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15.1 - Absorption and transport

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  1. Front
    Fed state (postprandial)
    Back
    State after eating
  2. Front
    What happens to dietary lipids in the fed state
    Back
    Lipids enter the intestine, are packaged into chylomicrons, and are transported to tissues for use or storage
  3. Front
    Major destinations of chylomicron lipids in the fed state
    Back
    Muscle for energy use and adipose tissue for storage
  4. Front
    Role of the liver in the fed state
    Back
    Can synthesize new fat de novo and can oxidize fatty acids
  5. Front
    TAG
    Back
    Triacylglycerol, the main fat form
  6. Front
    FA
    Back
    Fatty acid, a building block of fat
  7. Front
    Acyl-CoA
    Back
    Activated fatty acid
  8. Front
    Big idea of lipid handling after a meal
    Back
    Lipids are transported from intestine to tissues, where they are stored or used
  9. Front
    Major dietary lipid
    Back
    Triacylglycerols, about 90 percent of dietary lipids
  10. Front
    Other dietary lipids
    Back
    Cholesterol and cholesterol esters, phospholipids, and free fatty acids
  11. Front
    Central problem of lipid digestion and transport
    Back
    Lipids are hydrophobic and the body environment is aqueous
  12. Front
    Why lipid digestion, absorption, and transport are challenging
    Back
    Lipids do not dissolve in water
  13. Front
    Organs involved in lipid digestion
    Back
    Gallbladder and pancreas
  14. Front
    What the gallbladder releases
    Back
    Bile salts
  15. Front
    What the pancreas releases for lipid digestion
    Back
    Lipase, colipase, and bicarbonate
  16. Front
    Function of bicarbonate in lipid digestion
    Back
    Neutralizes stomach acid and creates optimal pH for enzymes
  17. Front
    Amphipathic
    Back
    Having both hydrophobic and hydrophilic parts
  18. Front
    Function of bile salts
    Back
    Emulsify dietary fat
  19. Front
    Emulsify
    Back
    Break large fat droplets into small droplets
  20. Front
    Why emulsification is important
    Back
    It increases surface area for enzyme action
  21. Front
    Function of pancreatic lipase
    Back
    Breaks triacylglycerol into smaller molecules
  22. Front
    Function of colipase
    Back
    Helps lipase bind to fat
  23. Front
    Main products of TAG digestion
    Back
    Free fatty acids and 2-monoacylglycerol
  24. Front
    2-MG
    Back
    2-monoacylglycerol
  25. Front
    Barriers lipids must cross during absorption
    Back
    Unstirred water layer and enterocyte membrane
  26. Front
    Micelles
    Back
    Structures formed when bile salts surround lipids to carry them through water to the enterocyte surface
  27. Front
    Function of micelles
    Back
    Transport lipids through the aqueous environment to intestinal cells
  28. Front
    What enters enterocytes during lipid absorption
    Back
    Fatty acids and 2-monoacylglycerol
  29. Front
    Fate of bile salts after absorption
    Back
    About 95 percent are reabsorbed and returned to the liver
  30. Front
    Enterohepatic circulation
    Back
    Reabsorption of bile salts and their return to the liver
  31. Front
    SCFA and MCFA absorption route
    Back
    They go directly into portal blood and then to the liver
  32. Front
    Do short- and medium-chain fatty acids require bile salts for absorption
    Back
    No
  33. Front
    LCFA absorption
    Back
    Long-chain fatty acids enter enterocytes by a more complex process
  34. Front
    Mechanisms for LCFA entry into enterocytes
    Back
    Passive diffusion and transporters such as FATP
  35. Front
    FATP
    Back
    Fatty acid transport protein
  36. Front
    FABP
    Back
    Fatty acid binding protein
  37. Front
    Function of FABP
    Back
    Binds fatty acids inside the enterocyte and carries them to the ER
  38. Front
    Where digested lipids are rebuilt
    Back
    Inside the smooth ER of enterocytes
  39. Front
    What happens in lipid re-synthesis
    Back
    FA and MAG are reassembled into TAG
  40. Front
    Why lipids are rebuilt after digestion
    Back
    They are re-synthesized into triacylglycerols for transport
  41. Front
    What is packaged with TAG to form chylomicrons
    Back
    Other lipids and proteins
  42. Front
    Chylomicron
    Back
    A lipoprotein that transports dietary fat from intestine to tissues
  43. Front
    Transport route of chylomicrons
    Back
    They enter lymph, then blood, then reach tissues
  44. Front
    Why lipids are transported as chylomicrons
    Back
    They cannot travel freely in blood because they are hydrophobic
  45. Front
    Structure of a chylomicron
    Back
    A hydrophobic core surrounded by an amphipathic surface
  46. Front
    Chylomicron core contents
    Back
    Triacylglycerol and cholesteryl esters
  47. Front
    Hydrophobic components of chylomicrons
    Back
    TAG and cholesteryl esters in the core
  48. Front
    Chylomicron surface contents
    Back
    Phospholipids, cholesterol, and apoproteins
  49. Front
    Amphipathic components of chylomicrons
    Back
    Phospholipids, cholesterol, and apoproteins on the surface
  50. Front
    Apo B-48
    Back
    Structural apoprotein of chylomicrons made in the intestine
  51. Front
    Apo C-II
    Back
    Apoprotein that activates lipoprotein lipase
  52. Front
    Apo E
    Back
    Apoprotein that helps liver uptake of remnants
  53. Front
    Source of Apo C-II and Apo E for chylomicrons
    Back
    They are obtained from HDL
  54. Front
    MTP
    Back
    Microsomal triglyceride transfer protein
  55. Front
    Function of MTP
    Back
    Required for chylomicron assembly
  56. Front
    Detailed chylomicron transport pathway
    Back
    Lymph to thoracic duct to blood to peripheral tissues
  57. Front
    Why chylomicrons go through lymph first
    Back
    They initially avoid portal blood
  58. Front
    How chylomicron entry differs from glucose absorption
    Back
    Chylomicrons avoid portal blood initially, unlike glucose
  59. Front
    Main problem in tissue delivery of chylomicron TAG
    Back
    Cells need fatty acids, but chylomicrons carry triacylglycerol
  60. Front
    LPL
    Back
    Lipoprotein lipase, the enzyme that hydrolyzes TAG to fatty acids and glycerol
  61. Front
    Location of LPL
    Back
    Attached to capillary endothelium
  62. Front
    Where LPL is made
    Back
    Adipose tissue, muscle, heart, and mammary gland
  63. Front
    What LPL does to chylomicrons
    Back
    Hydrolyzes about 90 percent of their TAG to fatty acids and glycerol
  64. Front
    Main activator of LPL
    Back
    Apo C-II
  65. Front
    Effect of insulin on LPL
    Back
    Insulin increases LPL activity
  66. Front
    Physiologic meaning of insulin increasing LPL
    Back
    After eating, more fatty acids are delivered for storage
  67. Front
    What determines LPL activity
    Back
    Tissue type and insulin
  68. Front
    Core takeaway from LPL kinetics
    Back
    LPL activity depends on tissue type and insulin
  69. Front
    What happens after LPL releases fatty acids
    Back
    Fatty acids enter tissues
  70. Front
    Adipose tissue role in the fed state
    Back
    Stores fatty acids as triacylglycerol
  71. Front
    First step for fatty acids in adipose tissue
    Back
    They are activated to acyl-CoA
  72. Front
    Activated fatty acid in adipose tissue
    Back
    Acyl-CoA
  73. Front
    Source of glycerol-3-phosphate in adipose tissue
    Back
    From glucose via DHAP
  74. Front
    DHAP
    Back
    Dihydroxyacetone phosphate
  75. Front
    What adipose tissue synthesizes in the fed state
    Back
    Triacylglycerol
  76. Front
    Key function of adipose tissue in lipid metabolism
    Back
    Storage organ for TAG
  77. Front
    Role of muscle and heart in lipid metabolism
    Back
    Use fatty acids for energy
  78. Front
    Do heart and muscle use fatty acids when glucose is available
    Back
    Yes, they still use fatty acids
  79. Front
    Sources of fatty acids to the liver
    Back
    De novo synthesis, chylomicron remnants, and portal vein
  80. Front
    Approximate contribution of de novo synthesis to liver fatty acids
    Back
    About 60 percent
  81. Front
    What the liver does with chylomicron remnants
    Back
    Processes them after LPL action
  82. Front
    First step of dietary lipid handling
    Back
    Emulsification by bile salts
  83. Front
    Second step of dietary lipid handling
    Back
    Digestion of TAG by lipase to FA and MAG
  84. Front
    Third step of dietary lipid handling
    Back
    Micelle formation to deliver lipids to enterocytes
  85. Front
    Fourth step of dietary lipid handling
    Back
    Absorption of FA and MAG into enterocytes
  86. Front
    Fifth step of dietary lipid handling
    Back
    Reassembly of FA and MAG into TAG
  87. Front
    Sixth step of dietary lipid handling
    Back
    Packaging into chylomicrons
  88. Front
    Seventh step of dietary lipid handling
    Back
    Transport through lymph and then blood
  89. Front
    Eighth step of dietary lipid handling
    Back
    LPL releases fatty acids to tissues
  90. Front
    Ninth step of dietary lipid handling
    Back
    Fatty acids are stored in adipose tissue or used by muscle for energy
  91. Front
    Function of Apo C-II
    Back
    Activates lipoprotein lipase
  92. Front
    Why do chylomicrons go through lymph first
    Back
    To avoid portal blood initially
  93. Front
    What happens to glycerol after LPL action
    Back
    It is released when TAG is hydrolyzed by LPL
  94. Front
    Digestion overview
    Back
    Bile salts emulsify fats and lipase converts TAG to FA and 2-MG
  95. Front
    Absorption overview
    Back
    Micelles deliver lipids to enterocytes, SCFA enter portal blood directly, and LCFA are reassembled
  96. Front
    Transport overview
    Back
    TAG are packaged into chylomicrons and travel through lymph then blood
  97. Front
    What happens to TAG in blood
    Back
    LPL on capillaries hydrolyzes TAG to fatty acids
  98. Front
    Regulation of LPL
    Back
    Apo C-II activates it and insulin increases its activity
  99. Front
    Fate of fatty acids in adipose tissue
    Back
    Stored as TAG
  100. Front
    Fate of fatty acids in muscle
    Back
    Oxidized for energy

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