Transport Physiology Flashcards
Membrane permeability is?
The selective passage of hydrophilic solutes across the hydrophobic barrier. It is mediated by the presence of membrane transport proteins that span the phospholipid bilayer.
electrochemical potential
electrochemical potential of a solute is the partial molar free energy of the solute or the potential to do work when a difference in electrochemical potential exists across the cell membrane
At thermodynamic equilibrium
unidirectional solute transport is equal and opposite across the cell membrane, NO net transport occurs
non-equilibrium solute steady state occurs when
solute is in electrochemical dis-equilibrium across the cell membrane, but unidirectional solute transport is equivalent
passive solute transport (channels and uniporters)
mediate net solute transport across the cell membrane in a direction from higher solute electrochemical potential to lower electrochemical potential or “down” the solute electrochemical potential gradient
active solute transport (ion-translocating pumps, symporters, antiporters)
mediate net solute transport across the cell membrane in a direction from lower solute electrochemical potential to higher electrochemical potential or “up” the solute electrochemical potential gradient.
Primary & secondary active transport
hydrolysis of phosphate bonds of ATP in primary active transport
secondary active transport arises from a coupling to a second solute, moving down its electrochemical potential gradient in secondary active transport (Secondary active transport mechanisms include the symporters in the same direction and antiporters in the opposite direction). carrier-mediated, secondary active transport couples the transport of the “driving” solute “down” its electrochemical potential gradient across the membrane to the transport of the “driven” solute “up” or against its electrochemical potential gradient. Carrier mediated, secondary active transport may couple transport of the “driving” and “driven” solute in the same or opposite direction where carrier mediated cotransport (symport) couples solute transport in the same direction and carrier mediated countertransport (antiport) couples solute transport in the opposite direction.
Na/K ATPase is what kind of transport?
Primary active transport
The basolateral Na+/K+ ATPase, or “sodium pump” uses primary active transport to pump three sodium ions out of the cell in exchange for two K+ ions
Mediated transport
membrane transport proteins mediate the trans-membrane transfer of solutes, reversibly, in either direction, without solute interaction with the lipids of the cell membrane. Mediated transport occurs by three different transport mechanisms: Channels, Carriers and Pumps, which are distinguished by differences in functional properties (substrate specificity and affinity) and ability to mediate active or passive transport.
Non-mediated transport = simple diffusion
transfer of solutes across the membrane without interaction with membrane transport proteins.
Channel mediated transport
is passive and mediates transport of solutes in a direction across the membrane down the solute electrochemical gradient
Carrier mediated transport may result from three different transport mechanisms
Facilitated Diffusion or Uniport
Cotransport (symport)
Countertransport (antiport)
Facilitated Diffusion or Uniport
Carrier mediated transport by Facilitated Diffusion or Uniport is passive
Carrier mediated transport by Cotransport (symport) is
active. The source of energy driving active solute transport by cotransport arises from coupled transport of two or more solutes across the membrane where one solute acts as the driving force moving down its electrochemical potential gradient and is coupled to the active transport of a second solute moving up or against its electrochemical potential gradient. Cotransport occurs where transport of both the “driving” and “driven” solute are in the same direction across the membrane.
Carrier mediated transport by Countertransport (antiport) is
active and mediates the trans- membrane accumulation of solute inside or outside the cell against the solute electrochemical potential. Countertransport occurs where transport of the “driving” and “driven” solute are in the opposite direction across the membrane.
Pump mediated transport is
active and mediates the trans-membrane accumulation of solute inside or outside the cell against the solute electrochemical potential gradient. Example is Na/K ATPase.
What works faster, channel or carrier-mediated facilitated diffusion?
channels mediate a rate of transport of inorganic ions many orders of magnitude faster than the rate of transport of organic solutes by carrier-mediated facilitated diffusion.
Think about it, channels do not open & close–THEY ARE ALWAYS OPEN!
Describe the mechanism for the Na/K ATPase
The molecular mechanism of the Na/K ATPase may be defined by discrete steps of a complete transport cycle in which one molecule of ATP is hydrolyzed, 3 Na are extruded from the cell and 2 K are accumulated into the cell. Interestingly, the Na/ K ATPase mediates the transfer of 3 cations out of the cell and 2 cations into the cell, which results in net positive charge transfer out of the cell and contributing to the generation of an inside-negative voltage difference across the cell membrane. It is Primary active transport!
Describe the slopes for carrier mediated & facilitated diffusion vs simple diffusion
Simple diffusion is a straight line slope, about 1/2
Carrier & facilitated diffusion looks like an enzyme curve
see pg. 30
transport stoichiometry
is defined as the number substrate molecules transported in one complete cycle of molecular events mediated by the transport protein and resulting in transfer of substrate across the membrane (e.g., stoichiometric coupling shown for the Na/K ATPase is 3Na : 2K)
Competitive inhibition
increasing concentration of a transported substrate decreases transport inhibition by a fixed concentration of inhibitor (competitive)
Non-Competitive inhibition
increasing concentration of a transported substrate has no effect on transport inhibition by a fixed concentration of inhibitor (noncompetitive).
Electrogenicity
Electrogenicity is the net transfer of positive or negative charge associated with 1 cycle of the transport protein.
