Organic Chemistry

phosphatidylserine

008 CHAPTER 26 Lipids

26.4 Membranes

For biological systems to operate, some parts of organisms must be separated from

other parts. On a cellular level, the outside of the cell must be separated from the in-

side. “Greasy”lipid membranesserve as the barrier. In addition to isolating the cell’s

contents, these membranes allow the selective transport of ions and organic molecules

into and out of the cell.

Phospholipids

Phosphoacylglycerols(also called phosphoglycerides) are the major components

of cell membranes. They are similar to triacylglycerols except that a terminal OH

group of glycerol is esterified with phosphoric acid rather than with a fatty acid,

forming a phosphatidic acid. Because phosphoacylglycerols are lipids that contain

a phosphate group, they are classified as phospholipids. The C-2 carbon of glycerol

in phosphoacylglycerols has the Rconfiguration.

Phosphatidic acids are the simplest phosphoacylglycerols and are present only in

small amounts in membranes. The most common phosphoacylglycerols in membranes

have a second phosphate ester linkage. The alcohols most commonly used to form this

second ester group are ethanolamine, choline, and serine. Phosphatidylethanolamines

are also called cephalins, and phosphatidylcholines are called lecithins. Used as

emulsifying agents, lecithins are added to foods such as mayonnaise to prevent the

aqueous and fat components from separating.

Phosphoacylglycerols form membranes by arranging themselves in a lipid bilayer.

The polar heads of the phosphoacylglycerols are on the outside of the bilayer, and the

fatty acid chains form the interior of the bilayer. Cholesterol—a membrane lipid dis-

cussed in Section 26.9—is also found in the interior of the bilayer (Figure 26.1). A

typical bilayer is about 50Å thick. [Compare the bilayer with the micelles formed by

soap in aqueous solution (Section 17.13).]

CH 2 O

CH 2 O

O−

CH O

O

O

O

R configuration

CR^1

POH

CR^2

a phosphatidic acid

3-D Molecule:

Phosphatidic acid

CH 2 O

CH 2 O

O−

CH O

O

O

O

a phosphatidylethanolamine

a cephalin

C R^1

P OCH 2 CH 2 NH 3

C R^2

CH 2 O

CH 2

CH 3

CH 3

O

O−

CH O

O

O

O

a phosphatidylcholine

a lecithin

C R^1

P OCH 2 CH 2 NCH 3

C R^2

+ +

CH 2 O

CH 2

NH 3

O

O−

CH O

O

O

O

a phosphatidylserine

C R^1

P OCH 2 CHCOO−

C R^2

+