Organisation of Cellular Lipids and Membranes

Organisation of Cellular Lipids and Membranes

Organisation of Cellular Lipids and Membranes

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Lipids and Biological Function

  • Structural: Plasma membrane (phospholipids), intracellular organelles
  • Energy: Energy stores and insultation (white adipose tissue) (brown adipose tissue - thermogenesis)
  • Carriers: Absorption and transport of fat soluble vitamins (A, D, E and K)
  • Messengers: Cell signalling
  • Prescursors: Chloesterol for many hormones
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Lipids - Simple and Complex

  • Simple
    • Fats - esters of fatty acids with glycerol - oils are fats in the liquid state
    • Waxes - esters of fatty acids with higher molecular weight monohydric alcohols
  • Complex 
    • Phospholipids, glycolipids, sulpholipids, aminolipids
  • Precursor and derived lipids
    • Fatty acids
    • Glycerol
    • Steroids
    • Hormones
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Lipid Function

  • Triglycerides
    • Main storage forms of fatty acids
  • Phospholipids, cholesterol, glycolipids
    • Constituents of membranes
  • Sterols - cholesterol
    • Major constituent of plasma membrane, plasma lipoproteins
    • Precursor to vitamin D and many steroid hormones
  • Eicosanoids, e.g. Prostaglandins
    • Immune system
    • Inflammation
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Fatty Acids

  • Occur mainly as esters in natural fats and oils
  • Do occur as free fatty acids
    • Blood
  • Are either:
    • Saturated
      • No double bonds in the acyl chain
    • Unsaturated
      • 1 or more double bonds in the acyl chain
  • Saturated fatty acids end in -anoic
  • Unsaturated fatty acids end in -enoic
  • Number of carbons, number of double bonds, lowest carbon number that shows position of the double bond, e.g. Octadecenoic acid 18:1;9
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Saturated Fatty Acids in the Diet

  • Butyric acid
    • 4 carbon atoms
    • Present in certain fats such as butter
  • Palmitic acid
    • 16 carbon atoms
    • Common in all animal and plant fats
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Unsaturated Fatty Acids in the Diet

  • Oleic acid
    • One double bond
    • Most common fatty acids in natural fats
  • Linoleic acid
    • Two/three double bonds
    • Corn, peanut, soybean
  • Timnodonic acid - Eicosapentenoic acid
    • 5 double bonds
    • Fish oils
    • Omega 3
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Chain Length and Saturation

  • Melting points of even numbered carbon fatty acids
    • Increase with chain length
    • Decrease with increasing unsaturation
  • Example
    • A triglyceride with 3 chains, each with 12 carbons and no double bonds is solid at 37 degrees
    • If the fatty acids are 18:2 the lipid is liquid at <0 degrees
  • Membrane lipids
    • More unsaturated than storage lipids
      • More fluidic
    • Cholesterol
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Triglycerides (Triacylglycerols)

  • Main storage forms of fatty acids
  • Esters of glycerol and fatty acids
  • Mono- and di- acylglycerides also exist

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Fats and Oils

  • Fats have all the available bonding sites taken up by hydrogen
    • Saturated
  • Oils have fewer hydrogens on account of the double bonds
    • Unsaturated
    • CH=CH-
    • These are liquid at room temperature
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Plasma Membrane

  • Plasma membrane is essential for cell integrity
    • Target for drugs
    • Barrier for drugs

Phospholipids

  • Backbone on which phospholipids are built:
    • Glycerol
    • Called phosphoglycerides - glycerol backbone, 2 fatty acids and a phosphorylated alcohol
  • Sphingosine
    • Sphingomyelin
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Phospholipids

  • Substituted lipids
    • One unit glycerol
    • Two units of fatty acids
  • Derivatives of diacylglycerol-3-phosphate
  • Phosphate group often linked with N2 containing group
  • Phosphate group has tendency to loose hydrogen ion so that one oxygen becomes negative
  • Nitrogen tends to attract a hydrogen ion - one nitrogen becomes positively charged
    • End becomes polar and water soluble
    • Tail is non-polar and insoluble
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Sphingolipids

  • Prominent on plasma membrane of cells
  • Backbone is sphingosine not glycerol

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Phosphatidylinositols

  • Precursor for phosphatidylinositol 4,5-bisphosphate (PIP2)
    • Cleaved to form 
      • Diacylglycerol (DAG)
      • Inositol triphosphate (IP3)
    • Both important secondary messengers
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Steroids

  • Classified as lipids because of solubility
  • Markedly different in structure
  • Four interlocking rings of carbon atoms
    • Side groups attached to rings
  • Cholesterol
    • Constituent of biological membranes
      • Plasma membrane
    • Prescursor of large number of steroids
      • Bile acids
      • Sex hormones
      • Adrenocortical hormones
      • Vitamin D
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Glycolipids

  • Found everywhere in and on the cell surface
    • Especially prominent in outer leaflet of the plasma membrane
  • Major glycolipids - Glycosphingolipids
    • Ceramide and 1-n sugars
      • Galactosylceramide
        • Converted to sulphogalactosylceramide - present in high amounts in myelin
      • Gangliosides
        • GM1 - receptor for cholera toxin in intestine
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Eicosanoids

  • All derived from eicosa (-C20) polyenoic fatty acids
  • Present in all animal tissue
  • Important physiological and pharmacological actions
    • Prostaglandins - act as local hormones
      • Muscle contraction, inflammation
      • Pharmacy
        • NSAIDS - COX-2 inhibitors
    • Leukotrienes
      • Asthma
      • Leukotriene receptor antagonist
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Properties of Lipids

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