section 10.2

Cell Membrane Constituents

159

(b)

Phosphosphingolipids

T h e se lipids contain a long unsatu rated hydrocarbon chain am ino alchohol know n a s s p h in g o s in e :

HOH,C'

A derivative ot sphingosine, in which a fatty a d d is linked by an am ide linkage, is c e r a m ld e :

Fatty acid

residue

T he product obtained w hen th e alchohol hydroxyl group of sphingosine is esterified with phosphorylcholine is

s p h in g o m y e lin .The conform ations of phosphatidylcholine a n d sphingom yelin a re similiar.

(c)

G ly c o s p h in g o lip ld s (c e re b ro s id e s )

A derivative of ceram ide th at contains a m onosaccharide unit (either glucose o r g alacto se) linked in a B-glycosidic linkage is a cerebroside.

T h ese neutral hprds occu r m ost abundantly in th e brain an d myelin sh eath of nerves. T h e sp ed fic galactocerebroside, ph ren o sin e contains

a z-hydroxy-24-carbon fatty acid residue. Following is a structure of a glucocerebroside.

FIGURE 10-5

(

Continued

)

membrane viscosity. Thus, the lipid composition of the

membrane at physiological temperatures can have signif-

icant effects on fluidity and permeability. Some correla-

tion appears to exist between the concentrations of sphin-

gomyelin and cholesterol in different membranes. Plasma

membranes (e.g., red blood cells and myelin sheaths) are

rich in both; the inner membrane of mitochondria contains

neither.

Membrane proteins show considerable mobility in the

plane of the bilayer (lateral motion). There is no evidence

that proteins migrate from one side of the bilayer to the

other. The frequency of reorientation of lipid components

(flip-flop migration) is extremely slow or nonexistent, for

thermodynamic reasons.

Artificial membrane systems (

liposomes

) have in-

creased our understanding of the dynamic nature of lipid