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Sugars and Starches

Recognized as sugars and starches, carbohydrates are a class of molecules that primarily function to provide energy for various cellular processes.
A carbohydrate is a nutrient used by the cell for energy and structural support. They are sugars made of carbon, hydrogen, and oxygen. The basic chemical formula of a carbohydrate is (CH2O)n. The most basic function of carbohydrates is to provide energy for chemical reactions within the cell. There are three groups of carbohydrates: monosaccharides, disaccharides, and polysaccharides. A monosaccharide is a single molecule of sugar and is, therefore, termed "simple sugar." The prefix mono means "one," and sacchar means "sweet." Monosaccharides are the simplest carbohydrates and provide quick energy. Some important monosaccharides include glucose, fructose, ribose, and deoxyribose.


Two important monosaccharides are glucose and fructose. They have similar structures and the same number of carbon atoms. However, fructose contains a ketone functional group indicated by the presence of a CH2OH bonded to the carbonyl group (CO), while glucose contains an aldehyde functional group. This group does not have a CH2OH molecule bonded to the carbonyl group.
Two monosaccharides can covalently bond together to form a disaccharide, such as sucrose (table sugar) and lactose, through the process of a condensation reaction. The prefix di means "two." Monosaccharides can also covalently bond together to form a much larger polymer, known as a polysaccharide. The prefix poly means "many." These straight or branched chains of many sugar monomers are stored energy sources that the cell can use later. Polysaccharides are stored in plants as amylose. They are also stored as starch in potatoes and as cellulose, or fiber, in the stems of plants such as celery. Cellulose and starch differ in the bonding patterns between monomers. Cellulose is a tough, structural material in plants that is indigestible by humans. Starch, on the other hand, is easily digested by humans.
Starch is a common polysaccharide, or large sugar molecule, made of smaller subunits. The macromolecule consists of a long chain of subunits called monosaccharides that are covalently bonded together. Like many polysaccharides, starch is composed of monomers of glucose, which are formed into a polysaccharide through a series of condensation reactions.
In animals, polysaccharides are stored as glycogen in the liver and in muscle cells. When the blood sugar level decreases in the body, liver cells degrade the glycogen and release glucose. Another polysaccharide is chitin, which is a structural material for the hard parts of invertebrate animals and the cell walls of many fungi. Chitin is composed of nitrogen-containing groups that are attached to glucose monomers.