Fatty Acids Are Key Constituents of Lipids

the boundary of a cell. [(Left) Photonica.]
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.1. Many Common Features Underlie the Diversity of Biological Membranes
Membranes are as diverse in structure as they are in function. However, they do have in common a number of important
attributes:
1. Membranes are sheetlike structures, only two molecules thick, that form closed boundaries between different
compartments. The thickness of most membranes is between 60 Å (6 nm) and 100 Å (10 nm).
2. Membranes consist mainly of lipids and proteins. Their mass ratio ranges from 1:4 to 4:1. Membranes also contain
carbohydrates that are linked to lipids and proteins.
3. Membrane lipids are relatively small molecules that have both hydrophilic and hydrophobic moieties. These lipids
spontaneously form closed bimolecular sheets in aqueous media. These lipid bilayers are barriers to the flow of polar
molecules.
4. Specific proteins mediate distinctive functions of membranes. Proteins serve as pumps, channels, receptors, energy
transducers, and enzymes. Membrane proteins are embedded in lipid bilayers, which create suitable environments for
their action.
5. Membranes are noncovalent assemblies. The constituent protein and lipid molecules are held together by many
noncovalent interactions, which are cooperative.
6. Membranes are asymmetric. The two faces of biological membranes always differ from each other.
7. Membranes are fluid structures. Lipid molecules diffuse rapidly in the plane of the membrane, as do proteins, unless
they are anchored by specific interactions. In contrast, lipid molecules and proteins do not readily rotate across the
membrane. Membranes can be regarded as two-dimensional solutions of oriented proteins and lipids.
8. Most cell membranes are electrically polarized, such that the inside is negative [typically - 60 millivolts (mV)].
Membrane potential plays a key role in transport, energy conversion, and excitability (Chapter 13).
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.2. Fatty Acids Are Key Constituents of Lipids
Among the most biologically significant properties of lipids are their hydrophobic properties. These properties are
mainly due to a particular component of lipids: fatty acids, or simply fats. Fatty acids also play important roles in signaltransduction pathways (Sections 15.2 and 22.6.2).
12.2.1. The Naming of Fatty Acids
Fatty acids are hydrocarbon chains of various lengths and degrees of unsaturation that terminate with carboxylic acid
groups. The systematic name for a fatty acid is derived from the name of its parent hydrocarbon by the substitution of oic
for the final e. For example, the C18 saturated fatty acid is called octadecanoic acid because the parent hydrocarbon is
octadecane. A C18 fatty acid with one double bond is called octadecenoic acid; with two double bonds, octadecadienoic
acid; and with three double bonds, octadecatrienoic acid. The notation 18:0 denotes a C18 fatty acid with no double
bonds, whereas 18:2 signifies that there are two double bonds. The structures of the ionized forms of two common fatty
acids palmitic acid (C16, saturated) and oleic acid (C18, monounsaturated) are shown in Figure 12.2.
Fatty acid carbon atoms are numbered starting at the carboxyl terminus, as shown in the margin. Carbon atoms 2 and 3
are often referred to as α and β, respectively. The methyl carbon atom at the distal end of the chain is called the ωcarbon atom. The position of a double bond is represented by the symbol ∆ followed by a superscript number. For
example, cis-∆ 9 means that there is a cis double bond between carbon atoms 9 and 10; trans-∆ 2 means that there is a
trans double bond between carbon atoms 2 and 3. Alternatively, the position of a double bond can be denoted by
counting from the distal end, with the ω-carbon atom (the methyl carbon) as number 1. An ω-3 fatty acid, for example,
has the structure shown in the margin. Fatty acids are ionized at physiological pH, and so it is appropriate to refer to
them according to their carboxylate form: for example, palmitate or hexadecanoate.
12.2.2. Fatty Acids Vary in Chain Length and Degree of Unsaturation
Fatty acids in biological systems usually contain an even number of carbon atoms, typically between 14 and 24 (Table
12.1). The 16- and 18-carbon fatty acids are most common. The hydrocarbon chain is almost invariably unbranched in
animal fatty acids. The alkyl chain may be saturated or it may contain one or more double bonds. The configuration of
the double bonds in most unsaturated fatty acids is cis. The double bonds in polyunsaturated fatty acids are separated by
at least one methylene group.
The properties of fatty acids and of lipids derived from them are markedly dependent on chain length and degree of
saturation. Unsaturated fatty acids have lower melting points than saturated fatty acids of the same length. For example,
the melting point of stearic acid is 69.6°C, whereas that of oleic acid (which contains one cis double bond) is 13.4°C.
The melting points of polyunsaturated fatty acids of the C18 series are even lower. Chain length also affects the melting
point, as illustrated by the fact that the melting temperature of palmitic acid (C16) is 6.5 degrees lower than that of stearic
acid (C18). Thus, short chain length and unsaturation enhance the fluidity of fatty acids and of their derivatives.
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.2. Fatty Acids Are Key Constituents of Lipids
Figure 12.2. Structures of Two Fatty Acids. Palmitate is a 16-carbon, saturated fatty acid, and oleate is an 18-carbon
fatty acid with a single cis double bond.
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.2. Fatty Acids Are Key Constituents of Lipids
Table 12.1. Some naturally occurring fatty acids in animals
Number of
carbons
Number of
double bonds
Common name Systematic name
Formula
12
0
Laurate
n-Dodecanoate
CH3(CH2)10COO-
14
0
Myristate
n-Tetradecanoate
CH3(CH2)12COO-
16
0
Palmitate
n-Hexadecanoate
CH3(CH2)14COO-
18
0
Stearate
n-Octadecanoate
CH3(CH2)16COO-
20
0
Arachidate
n-Eicosanoate
CH3(CH2)18COO-
22
0
Behenate
n-Docosanoate
CH3(CH2)20COO-
24
0
Lignocerate
n-Tetracosanoate
CH3(CH2)22COO-
16
1
Palmitoleate
cis-∆ 9-Hexadecenoate
CH3(CH2)5CH=CH(CH2)
7COO
18
1
Oleate
cis-∆ 9-Octadecenoate
CH3(CH2)7CH=CH(CH2)
7COO
18
2
Linoleate
cis,cis-∆ 9,∆ 12-Octadecadienoate
-
-
CH3(CH2)4(CH=CHCH2)2
(CH2)6COO-
18
20
3
4
Linolenate
Arachidonate
all-cis-∆ 9,∆ 12,∆ 15Octadecatrienoate
CH3CH2(CH=CHCH2)3
all-cis-∆ 5,∆ 8,∆ 11,∆ 14Eicosatetraenoate
CH3(CH2)4(CH=CHCH2)4
(CH2)6COO(CH2)2COO-