Essential Cell Biology 5th edition

54 CHAPTER 2 Chemical Components of Cells
Smaller oligosaccharides can be covalently linked to proteins to form glycoproteins,
or to lipids to form glycolipids (Panel 2–5, pp. 74–75), which
are both found in cell membranes. The sugar side chains attached to
glycoproteins and glycolipids in the plasma membrane are thought to
help protect the cell surface and often help cells adhere to one another.
Differences in the types of cell-surface sugars form the molecular basis
for the human blood groups, information that dictates which blood types
can be used during transfusions.
Fatty Acid Chains Are Components of Cell Membranes
A fatty acid molecule, such as palmitic acid, has two chemically distinct
regions. One is a long hydrocarbon chain, which is hydrophobic and not
very reactive chemically. The other is a carboxyl (–COOH) group, which
behaves as an acid (carboxylic acid): in an aqueous solution, it is ionized
(–COO – ), extremely hydrophilic, and chemically reactive (Figure
2–21). Molecules—such as fatty acids—that possess both hydrophobic
and hydrophilic regions are termed amphipathic. Almost all the fatty acid
molecules in a cell are covalently linked to other molecules by their carboxylic
acid group (see Panel 2–5, pp. 74–75).
The hydrocarbon tail of palmitic acid is saturated: it has no double bonds
between its carbon atoms and contains the maximum possible number
of hydrogens. Some other fatty acids, such as oleic acid, have unsaturated
tails, with one or more double bonds along their length. The double
bonds create kinks in the hydrocarbon tails, interfering with their ability
to pack together. Fatty acid tails are found in cell membranes, where
the tightness of their packing affects the fluidity of the membrane. The
many different fatty acids found in cells differ only in the length of their
hydrocarbon chains and in the number and position of the carbon–
carbon double bonds (see Panel 2–5).
Fatty acids serve as a concentrated food reserve in cells: they can be broken
down to produce about six times as much usable energy, gram for
gram, as glucose. Fatty acids are stored in the cytoplasm of many cells
in the form of fat droplets composed of triacylglycerol molecules—compounds
made of three fatty acid chains covalently joined to a glycerol
molecule (Figure 2–22 and see Panel 2–5). Triacylglycerols are the animal
fats found in meat, butter, and cream, and the plant oils such as
corn oil and olive oil. When a cell needs energy, the fatty acid chains
hydrophilic
carboxylic
acid head
O
C
O _
Figure 2–21 Fatty acids have both
hydrophobic and hydrophilic components.
The hydrophobic hydrocarbon chain is
attached to a hydrophilic carboxylic acid
group. Different fatty acids have different
hydrocarbon tails. Palmitic acid is shown
here. (A) Structural formula, showing the
carboxylic acid head group in its ionized
form, as it exists in water at pH 7. (B) Balland-stick
model. (C) Space-filling model
(Movie 2.2).
hydrophobic
hydrocarbon tail
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 3
(A) (B) (C)