Physiology 3120 Lecture Notes - Active Transport, Cotransporter, Osmosis

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26 Nov 2011
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Human Physiology
Friday, September 18, 2009
Membrane Transport Mechanisms
Secondary active transport
Concentration gradients of Na and K can be used to power other mechanisms (moving
substances down the gradient releases energy, which can be used for other things)
Na/K pump on basal membrane (away from external environment)
On luminal side, sodium allowed to come in down its concentration gradient
Glucose brought into the cell along with Na using the sodium/glucose cotransporter (ratio
of transport?)
Also have sodium/hydrogen exchanger (Na comes in down its gradient, which provides
energy to pump H+ ions out)
Indirectly requires ATP (requires ATP to maintain the concentration gradients)
Polarized transport
Different mechanisms on one side compared to the other side
Allow for unidirectional transport in/out of body fluids
Similarities & differences between all transport mechanisms (study tip!!)
Osmosis: movement of water down its concentration gradient due to its thermal motion (i.e. diffusion of
Water concentration is determined by the number of solute particles in solution (not on their
High solute = low water concentration
Low solute = high water concentration
Requires a semi-permeable membrane that allows the water to cross (aquaporins) but not the
Osmotic pressure: pressure required to stop movement of water (i.e. osmosis)
If membrane was permeable to both solute and solvent. . .solute will move down its
concentration gradient (and no osmosis)
If solute concentration is increased. . .osmotic pressure would increase by about the same amount
(have to account for the effects of gravity)
Affected by:
Permeability of membrane
Concentration gradient of solutes
Pressure gradient across membrane
Units of osmosis
Osmol: number of particles in solution that cause osmosis (osmotically active particles)
Osmolality: # of osmol/kg of water
Osmolarity: # of osmol/L of solution
Osmolarity & osmolality are used interchangeably
Osmolality of a 1M solution of NaCl is 2osmol/kg water (dissociates into one mole each
of sodium and chloride)