Transport Physiology

Question 1

Membrane permeability is?

Answer 1

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.

Question 2

electrochemical potential

Answer 2

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

Question 3

At thermodynamic equilibrium

Answer 3

unidirectional solute transport is equal and opposite across the cell membrane, NO net transport occurs

Question 4

non-equilibrium solute steady state occurs when

Answer 4

solute is in electrochemical dis-equilibrium across the cell membrane, but unidirectional solute transport is equivalent

Question 5

passive solute transport (channels and uniporters)

Answer 5

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

Question 6

active solute transport (ion-translocating pumps, symporters, antiporters)

Answer 6

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.

Question 7

Primary & secondary active transport

Answer 7

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.

Question 8

Na/K ATPase is what kind of transport?

Answer 8

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

Question 9

Mediated transport

Answer 9

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.

Question 10

Non-mediated transport = simple diffusion

Answer 10

transfer of solutes across the membrane without interaction with membrane transport proteins.

Question 11

Channel mediated transport

Answer 11

is passive and mediates transport of solutes in a direction across the membrane down the solute electrochemical gradient

Question 12

Carrier mediated transport may result from three different transport mechanisms

Answer 12

Facilitated Diffusion or Uniport

Cotransport (symport)

Countertransport (antiport)

Question 13

Facilitated Diffusion or Uniport

Answer 13

Carrier mediated transport by Facilitated Diffusion or Uniport is passive

Question 14

Carrier mediated transport by Cotransport (symport) is

Answer 14

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.

Question 15

Carrier mediated transport by Countertransport (antiport) is

Answer 15

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.

Question 16

Pump mediated transport is

Answer 16

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.

Question 17

What works faster, channel or carrier-mediated facilitated diffusion?

Answer 17

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!

Question 18

Describe the mechanism for the Na/K ATPase

Answer 18

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!

Question 19

Describe the slopes for carrier mediated & facilitated diffusion vs simple diffusion

Answer 19

Simple diffusion is a straight line slope, about 1/2

Carrier & facilitated diffusion looks like an enzyme curve

see pg. 30

Question 20

transport stoichiometry

Answer 20

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)

Question 21

Competitive inhibition

Answer 21

increasing concentration of a transported substrate decreases transport inhibition by a fixed concentration of inhibitor (competitive)

Question 22

Non-Competitive inhibition

Answer 22

increasing concentration of a transported substrate has no effect on transport inhibition by a fixed concentration of inhibitor (noncompetitive).

Question 23

Electrogenicity

Answer 23

Electrogenicity is the net transfer of positive or negative charge associated with 1 cycle of the transport protein.