Carbohydrates
Carbohydrates are molecules that have the chemical structure (CH2)n. The basic unit, or monomer, of a carbohydrate is a monosaccharide, or simple sugar (Figure 2.10). Glucose is a typical monosaccharide, with the chemical formula C6H12O6. Because of the hydroxyl (OH) groups, which have a slight positive charge on the hydrogen due to its covalent bond with the electronegative oxygen, glucose is water soluble.
Figure 2.10. The Structure of Glucose
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When two monosaccharides are joined together through dehydration synthesis, a dissacharide is formed (Figure 2.11). Maltose is composed of two glucose molecules.
Figure 2.11. Disaccharides
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When many monomers are joined together a polysaccharide is formed. One of the two main functions of polysaccharides is energy storage. Energy is stored in the chemical bonds of the polysaccharides starch and glycogen, which are polymers of glucose molecules. Starch is the energy storage molecule of plants. Plants produce sugars through photosynthesis to meet their own energy needs. Any sugars that they do not immediately need are converted to starch for storage. When primary consumers eat plant material, they ingest starch, which is broken down through the process of digestion and the chemical bond energy released. We will learn more about this in lesson 4. Animals also store energy in the form of a carbohydrate, glycogen. However, glycogen is only used for short-term storage. In humans, glycogen is produced and broken down primarily in the liver.
Carbohydrates also have a structural role in some organisms. Plants use the polysaccharide cellulose as a major component of their cell walls. Cellulose also is a polymer of glucose monomers. However, the glucose molecules are bonded together differently, and the enzyme amylase, which can digest starch, cannot break the chemical bonds in cellulose. Therefore, in humans, cellulose acts as roughage, an important part of the diet that keeps material moving through the digestive system. It is not broken down during digestion. Animals that eat large amounts of plant material, such as cows and koalas, have organisms, bacteria and protists, living symbiotically in their digestive tract. These organisms do produce enzymes that can break the chemical bonds between glucose molecules in cellulose and allow the animal to obtain nutrition from that polysaccharide. The polysaccharide chitin also provides structure. It is a major component of the cell walls in fungi, and of the exoskeleton in arthropods.
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