9.3. Carbon and Living Things http://www.ck12.org
Lipids
Lipidsare biochemical compounds such as fats and oils. Organisms use lipids to store energy. In addition to carbon
and hydrogen, lipids contain oxygen.
Fatty Acids
Lipids are made up of long carbon chains called fatty acids. Like hydrocarbons, fatty acids may be saturated or
unsaturated.Figure9.21 shows structural formulas for two small fatty acids. One is saturated and one is unsaturated.
- In saturated fatty acids, there are only single bonds between carbon atoms. As a result, the carbons are
saturated with hydrogen atoms. Saturated fatty acids are found in fats. Fats are solid lipids that animals use to
store energy. - In unsaturated fatty acids, there is at least one double bond between carbon atoms. As a result, some carbons
are not bonded to as many hydrogen atoms as possible. Unsaturated fatty acids are found in oils. Oils are
liquid lipids that plants use to store energy.
FIGURE 9.21
Both of these fatty acid molecules have
six carbon atoms and two oxygen atoms.
How many hydrogen atoms does each
fatty acid have?Phospholipids
Some lipids contain the element phosphorus as well as oxygen, carbon, and hydrogen. These lipids are called
phospholipids. Two layers of phospholipid molecules make up most of the cell membrane in the cells of living
things.
Figure9.22 shows how phospholipid molecules are arranged in a cell membrane. One end (the head) of each
phospholipid molecule is polar and attracts water. This end is called hydrophilic ("water loving"). The other end
(the tail) is nonpolar and repels water. This end is called hydrophobic ("water hating"). The nonpolar tails are on the
inside of the membrane. The polar heads are on the outside of the membrane. These differences in polarity allow
some molecules to pass through the membrane while keeping others out. You can see how this works in the video at
the URL below.
http://www.beyondbooks.com/lif71/4b.asp
Nucleic Acids
Nucleic acidsare biochemical molecules that contain oxygen, nitrogen, and phosphorus in addition to carbon and
hydrogen. There are two main types of nucleic acids. They are DNA (deoxyribonucleic acid) and RNA (ribonucleic
acid).