Steatorrhea and Lipid Absorption
August 29th 2008 05:04
LIPID ABSORPTION AND TRANSPORT INTO THE BLOOD
Small to medium fatty acid chains, together with glycerol goes directly into the intestinal walls and into the bloodstream. This is possible since the cell membranes are lipid-soluble so lipids can pass through them.
However for monoglycerides, free cholesterol, phospholipids, and long fatty acid chains, different event happens. These substances retain their association with bile acids, forming structures called micelles (Figure 7). Micelles are essentially small aggregates (4-8 nm in diameter) of mixed lipids and bile acids suspended within the ingesta. As the ingesta is mixed, micelles bump into the brush border of small intestinal enterocytes, and the lipids, including monoglyceride and fatty acids, are taken up into the epithelial cells.
Figure 7. Micelles
The major products of lipid digestion - fatty acids and 2-monoglycerides - enter the enterocyte by simple diffusion across the plasma membrane. A considerable fraction of the fatty acids and other lipids also enter the enterocyte via a specific transporter protein in the membrane.
Once inside the enterocyte, fatty acids and monoglyceride are transported into the endoplasmic reticulum, where they are used to resynthesize triglyceride. Beginning in the endoplasmic reticulum and continuing in the Golgi, triglyceride is packaged with cholesterol, lipoproteins and other lipids into particles called chylomicrons. This is occurring in the absorptive enterocytes of the small intestine.
Chylomicrons are extruded from the Golgi into exocytotic vesicles, which are transported to the basolateral aspect of the enterocytes. The vesicles fuse with the plasma membrane and undergo exocytosis, dumping the chylomicrons into the space outside the cells.
Instead of being absorbed directly into capillary blood, chylomicrons are transported first into the lymphatic vessel that penetrates into each villus. Chylomicron-rich lymph then drains into the system lymphatic system, which rapidly flows into blood. Blood-borne chylomicrons are rapidly disassembled and their constitutent lipids utilized throughout the body (Figure 8).
Small to medium fatty acid chains, together with glycerol goes directly into the intestinal walls and into the bloodstream. This is possible since the cell membranes are lipid-soluble so lipids can pass through them.
However for monoglycerides, free cholesterol, phospholipids, and long fatty acid chains, different event happens. These substances retain their association with bile acids, forming structures called micelles (Figure 7). Micelles are essentially small aggregates (4-8 nm in diameter) of mixed lipids and bile acids suspended within the ingesta. As the ingesta is mixed, micelles bump into the brush border of small intestinal enterocytes, and the lipids, including monoglyceride and fatty acids, are taken up into the epithelial cells.
Figure 7. Micelles
The major products of lipid digestion - fatty acids and 2-monoglycerides - enter the enterocyte by simple diffusion across the plasma membrane. A considerable fraction of the fatty acids and other lipids also enter the enterocyte via a specific transporter protein in the membrane.
Once inside the enterocyte, fatty acids and monoglyceride are transported into the endoplasmic reticulum, where they are used to resynthesize triglyceride. Beginning in the endoplasmic reticulum and continuing in the Golgi, triglyceride is packaged with cholesterol, lipoproteins and other lipids into particles called chylomicrons. This is occurring in the absorptive enterocytes of the small intestine.
Chylomicrons are extruded from the Golgi into exocytotic vesicles, which are transported to the basolateral aspect of the enterocytes. The vesicles fuse with the plasma membrane and undergo exocytosis, dumping the chylomicrons into the space outside the cells.
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