Unit 2 - 2

Lipids are a diverse group of hydrophobic
molecules
• Lipids are the one class of large biological
molecules that do not form polymers
• The unifying feature of lipids is having little or
no affinity for water
• Lipids are hydrophobic becausethey consist
mostly of hydrocarbons, which form nonpolar
covalent bonds
• The most biologically important lipids are fats,
phospholipids, and steroids
Fats
• Fats are constructed from two types of smaller
molecules: glycerol and fatty acids
• Glycerol is a three-carbon alcohol with a
hydroxyl group attached to each carbon
• A fatty acid consists of a carboxyl group
attached to a long carbon skeleton
Figure 5.10
Fatty acid
(in this case, palmitic acid)
Glycerol
(a) One of three dehydration reactions in the synthesis of a fat
Ester linkage
(b) Fat molecule (triacylglycerol)

Figure 5.10a
Fatty acid
(in this case, palmitic acid)
Glycerol
(a) One of three dehydration reactions in the synthesis of a fat
• Fats separate from water because water
molecules form hydrogen bonds with each
other and exclude the fats
• In a fat, three fatty acids are joined to
glycerol by an ester linkage, creating a
triacylglycerol, or triglyceride
Ester linkage
(b) Fat molecule (triacylglycerol)

• Fatty acids vary in length (number of carbons)
and in the number and locations of double
bonds
• Saturated fatty acids have the maximum
number of hydrogen atoms possible and no
double bonds
• Unsaturated fatty acids have one or more
double bonds
© 2011 Pearson Education, Inc.

(a) Saturated fat
Structural formula of a saturated fat molecule

(b) Unsaturated fat
Structural formula of an unsaturated fat

molecule
• Fats made from saturated fatty acids are
called saturated fats, and are solid at room
temperature
• Most animal fats are saturated
• Fats made from unsaturated fatty acids are
called unsaturated fats or oils, and are liquid
at room temperature
• Plant fats and fish fats are usually unsaturated
• A diet rich in saturated fats may contribute to
cardiovascular disease through plaque deposits
• Hydrogenation is the process of converting
unsaturated fats to saturated fats by adding
hydrogen
• Hydrogenating vegetable oils also creates
unsaturated fats with trans double bonds
• These trans fats may contribute more than
saturated fats to cardiovascular disease
© 2011 Pearson Education, Inc.

• Certain unsaturated fatty acids are not
synthesized in the human body
• These must be supplied in the diet
• These essential fatty acids include the omega-3
fatty acids, required for normal growth, and
thought to provide protection against
cardiovascular disease
• The major function of fats is energy storage
• Humans and other mammals store their fat in
adipose cells
• Adipose tissue also cushions vital organs and
insulates the body
Phospholipids
• In a phospholipid, two fatty acids and a
phosphate group are attached to glycerol
• The two fatty acid tails are hydrophobic, but
the phosphate group and its attachments
form a hydrophilic head
Choline
Hydrophilic head
Phosphate
Glycerol
Hydrophobic tails
Fatty acids
Hydrophilic
head
Hydrophobic
tails
(a) Structural formula (b) Space-filling model (c) Phospholipid symbol

• When phospholipids are added to water, they
self-assemble into a bilayer, with the
hydrophobic tails pointing toward the interior
• The structure of phospholipids results in a
bilayer arrangement found in cell membranes
• Phospholipids are the major component of all
cell membranes
Hydrophilic WATER
head
Hydrophobic
tail WATER
Steroids
• Steroids are lipids characterized by a carbon
skeleton consisting of four fused rings
• Cholesterol, an important steroid, is a
component in animal cell membranes
• Although cholesterol is essential in animals,
high levels in the blood may contribute to
cardiovascular disease
The natural steroid hormones are generally
synthesized from cholesterol in
the gonads and adrenal glands. These forms of
hormones are lipids.

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Unit 2 - 2

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Lipids are a diverse group of hydrophobic

(b) Fat molecule (triacylglycerol)

(b) Fat molecule (triacylglycerol)

© 2011 Pearson Education, Inc.

Structural formula of a saturated fat molecule

Structural formula of an unsaturated fat

© 2011 Pearson Education, Inc.

(a) Structural formula (b) Space-filling model (c) Phospholipid symbol

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