Phospholipids

The group of lipids known as phospholipids includes a number of different categories of lipids, all of which contain a phosphate group. The most common type of phospholipid molecule is one in which the three-carbon alcohol molecule glycerol is attached to two fatty acid molecules; the third carbon atom of the glyc-erol molecule is attached to a phosphate group, and the phosphate group in turn is bound to other molecules. If the phosphate group is attached to a nitrogen-containing choline molecule, the phospholipid molecule thus formed is known as lecithin (or phosphatidylcholine). Figure 2.20 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.

Phosphate group

(polar)

Fatty acid chains bonded to glycerol (nonpolar)

Nitrogen-containing choline group (polar)

Polar

(hydrophilic) portion

Nonpolar (hydrophobic) portion

Figure 2.20 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 saw-toothed lines the nonpolar portion of the molecule.

Polar

(hydrophilic) portion

Lecithin Polar Unpolar

Nonpolar (hydrophobic) portion

Figure 2.20 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 saw-toothed lines the nonpolar portion of the molecule.

Chemical Composition of the Body

Since the nonpolar ends of phospholipids are hydrophobic, they tend to group together when mixed in water. This allows the hydrophilic parts (which are polar) to face the surrounding water molecules (fig. 2.21). Such aggregates of molecules are called micelles. The dual nature of phospholipid molecules (part polar, part nonpolar) allows them to alter the interaction of water molecules and thus decrease the surface tension of water. This function of phospholipids makes them surfactants (surface-active agents). The surfactant effect of phospholipids prevents the lungs from collapsing due to surface tension forces (see chapter 16). Phospholipids are also the major component of cell membranes, as will be described in chapter 3.

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  • eliano
    WHAT ARE THE The polar parts of the phospholipids face the?
    8 years ago

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