Download Different transport mechanisms Aqueous diffusion Lipid

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
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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
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