Essential Cell Biology 5th edition

Small Molecules in Cells
55
can be released from triacylglycerols and broken down into two-carbon
units. These two-carbon units are identical to those derived from the
breakdown of glucose, and they enter the same energy-yielding reaction
pathways, as described in Chapter 13.
glycerol
glycerol
Fatty acids and their derivatives, including triacylglycerols, are examples
of lipids. Lipids are loosely defined as molecules that are insoluble in
water but soluble in fat and organic solvents such as benzene. They typically
contain long hydrocarbon chains, as in the fatty acids, or multiple
linked aromatic rings, as in the steroids (see Panel 2–5).
The most unique function of fatty acids is in the establishment of the lipid
bilayer, the structure that forms the basis for all cell membranes. These
thin sheets, which enclose all cells and surround their internal organelles,
are composed largely of phospholipids (Figure 2–23).
Like triacylglycerols, most phospholipids are constructed mainly from fatty
acids and glycerol. In these phospholipids, however, the glycerol is joined
to two fatty acid chains, rather than to three as in triacylglycerols. The
remaining –OH group on the glycerol is linked to a hydrophilic phosphate
group, which in turn is attached to a small hydrophilic compound such as
choline (see Panel 2–5, pp. 74–75). With their two hydrophobic fatty acid
tails and a hydrophilic, phosphate-containing head, phospholipids are
strongly amphipathic. This characteristic amphipathic composition and
shape gives them very different physical and chemical properties from
triacylglycerols, which are predominantly hydrophobic. In addition to
phospholipids, cell membranes contain differing amounts of other lipids,
including glycolipids, which are structurally similar to phospholipids but
contain one or more sugars instead of a phosphate group.
Thanks to their amphipathic nature, pure phospholipids readily form
membranes in water. These lipids can spread over the surface of water
to form a monolayer, with their hydrophobic tails facing the air and their
hydrophilic heads in contact with the water. Alternatively, two of these
phospholipid layers can readily combine tail-to-tail in water to form the
phospholipid sandwich that is the lipid bilayer (see Chapter 11).
saturated
fatty acid tails
(A)
unsaturated
fatty acid tails
(B)
Figure 2–22 The properties of fats
depend on the length and saturation
of the fatty acid chains they carry. Fatty
acids are stored in the cytosol of many cells
in the form of droplets of triacylglycerol
molecules ECB5 made E2.20/2.22 of three fatty acid chains
joined to a glycerol molecule. (A) Saturated
fats are found in meat and dairy products.
(B) Plant oils, such as corn oil, contain
unsaturated fatty acids, which may be
monounsaturated (containing one double
bond) or polyunsaturated (containing
multiple double bonds). The presence of
these double bonds causes plant oils to
be liquid at room temperature. Although
fats are essential in the diet, saturated fats
raise the concentration of cholesterol in
the blood, which tends to clog the arteries,
increasing the risk of heart attacks and
strokes.
hydrophilic
head
polar
group
phosphate
glycerol
water
two
hydrophobic
fatty acid
tails
fatty acid
fatty acid
phospholipid
bilayer,
or membrane
(A)
phospholipid molecule
(B)
Figure 2–23 Phospholipids can aggregate to form cell membranes. Phospholipids contain two hydrophobic fatty acid tails and
a hydrophilic head. (A) Phosphatidylcholine is the most common phospholipid in cell membranes. (B) Diagram showing how, in an
aqueous environment, the hydrophobic tails of phospholipids pack together to form a lipid bilayer. In the lipid bilayer, the hydrophilic
heads of the phospholipid molecules are on the outside, facing the aqueous environment, and the hydrophobic tails are on the inside,
where water is excluded.
ECB5 e2.21/2.23