Carbs and Proteins

There are three principal kinds of carbohydrates and each are classified according to the number of sugar molecules they contain. Monosaccharides, such as ribose, glucose, and fructose, contain only one sugar molecule. Disaccharides consist of two sugar molecules linked covalently. Familiar examples are sucrose (table sugar), maltose (malt sugar), and lactose (milk sugar). Polysaccharides, such as cellulose and starch, contain many sugar molecules linked together.

Monosaccharides are organic compounds composed of carbon, hydrogen, and oxygen. They can be described by the chemical formula (CH2O) n, where n may be as small as 3 or as large as 8. They are characterized by hydroxyl groups and an aldehyde or ketone group. These functional groups make sugars highly soluble in aqueous solution. In solution the aldehyde or ketone group has a tendency to react with one of the hydroxyl groups, producing a ring structure. The alpha glucose, which is the base of many polymers, is an energy source in organisms.

Although glucose is the common transport sugar for vertebrates, sugars are often transported in other organisms as disaccharides. Sucrose, commonly called cane sugar, is the form in which sugar is transported in plants from the photosynthetic cells to other parts of the plant body. Sucrose is composed of the monosaccharides glucose and fructose. In the synthesis of a disaccharide molecule from two monosaccharides, a molecule of water is removed in the process of forming the new bond between the two monosaccharides. This type of chemical reaction, which occurs in the synthesis of most organic polymers from their subunits, is known as condensation. When a disaccharide is split into its monosaccharide units, which happens when it is used as an energy source, the molecule of water is added again. This process is called hydrolysis.

Polysaccharides are made up of monosaccharides linked together in long chains. Some of them are storage forms of sugar. Starch is the principal food storage form in most plants. Starch occurs in two forms, amylose and amylopectin. Both consist of glucose units linked together. Glycogen is the principal storage form for sugar in higher animals. Glycogen has a structure similar to that of amylopectin except that it is more highly branched. The principal structural molecule in plants is cellulose. Cellulose is a polymer composed of monomers of glucose, just as starch and glycogen are.