| COMPOSITION OF BIOMEMBRANES
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| Eukaryotic cells have a plasma membrane, as well as a number of intracellular membranes that define compartments with specialized functions; differences in both membrane protein and lipid composition distinguish these organelles (Table 7.2). In addition to the major phospholipids described in Figure 7.1, other important membrane lipids include phosphatidylinositol, cardiolipin, sphingolipids (sphingomyelin and glycolipids), and cholesterol, which are described in detail in later chapters.
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| page 78 |  | | page 79 |
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Table 7-2.
Phospholipid composition of organelle membranes from rat liver. |
| Body_ID: None |
| Phospholipid composition of organelle membranes from rat liver |
| Body_ID: T007002.50 |
| | Percentage of total phospholipids in membranes from different organelles |
| Body_ID: T007002.100 |
| | Mitochondria | Microsomes | Lysosomes | Plasma membrane | Nuclear membrane | Golgi membrane |
| Body_ID: T007002.150 |
| Cardiolipin | 18 | 1 | 1 | 1 | 4 | 1 |
| Body_ID: T007002.200 |
| Phosphatidylethanolamine | 35 | 22 | 14 | 23 | 13 | 20 |
| Body_ID: T007002.250 |
| Phosphatidylcholine | 40 | 58 | 40 | 39 | 55 | 50 |
| Body_ID: T007002.300 |
| Phosphatidylinositol | 5 | 10 | 5 | 8 | 10 | 12 |
| Body_ID: T007002.350 |
| Phosphatidylserine | 1 | 2 | 2 | 9 | 3 | 6 |
| Body_ID: T007002.400 |
| Phosphatidic acid | - | 1 | 1 | 1 | 2 | <1 |
| Body_ID: T007002.450 |
| Sphingomyelin | 1 | 1 | 20 | 16 | 3 | 8 |
| Body_ID: T007002.500 |
| Phospholipids (mg/mg protein) | 0.175 | 0.374 | 0.156 | 0.672 | 0.500 | 0.825 |
| Body_ID: T007002.550 |
| Cholesterol (mg/mg protein) | 0.003 | 0.014 | 0.038 | 0.128 | 0.038 | 0.078 |
| Body_ID: T007002.600 |
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| Body_ID: T007002.650 |
This table shows the phospholipid composition (%) of various organelle membranes together with weight ratios of phospholipids and cholesterol to protein.
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| Cardiolipin (diphosphatidyl glycerol) is a significant component of the mitochondrial inner membrane, while sphingomyelin, phosphatidylserine and cholesterol are enriched in the plasma membrane (Table 7.2). The protein to lipid ratio also differs among various biomembranes, ranging from about 80% (dry weight) lipid in the myelin sheath that insulates nerve cells, to about 20% lipid in the inner mitochon-drial membrane. Lipids affect the structure of the membrane, the activity of membrane enzymes and transport systems, and membrane function in processes such as cellular recognition and signal transduction. Each organelle membrane also has unique proteins and enzymes that may be used as markers for the purity of isolated sub-cellular fractions.
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