E. CARBOHYDRATES
Most carbohydrates have the empirical formula C(H2O)n. Carbohydrates are composed of covalently bonded atoms of carbon, hydrogen, and oxygen.
1. Monosaccharides
The basic unit of a carbohydrate is a monosaccharide or simple sugar. Monosaccharides can be burned (oxidized) to yield carbon dioxide, water, and energy. The principle source of energy for organisms is glucose. Structurally a sugar consists of a carbon backbone of three or more carbon atoms with either an aldehyde or carbonyl group on one carbon and hydroxyl groups on each of the other carbons.
The most common monosaccharide is glucose, C6H12O6. Glucose is the form of sugar generally transported in the human body. A disaccharideis formed by joining two monosaccharides together. The two monosaccharides are linked by a reaction called a dehydration or condensation reaction.
• A monomer is a relatively simple and small molecule; many of them can be
linked together to form a polymer.
• A polymer is a large molecule composed of many similar or identical
molecular subunits.
• A polysaccharide consists of many monosaccharides joined together by
condensation reactions.
Condensation reaction: the joining of two smaller organic compounds resulting in the formation of a larger organic molecule and the release of a water molecule. The condensation reaction, a synthesis reaction, is important because it is the reaction that puts together polymers from monomer units. Synthesis reactions require energy to complete.
C6H12O6 + C6H12O6 –> C12H22O11 + H2O
glucose + fructose –> sucrose + water
Hydrolytic cleavage (hydrolysis): With the addition of water, the splitting of a large organic molecule into two smaller organic molecules. Hydrolysis reactions liberate energy. Hydrolytic cleavage, or hydrolysis, is the opposite of a dehydration reaction. For example, in the human digestive system, sucrose (disaccharide) is split into glucose and fructose (two monosaccharides).
2. Disaccharides
Two monosaccharides that are joined by a blycosidic linkage, a covalent bond between two monosaccharides.
glucose + glucose = maltose glucose + fructose = sucrose
2C6H12O6 àà C12H22O11 + H2O
3. Polysaccharides
Glycosidic linkages can be oriented in space. Two monomers can be joined either by an alpha or beta linkage. By a series of dehydration reactions, many monosaccharides can be put together to form a polysaccharide. Three examples of polysaccharides are starch, glycogen, and cellulose. In starch and glycogen the monomers are joined by alpha linkages; in cellulose the glucose monomers are joined by beta linkages.
a. Starch
Starch is the storage plsacchride in plants and is an important reservoir for energy. There are two common types of starch.
1) Amylose: the simplest starch. Consisting of unbranched chains of hundreds of glucose molecules.
2) Amylopectrin: large molecule consisting of short glucose chains with other glucose chains branching off the main chain.
b. Glycogen
Glucogen is the storage polysaccharide in animals. Glycogen is composed of branching glucose chains, with more branches then amylopectrin. It is found in the liver and muscles and acts as a temporary storage form of glucose. The liver removes the excess glucose from the bloodstream, converts the glucose monomers to glycogen via condensation reactions, and stores it as glycogen. When vertebrates need glucose for energy, glycogen is converted by hydrolytic cleavage back to glucose.
c. Cellulose
Cellulose is a structural polysaccharide and is the major building material made by plants. It is the most abundant organic material in earth. Cellulose is made up of long, straight glucose molecules. Cellulose is called a structural polysaccharide because it gives the plant cell its shape, is not soluble, and is very strong. Cellulose is flexible when the plant cell is young. As the cell grows, the cellulose becomes thicker and more rigid.
Cellulose is indigestible to animals because the linkages are 1-4 beta linkages, and our enzyme can only break down 1-4 alpha linkages because the shapes are different. Cellulose is the so-called fiber in our diets. Some bacteria, protests, fungi, and lichens can break down cellulose. For example, bacteria and protests found in the stomachs of termites and grazing animals break down the cellulose in the grass and wood to provide the animal with glucose.
d. Other structural polysaccharides
1) Pectin and andarrageenan: these are extracted from algae. Pectin and carrageenan are put into food items such as jellies, jams, yogurt, ice cream, and milkshakes to give them a jelly-like or creamy consistency.
2) Chitin: Chitin is principal component of the exoskeletons of insects and other arthropods, including lobsters. Chitin is very soft but is combined with CaCO3 (calcium carbonate or limestone) to become hard. Most animals cannot digest chitin.