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Updated: 03/11/08 02:32 PM |
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| HOME | HEAL | EDUCATE | RESEARCH | DIRECTORY | OUTREACH |
Authors: D.R. Saunders, C.E. Rubin, and J.D. Ostrow
Download Chapter ♦ Download Lecture Slides & Notes
Download Chapter ♦ Download Lecture Slides & Notes
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F. Properties of the Membrane |
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| Paracellular junctions: like porous plastic around a six-pack |
Intestinal cell membranes have lipoidal properties. Non-lipid molecules such as monosaccharides, amino acids, di- and tripeptides and electrolytes are polar and water soluble; they can be absorbed in only two ways: either through the lipid membrane assisted by carrier proteins (transcellular transport), or through gaps in the intercellular ("tight") junctions (paracellular junction). The latter behave like watery pores which are more permeable to Na+, and K+ than they are to Cl- and HCO3-; movement of ions larger than K+ , such as Ca++, is restricted. These tight junctions are impermeable to molecules as large as glucose.
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| Water transport is via aquaporins |
Water transport is mainly transcellular via aquaporins in the epithelial cell membranes, and it is driven by concentration gradients of the solutes transported across the membrane. When water permeability is high as in epithelia such as the intestine and the gall bladder, the transported fluids are iso osmotic and they equilibrate rapidly with plasma. Dissolved gases such as CO2 diffuse rapidly across intestinal epithelia.
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Simple diffusion
Facilitated diffusion Carriers are like enzymes Channels are ion-specific ABC transporters use energy from ATP |
Transport of electrically neutral solutes may be transported passively (without requiring metabolic energy); only the concentration gradient is relevant. The transport of ions, on the other hand, may depend on chemical and electrical gradients. Other solutes need to interact with membrane transport proteins in order to be transported through channels or carriers. Carrier proteins act like enzymes as they bind and transfer specific solutes across membranes. Carriers may transport a single solute, or two - three solutes in the same direction (co-transporter), or in opposite directions (exchangers). A channel is a transmembrane protein pore that provides rapid transport for a specific ion. The activity of a channel can be modulated by Ca++, ATP, etc.
Active (uphill) transport requires energy such as the phosphorylation by ATP of Na+/K+ pumps (primary active transport). Energy may be derived indirectly by the active transport of an ion such as Na+ whose transport becomes coupled with that of another solute such as glucose (secondary active transport). The coupled transport is mediated by the carrier protein (Sodium-Glucose Linked Transporter). Water-insoluble molecules such as monoglyceride and cholesterol can only move from the lumen into the absorptive cell by passing through the water layer covering these cells and then passing through the lipid microvillous membrane. The lipid molecules are chaperoned through the water layer dissolved in mixed micelles of bile salts and monoglyceride. Dissociation of the mixed micelle at the cell surface releases the lipid molecule for uptake into the enterocyte. |
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Next Section (G): Normal Water & Electrolyte Absorption » |
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