HUMAN BIOLOGY

30 Chapter 2

Lipids: fats and their Chemical relatives

n    Cells use lipids to store energy, as structural materials, and

as signaling molecules.

Oil and water don’t mix. Why? Oils are a type of lipid, and

a lipid is a nonpolar hydrocarbon. A lipid’s large nonpolar

region makes it hydro phobic, so it does not dissolve eas-

ily in water. Lipids do easily dissolve in other nonpolar

substances. For example, you can dissolve melted butter

in olive oil. Here we are interested first in fats and phos-

pholipids, both of which have chemical “tails” called fatty

acids. We will also consider sterols, which have a backbone

of four carbon rings.

fats are energy-storing

lipids

The lipids called fats have as many

as three fatty acids, all attached to

glycerol. Each fatty acid has a back-

bone of up to thirty-six carbons and

a carboxyl group (—COOH) at one

end. Hydrogen atoms occupy most

or all of the remaining bonding

sites. A fatty acid typically stretches

out like a flexible tail (Figure 2.19).

In saturated fats, the fatty acid

backbones have only single cova-

lent bonds. Animal fats are satu-

rated and solid at room temperature.

Examples are butter, lard, or chicken fat.

The fatty acid tails of unsaturated fats have

one or more double covalent bonds. Such

strong bonds make rigid kinks that prevent

unsaturated fats from packing tightly. Most

vegetable oils such as canola oil, peanut oil,

corn oil, and olive oil are unsaturated. They

stay liquid at room temperature.

Butter, lard, oils, and other dietary fats

consist mostly of triglycerides. These fats

have three fatty acid tails attached to a glyc-

erol backbone (Figure 2.20). Tri glycerides

are the most common lipids in the body as

well as its richest source of energy. Com-

pared to complex carbohydrates, they yield

more than twice as much energy when they are broken

down. This is because triglycerides have more removable

electrons than do carbohydrates—and energy is released

when electrons are removed. In the body, cells of fat-storing

tissues stockpile triglycerides as fat droplets.

Some unsaturated fats are unhealthy. Those known as

trans fats are an example. A double bond in “cis” fatty acids

keeps them kinked, but in trans fatty acids a double bond

keeps them straight (Figure 2.21). Some trans fatty acids

occur naturally in beef and lamb, but they can also form

in a manufacturing process (called hydrogenation) that for

many years was used to solidify vegetable oils for solid

margarines and shortenings used in many commercially

prepared foods. A diet high in trans fatty acids increases

2.10

Figure 2.19 Animated! Fats are built from fatty acids.

A Stearic acid has a carbon backbone fully saturated

with hydrogens. B Oleic acid, with a double bond in the

carbon backbone, is unsaturated. C Linolenic acid, with

three double bonds, is a polyunsaturated fatty acid.

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

H

CO

HO

HCH

HCH

HCH

HCH

HCH

HCH

HCH

CH

CH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

H

CCO

HO

HCH

HCH

HCH

HCH

HCH

HCH

HCH

CH

CH

H CH

CH

CH

HCH

CH

CH

HCH

HCH

H

CO

HO

A stearic acid B oleic acid C linolenic acid

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Figure 2.20 Animated! Triglycerides have

three fatty acid tails attached to glycerol.

CH

HCH

HCH

HCH

HCH

HCH

HCH

CH

CH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

H

CO CO CO

OO

CC

HH

H

O

C

H

H

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

H CH

H

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

HCH

H

H

+3H 2 O

A triglyceride

glycerol

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© iStockphoto.com/George Clerk

fat Lipid molecule that has
up to three fatty acid tails.

fatty acid Compound with a
backbone of carbon atoms
bonded to a carboxyl group.

lipid A nonpolar hydrocarbon.

phospholipid Complex lipid
that has a phosphate func-
tional group.

sterol Type of lipid that has
no fatty acid tail. Sterols
include cholesterol and ste-
roid hormones.

triglyceride Fat that has
three fatty acid tails attached
to a glycerol backbone.

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