Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Different transport mechanisms • Passive transport – Aqueous diffusion – Lipid diffusion • Specialized transport – Active transport – Facilitated transport – Phagocytosis/pinocytosis Lipid diffusion • Most common mechanism by which chemicals cross membrane barriers • Involves diffusion from aqueous phase on one side of membrane with a high concentration to aqueous phase on other side of membrane • Rate proportional to: – Concentration gradient (direct) – Lipid solubility (direct) • Oil-water partition coefficient • Affected by ionization state – Molecular size (inverse) – Surface area available for lipid diffusion (direct) • Type of kinetics: First-order (proportional to size of concentration gradient) • Not saturable and rate of lipid diffusion is unaffected by other chemicals (non-competitive) Aqueous diffusion • Diffusion of a chemical through the aqueous phase from a region of high concentration to a region of low • Example: most chemicals diffuse out of open capillary beds into extracellular space • Passive process (driven by concentration gradient) • Type of kinetics: first order (proportional to concentration • Not a saturable process • Not a competitive process Structure of phospholipids (phosphatidylcholine) Compounds that exist as both unionized and ionized forms (weak acids and bases) • Substances capable of accepting or donating a positively charged proton • Unionized form more lipid soluble, more easily absorbed • Predicted by Henderson Hasselbalch equation log [acid form HA/base form A-] = pKa - pH • pKa = pH at which 50% is in HA form and 50% is in A- form 1 Carrier-mediated transport • Transport across membrane utilizes membrane-bound proteins (carriers or transporters) • Facilitated transport (direction of transport down the conc. gradient) • Active transport (direction of transport up the conc. gradient) (requires ATP) • Since it is protein mediated, transport is – Structure-specific – Saturable, reaches a maximum rate – Potentially competitive with chemicals of similar structure • Type of kinetics: first order at low concentration, zero order at high concentration (reflects saturation) Examples of carrier-mediated transport • Important physiological role in transport of sugars, amino acids, some proteins, and nucleic acids, lipids, bile salts, vitamins, ions • Minor role in absorption and distribution – Intestinal mucosa (Heavy metals, some drugs) – Choroid plexus (2) • Organic acid and organic base • Important role in elimination of chemicals from the body – Kidney (2) • Organic acid and organic base – Liver (4) • Two for organic acids • One organic base • One neutral 2