The chemistry of carbon accounts for the formation of the very large number of organic molecules associated with living organisms. Carbon shares electrons with as many as four other atoms. Many times, carbon atoms share electrons with each other to form rings or chains of carbon atoms. These act as a skeleton for the biomo/ecu/escarbohydrates, lipids (fats), proteins, and nucleic acids. Biomolecules are polymers or chains of unit molecules joined together.
Carbohydrates
Carbohydrates (kar"bo-hi'drats) supply short-term but quick energy for all organisms, including humans. Carbohydrate molecules are characterized by the presence of the atomic grouping CH20, in which the ratio of hydrogen atoms (H) to oxygen atoms (0) is approximately 2: 1.
Monosaccharides and Oisaccharides
If the number of carbon atoms in a molecule is low (from three to seven), then the carbohydrate is a simple sugar, or monosaccharide (mon"o-sak'ah-rid). These molecules are often designated by the number of carbon atoms they contain; for example, glucose, with six carbon atoms, is called a hexose. Other common monosaccharides are fructose, found in fruits, and galactose, a constituent of milk. These three monosaccharides (glucose, fructose, and galactose) all occur as ring structures with the molecular formula C6H1206, but the exact shape of the ring differs, as does the arrangement of the hydrogen and the hydroxide groups attached to the ring.
A disaccharide (di -sak' ah - rid) (di -, two; saccharide, sugar) contains two monosaccharides. Synthesis of a
disaccharide is a condensation reaction because water is produced as the two monosaccharides join to form the disaccharide. Degradation of a disaccharide is a hydrolysis (hi-drol'T-sis) reaction because water is used to split a bond.
Maltose is a disaccharide that forms from two glucose molecules. When glucose and fructose join, the disaccharide sucrose forms. Sucrose, which is ordinarily derived from sugarcane and sugar beets, is commonly known as table sugar. Eating a candy bar provides quick energy because sucrose provides glucose. Cells use glucose as their primary energy source.
Polysaccharides
A polysaccharide (pol"e-sak'ah-rid) is a carbohydrate that contains a large number of monosaccharide molecules. Three polysaccharides are common in animals and plants: glycogen, starch, and cellulose. All of these are polymers of glucose, just as a necklace might be made up of only one type of bead. Even though all three polysaccharides contain only glucose, they are distinguishable from one another.
Glycogen (gli'ko-jen), a molecule having many side branches), is the storage form of glucose in humans. After a meal, the liver stores glucose as glycogen; between meals, the liver releases glucose so that the blood glucose level is always 0.1 %. If the blood contains more glucose, it spills over into the urine, signaling that the condition diabetes (di"ah-be'tez) exists.
The polymers starch and cellulose are found in plants.
Plants store glucose as starch, a polymer similar in structure to glycogen except that it has few side branches. Starch is an important source of glucose energy in the diet because it can be hydrolyzed to glucose by digestive enzymes. In cellulose, often called fiber, the glucose units are joined by a slightly different type of linkage from that of glycogen and starch.
For this reason, humans are unable to digest cellulose, and it passes through the digestive tract as roughage. The presence of fiber in the diet is necessary for good health and may help prevent colon cancer.
Glucose is an immediate source of energy in cells. It is the unit molecule for glycogen, starch, and cellulose. Glycogen stores energy in the body, starch is a dietary source of energy, and cellulose is fiber in the diet.
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