5 Steps to a 5 AP Biology, 2014-2015 Edition

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 saturatedorunsaturated.

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).

Steroidsare 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.

42 ❯ STEP 4. Review the Knowledge You Need to Score High

H

O C C C C C C H

H C O H H H H H

H H H H H

H

O C C C C C C H

H C O H H H H H

H H H H H

O C C C C C C H

H C O H H H H H

H H H H H

Figure 5.2 Fat structure (glycerol plus three fatty acids).

Figure 5.3 Steroid structures.

Steroids to Recognize!

H 3 C CH^3

H

Cholesterol Testosterone

CH 3 OH

CH 3

HO