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C2: Lipid Distribution in Cells

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    4679
  •  

    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