Human Physiology, 14th edition (2016)

36 Chapter 2

therefore hydrophobic. Although lipids are insoluble in water,

they can be dissolved in nonpolar solvents such as ether, ben-

zene, and related compounds.

Triglyceride (Triacylglycerol)

Triglyceride is the subcategory of lipids that includes fat and

oil. These molecules are formed by the condensation of 1 mol-

ecule of glycerol (a three-carbon alcohol) with 3 molecules of

fatty acids. Because of this structure, chemists currently prefer

the name triacylglycerol, although the name triglyceride is

still in wide use.

Each fatty acid molecule consists of a nonpolar hydro-

carbon chain with a carboxyl group (abbreviated COOH) on

one end. If the carbon atoms within the hydrocarbon chain

are joined by single covalent bonds so that each carbon atom

can also bond with 2 hydrogen atoms, the fatty acid is said to

be saturated. If there are a number of double covalent bonds

within the hydrocarbon chain so that each carbon atom can

bond with only 1 hydrogen atom, the fatty acid is said to be

unsaturated. Triglycerides contain combinations of differ-

ent saturated and unsaturated fatty acids. Those with mostly

saturated fatty acids are called saturated fats; those with

mostly unsaturated fatty acids are called unsaturated fats

( fig. 2.18 ).

Within the adipose cells of the body, triglycerides are

formed as the carboxyl ends of fatty acid molecules condense

with the hydroxyl groups of a glycerol molecule ( fig. 2.20 ).

Because the hydrogen atoms from the carboxyl ends of fatty

acids form water molecules during dehydration synthesis, fatty

acids that are combined with glycerol can no longer release

H^1 and function as acids. For this reason, triglycerides are

described as neutral fats.

monosaccharides is broken, a water molecule provides the
atoms needed to complete their structure. The water molecule
is split, and the resulting hydrogen atom is added to one of the
free glucose molecules as the hydroxyl group is added to the
other ( fig. 2.17 ).
When you eat a potato, the starch within it is hydrolyzed
into separate glucose molecules within the small intestine.
This glucose is absorbed into the blood and carried to the
tissues. Some tissue cells may use this glucose for energy.
Liver and muscles, however, can store excess glucose in
the form of glycogen by dehydration synthesis reactions in
these cells. During fasting or prolonged exercise, the liver
can add glucose to the blood through hydrolysis of its stored
glycogen.
Dehydration synthesis reactions not only build larger
carbohydrates from monosaccharides, they also build lip-
ids from their subunits (including fat from fatty acids and
glycerol; see fig. 2.20 ), proteins from their amino acid
subunits (see fig. 2.27 ), and polynucleotide chains from
nucleotide subunits (see fig. 2.31 ). Similarly, hydrolysis
reactions break down carbohydrates, lipids, proteins, and
polynucleotide chains into their subunits. In order to occur,
all of these reactions require the presence of the appropri-
ate enzymes.

Lipids

The category of molecules known as lipids includes several
types of molecules that differ greatly in chemical structure.
These diverse molecules are all in the lipid category by virtue
of a common physical property—they are all insoluble in polar
solvents such as water. This is because lipids consist primar-
ily of hydrocarbon chains and rings, which are nonpolar and

Figure 2.17 The
hydrolysis of starch. The
polysaccharide is first hydrolyzed
into ( a ) disaccharides (maltose)
and then into ( b ) monosaccharides
(glucose). Notice that as the
covalent bond between the
subunits breaks, a molecule of
water is split. In this way, the
hydrogen atom and hydroxyl group
from the water are added to the
ends of the released subunits.

H 2 O

H 2 O

H H

HO

O

O

H H

OH

O

H H

HO

(a)

(b)

O

O

H H

OH

O

H H

HO

O

O

H H

OH etc.

etc.

O

H H

HO

O

OOO

H H

O

H H

O

O

H H

O

H H

HO OH HO

O

H H

OH

O

Maltose

Maltose

+

+

+

+

+

Water +

Starch Water

Glucose Glucose