
Green plants use the energy of sunlight to convert carbon dioxide and water into carbohydrates. This process called photosynthesis, releases oxygen into the atmosphere and transforms light energy into the chemical energy stored in simple carbohydrates. Plants convert simple carbohydrates into sucrose, or common table sugar, which is particularly abundant in sugarcane and sugar beets. Cellulose plants principal structural component is starch.

Carbohydrates can be divided into three major groups: monosaccharides, disaccharides, and polysaccharides. Molecules of monosaccharides, or simple sugars, contain from three to nine carbon atoms, most commonly five or six.
Two of the most important simple sugars are glucose, also known as dextrose, grape sugar, or corn sugar; and fructose, or fruit sugar. Two simple sugar molecules are linked to each other in a disaccharide, or double sugar. The disaccharide sucrose consists of one molecule of glucose and one molecule of fructose bound together. Lactose, or milk sugar, and maltose, or malt sugar, are also disaccharides. Polysaccharides are large molecules, such as cellulose, starch, and glycogen, in which as many as 10,000 monosaccharide units are linked.
Many carbohydrates are isomers; that is, they have the same numbers of particular atoms but different structures. Glucose, fructose, and galactose (a monosaccharide part of lactose), for example, are all isomers with a formula of C6H12O6.
In most animals and humans, carbohydrates provide a quickly available reservoir of energy. Glucose, the sugar that circulates in the blood, is absorbed by the cells, where it participates in biochemical reactions that energise the metabolic processes. Glycogen, which consists of branching chains of glucose molecules, is stored in the liver and muscles and is broken down into glucose during times of muscular activity.
Two monosaccharides, ribose and deoxyribose are found in all cells. There they form the carbohydrate component of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), respectively.
Glucose is found in fruits and honey. It is also in the two most common naturally occurring disaccharides, sucrose and lactose, and it is the sole structural unit of the polysaccharides cellulose, starch, and glycogen. Glucose is produced commercially in large amounts from cornstarch by hydrolysis, a reaction with water. The largest amount of glucose is sold as corn syrup, though its crystalline form is sometimes sold under the name dextrose. Fructose is one of the constituents of the disaccharide sucrose and is also present in uncombined form in honey, apples, and tomatoes. It is produced from sucrose and is used by the food industry.
Lactose, which is composed of glucose and galactose, is one of the sugars found most commonly in human diets. It constitutes about 5 percent of the milk of all mammals.

Starch and Cellulose

The two best-known polysaccharides are starch and cellulose. Starches are a group of plant polysaccharides made up of glucose units; most starches are composed of two types of sub-units; a linear molecule called amylose and a branched molecule called amylopectin.
In humans, before starch can be used it must be broken into individual glucose units; this process is started in the mouth by enzymes called amylases that are present in saliva and continues in the intestinal tract. The product of amylase action is maltose, which is then hydrolysed to glucose as it is absorbed through the walls of the intestine.
Another important polysaccharide is glycogen, which is found in all animal tissues and is the primary animal storage form of carbohydrate. Glycogen is composed of glucose units. It resembles starch but is more highly branched.
Many foods rich in starch, such as potatoes, rice, bread, and pasta, have been considered fattening. Actually, 1 gram (1/30 ounce) of starch has only 4 calories, the same as protein and less than a gram of fat, which contains 9 calories. Many nutritionists and sports physicians now favour diets based on a higher percentage of starch, which they call complex carbohydrates, than most people are accustomed to. Some experts in the field of nutrition now consider complex carbohydrates a more efficient source of nutrients than protein and fats, with fewer toxic by-products from their digestion and less.

Further Reading : -

Simple and complex carbohydrates....whats the difference?
Carbohydrates and exercise, why is it important ?
A more indepth look at carbohydrates.