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C2: Lipid Distribution in Cells

To understand movement of lipids in an actual cell, a better understanding of lipid synthesis and trafficking in cells is important. The table below shows the distribution of four classes of lipids in a macrophage, a type of immune cells (Andreyev, A.Y. et al) while the following figure shows how the lipids composition of membranes organelle membranes.

Table: Distribution of Lipids in Resting Macrophage

 

Lipid Categories

Nucleus

Mitochon-dria

Endo. Reti.

Plasma Memb

microsome

cytosol

Whole cell

Glycero-phospholipids

149

152

 

 

150

 

 

151

 

 

142

 

 

109

 

 

155

 

 

Prenol lipids

 

 

5

 

 

5

 

 

5

 

 

5

 

 

5

 

 

5

 

 

5

 

 

Sphingolipids

 

 

48

 

 

47

 

 

48

 

 

48

 

 

48

 

 

47

 

 

48

 

 

Sterol lipids

 

 

13

 

 

12

 

 

12

 

 

13

 

 

11

 

 

5

 

 

12

 

 

Total

 

 

215

 

 

216

 

 

215

 

 

217

 

 

206

 

 

166

 

 

220

 

 


Figure:  Approximate Distribution Phospholipids in Resting Mammalian Cells

 

Lipids in membranes are often distributed asymmetrically.  The inner and outer leaflet of a biological membrane usually have different PL compositions.  For example, in red blood cell membranes, the outer leaflet is composed mostly of sphingomylein (SM) and PC, while the inner leaflet is composed mostly of PE and phosphatidyl serine (PS).  This phospholipid contains the amino acid serine linked through its side chain (-CH2OH) to phosphate in position 3 of diacylglyerol.   With a negative charge on the phosphate and carboxylate and a positive charge on the amine of PS, this phospholipid is acidic with a net negative charge.  All the PS is located in the inner leaftet!  This observation will become important latter on, when we discuss programmed cell death.  A dying cell will expose PS in the outer leaflet.  This is in fact one of the markers of a dying cell.

The membrane lipid composition in an average mammalian cell

Lipid %
PC 45-55
PE 15-25
PI 10-15
PS 5-10
PA 1-2
SM 5-10
cardiolipin (bis-PG) 2-5
cholesterol 10-20