Macromolecules
Up to this point we have considered only small molecules. Many of the molecules important to biological processes are HUGE. These are known as macromolecules. Most macromolecules are polymers, which are long chains of subunits called monomers. These subunits are often very similar to each other, and for all the diversity of polymers (and living things in general) there are only about 40 - 50 common monomers.
Making and breaking polymers
Joining two monomers is achieved by a process known as dehydration synthesis. One monomer gives up a hydroxyl (OH) group and one gives up a (H). These combine to make a water molecule. Hence the name dehydration synthesis.
Polymers are broken apart by a process known as hydrolysis. Bonds between monomers are broken by the addition of water. (3.3, pg 36)
There are four major categories of organic compounds found in living cells.
Carbohydrates
Carbohydrates are the sugars and their polymers. Simple sugars are called monosaccharides. These can be joined to form polysaccharides (3.5, pg 38). Glucose is an important monosaccharide. Sucrose, a disaccharide (consisting of two monosaccharides), is table sugar. (Note the ending "ose" common to most sugars.)
Polysaccharides may be made from thousands of simple sugars linked together. These large molecules may be used for storage of energy or for structure. First a couple of storage examples:
Starch is a storage polysaccharide of plants. Its is a giant string of glucoses. The plant can utilize the energy in starch by first hydrolyzing it, making the glucose available. Most animals can also hydrolyze starch. That's why we eat it.
Animals store glycogen as a supply of glucose. It is stored in the liver and muscles. (3.7, pg 39)
And some examples of structural carbohydrates:
Cellulose is a polysaccharide produced by plants. Its is a component of the cell walls. Cellulose is also a string of glucose molecules. Because the glucoses are joined together differently cellulose has a different shape, and therefor different properties, than starch or glycogen. The enzymes (we'll learn more about these soon) that are used to hydrolyze starch don't work on cellulose. Most organisms cannot digest cellulose and it passes right through them (roughage). Goats and termites don't really digest cellulose, they have bacteria that do it for them.
Chitin is an important polysaccharide used to make the exoskeletons of arthropods.
Lipids
Lipids are all similar in that they are (at least in part) hydrophobic. There are three important families of lipids: fats, phospholipids and steroids.
Fats
Fats are large molecules made of two types of molecules, glycerol and some type of fatty acid. The fatty acid has a long chain of carbon and hydrogen, usually referred to as the hydrocarbon tail, with a carboxyl group head. (The carboxyl group is why its called an acid). Glycerol has three carbons (3.8b, pg 40) so it can get three fatty acids. These can be the same three or different. This arrangement of three is why fats are called triglycerides.
Fats may be saturated or unsaturated. This has to do with the amount of hydrogen in the tail. Unsaturated fatty acids have some hydrogen missing, with double bonds replacing them. The double bond give the fatty acid a kink (3.8c, pg 40). Saturated fats are solid at room temperature and come from animals, unsaturated fats come from plants and are liquid at room temperature.
Fats are used as a high density energy storage in animals and in plants (seeds). It may also be used in animals for insulation.
Resource: www.olemiss.edu/courses/bisc102/macromol.html
