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C2: Lipid Distribution in Cells

  • Page ID
    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