HUMAN BIOLOGY

Chemistry of Life 31

What are lipids?

• Lipids are hydrophobic greasy or oily compounds.


• The type called triglycerides are major reservoirs of energy.


• Phospholipids are the main components of cell membranes.


• Sterols (such as cholesterol) are components of membranes and
  precursors of steroid hormones and other vital molecules.

taKe-home message

the risk of heart disease, which is why government regula-
tions now require they be phased out as ingredients in a
wide range of food products.

phospholipids are basic building blocks
of cell membranes

A phospholipid has a glycerol backbone, two fatty acid
tails, and a hydrophilic “head” with a phosphate group—
a phosphorus atom bonded to four oxygen atoms—and
another polar group (Figure 2.22A). Phospho lipids are the
main materials of cell membranes, which have two layers
of lipids (Figure 2.22B). The heads of one layer are dissolved
in the cell’s fluid interior, while the heads of the other layer
are dissolved in the surroundings. Sandwiched between
the two are all the fatty acid tails, which are hydrophobic.

Cholesterol and steroid hormones
are built from sterols

Sterols are among the lipids that have no fatty acid tails.
Sterols differ in the number, position, and type of their
functional groups, but they all have a rigid backbone of
four fused-together carbon rings (Figure 2.23A). Many
people associate the sterol cholesterol (Figure 2.23B) with
heart disease. However, normal amounts of this sterol are
essential in the body. For instance, the sterol cholesterol is
a vital component of membranes of every cell in your body.
Important derivatives of cholesterol include vitamin D
(essential for bone and tooth development), bile salts (which
help with fat digestion in the small intestine), and steroid
hormones such as estrogen and testosterone. In later chap-
ters we will discuss how steroid hormones influence repro-
duction, development, growth, and many other functions.

Figure 2.21 Some foods, such as butterfat and
beef, naturally contain small amounts of trans fats.
It is the arrangement of carbon atoms around the
carbon-carbon double bond (orange arrow) in the
middle of a trans fatty acid that distinguishes it from
other unsaturated fats.

hydrophobic

tails

O

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

O

CH 3

CH 2

O

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

CH

CH

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

CH 2

O

CH CH 2

O

O PO–

O

CH 2

CH 2 +N

CH 3

CH 3

CH 3

CH 2

CH 3

CC

two

hydrophobic

tails

A A phospholipid

hydrophilic

head

(orange)

B Arrangement in a cell membrane

Figure 2.22 Phospholipids contain a phosphate atom. A Structural

formula and B a simple diagram of a common phospholipid in human

cell membranes. (© Cengage Learning)

Figure 2.23 Cholesterol is the most common sterol in the

human body. Notice the carbon rings in its backbone.

CH 3

CH

CH 2

CH 2

CH 2

CH

CH 3

CH 3

CH 3 CH 3

OH

Cholesterol

Sterol

backbone

Carbon

rings

A

B

H—C—H

H—C—H

H—C—H

H—C—H

H—C^

C—H

H—C—H

H—C—H

H—C—H

H—C—H

H—C—H

H—C—H

H—C—H

H—C—H

H—C—H

H—C—H

C

HH

C

O OH

© Cengage Learning

© Cengage Learning

© iStockphoto.com/Bob Ingelhart

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