Biology (Holt)

Fatty acids can be saturated or unsaturated.

Figure 9 Structure of fats

Lipids

(LIHP ihdz)are nonpolar molecules that are not soluble in
water. They include fats, phospholipids, steroids, and waxes. Lipids
are an important part of the structure and functioning of cell mem-
branes. Phospholipids make up the lipid bilayer of cell membranes.
Steroids include cholesterol, which is found in animal cell mem-
branes. Other lipids include some light-absorbing compounds called
pigments,such as the plant pigment chlorophyll.
Fats are lipids that store energy. As Figure 9shows, a typical fat
contains three fatty acids bonded to a glycerol molecule backbone.
Glycerol is a three-carbon organic molecule. A fatty acid is a long
chain of carbon atoms, shown in green, with hydrogen atoms bonded
to them. Most carbon atoms in a fatty acid are bonded to either one
or two hydrogen atoms, shown in blue. Because bonds between car-
bon and hydrogen are rich in energy, fats can store a lot of energy.
In a saturated fatty acid, all of the carbon atoms in the chain are
bonded to two hydrogen atoms (except the carbon atom on the end,
which is bonded to three hydrogen atoms). Most animal fats—such
as those in butter, lard, and grease from cooked meats—contain
primarily saturated fatty acids. Saturated fatty acids are straight
molecules and are generally solid at room temperature.
In an unsaturated fatty acid, some of the carbon atoms are linked
by a “double” covalent bond, each with only one hydrogen atom,
producing kinks in the molecule, as shown in Figure 9. Most plant
oils, such as olive oil, and some fish oils contain mainly unsaturated
fatty acids and are generally liquid at room temperature.
Hydrogenated vegetable oils contain naturally unsaturated fatty
acids that have been saturated artificially by the addition of
hydrogen atoms. Thus, hydrogenated vegetable oils, such as those in
margarine and vegetable shortening, are generally solid at room
temperature.

Lipids

SECTION 3 Chemistry of Cells 35

Saturated fats, such as

butter, are solid at room

temperature.

Saturated fatty acid

H

C

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HO C

Unsaturated fatty acid

H

C

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HO C

Unsaturated fats, such as

olive oil, are liquid at room

temperature.

Real Life

Fat-free potato chips

fried in artificial fats

contain fewer calories

than those fried in

natural fats.

Unfortunately, some

artificial fats may reduce

vitamin absorption and

cause indigestion in some

people.

Finding Information

Research the benefits and

potential shortcomings of

artificial fats. 3B