Human Physiology, 14th edition (2016)

Chemical Composition of the Body 39

Figure 2.22 The structure

of lecithin. Lecithin is also called

phosphatidylcholine, where choline is

the nitrogen-containing portion of the

molecule. (Interestingly, choline is also

part of an important neurotransmitter

known as acetylcholine, discussed in

chapter 7.) The detailed structure of the

phospholipid ( top ) is usually shown in

simplified form ( bottom ), where the circle

represents the polar portion and the

sawtoothed lines the nonpolar portion of

the molecule.

2 CH O

OC

O

• O P O C H 2

O

CH 2

+N CH

H 3 C 3

CH 2

CH 3

H C

R

R

C

O

O

Fatty acid

chains bonded

to glycerol

(nonpolar)

Nitrogen-containing

choline group (polar)

Nonpolar (hydrophobic) portion

Polar

(hydrophilic)

portion

Phosphate
group
(polar)

molecule, the phospholipid thus formed is known as lecithin
(or phosphatidylcholine ). Figure 2.22 shows a simple way of
illustrating the structure of a phospholipid—the parts of the
molecule capable of ionizing (and thus becoming charged) are
shown as a circle, whereas the nonpolar parts of the molecule
are represented by sawtooth lines. Molecules that are part polar
and part nonpolar, such as phospholipids and bile acids (which
are derived from cholesterol), are described as amphipathic
molecules.
Phospholipids are the major component of cell mem-
branes; their amphipathic nature allows them to form a double
layer with their polar portions facing water on each side of
the membrane (chapter 3). When phospholipids are mixed in
water, they tend to group together so that their polar parts face
the surrounding water molecules ( fig. 2.23 ). Such aggregates
of molecules are called micelles. Bile acids (which are not
phospholipids, but are amphipathic molecules derived from
cholesterol) form similar micelles in the small intestine (chap-
ter 18, section 18.5). The amphipathic nature of phospholipids
(part polar, part nonpolar) allows them to alter the interaction
of water molecules and thus to decrease the surface tension of
water. This function of phospholipids makes them surfactants
(surface-active agents). The surfactant effect of phospholip-
ids prevents the lungs from collapsing due to surface tension
forces (chapter 16, section 16.2).

Figure 2.23 The formation of a micelle structure by

phospholipids such as lecithin. The hydrophilic outer layer of

the micelle faces the aqueous environment.