Lipids, Carbohydrates, and Proteins for AP Biology
Practice problems for these concepts can be found at: Chemistry Review Questions for AP Biology
Lipids are organic compounds used by cells as long-term energy stores or building blocks. Lipids are hydrophobic and insoluble in water because they contain a hydrocarbon tail of CH2s that is nonpolar and repellant to water. The most important lipids are fats, oils, steroids, and phospholipids.
Fats, which are lipids made by combining glycerol and three fatty acids (Figure 5.1), are used as long-term energy stores in cells. They are not as easily metabolized as carbohydrates, yet they are a more effective means of storage; for instance, one gram of fat provides two times the energy of one gram of carbohydrate. Fats can be saturated or unsaturated. Saturated fat molecules contain no double bonds. Unsaturated fats contain one (mono-) or more (poly-) double bonds, which means that they contain fewer hydrogen molecules per carbon than do saturated fats. Saturated fats are the bad guys and are associated with heart disease and atherosclerosis. Most of the fat found in animals is saturated, whereas plants tend to contain unsaturated fats. Fat is formed when three fatty acid molecules connect to the OH groups of the glycerol molecule. These connecting bonds are formed by dehydration synthesis reaction (Figure 5.2).
Steroids are lipids composed of four carbon rings that look like chicken-wire fencing in pictorial representations. One example of a steroid is cholesterol, an important structural component of cell membranes that serves as a precursor molecule for another important class of steroids: the sex hormones (testosterone, progesterone, and estrogen). You should be able to recognize the structures shown in Figure 5.3 for the AP exam.
A phospholipid is a lipid formed by combining a glycerol molecule with two fatty acids and a phosphate group (Figure 5.4). Phospholipids are bilayered structures; they have both a hydrophobic tail (a hydrocarbon chain) and a hydrophilic head (the phosphate group) (Figure 5.5). They are the major component of cell membranes; the hydrophilic phosphate group forms the outside portion and the hydrophobic tail forms the interior of the wall.
Carbohydrates can be simple sugars or complex molecules containing multiple sugars. Carbohydrates are used by the cells of the body in energy-producing reactions and as structural materials. Carbohydrates have the elements C, H, and O. Hydrogen and oxygen are present in a 2:1 ratio. The three main types of carbohydrates you need to know are monosaccharides, disaccharides, and polysaccharides.
A monosaccharide, or simple sugar, is the simplest form of a carbohydrate. The most important monosaccharide is glucose (C6H12O6), which is used in cellular respiration to provide energy for cells. Monosaccharides with five carbons (C5H10O5) are used in compounds such as genetic molecules (RNA) and high-energy molecules (ATP). The structural formula for glucose is shown in Figure 5.6.
A disaccharide is a sugar consisting of two monosaccharides bound together. Common disaccharides include sucrose, maltose, and lactose. Sucrose, a major energy carbohydrate in plants, is a combination of fructose and glucose; maltose, a carbohydrate used in the creation of beer, is a combination of two glucose molecules; and lactose, found in dairy products, is a combination of galactose and glucose.
A polysaccharide is a carbohydrate containing three or more monosaccharide molecules. Polysaccharides, usually composed of hundreds or thousands of monosaccharides, act as a storage form of energy and as structural material in and around cells. The most important carbohydrates for storing energy are starch and glycogen. Starch, made solely of glucose molecules linked together, is the storage form of choice for plants. Animals store much of their energy in the form of glycogen, which is most often found in liver and muscle cells. Glycogen is formed by linking many glucose molecules together.
Two important structural polysaccharides are cellulose and chitin. Cellulose, a compound composed of many glucose molecules, is used by plants in the formation of their cell walls. Chitin is an important part of the exoskeletons of arthropods such as insects, spiders, and shellfish.