Transport Physiology

Question 1

Membrane permeability

Answer 1

• selective passage of hydrophilic solutes across the hydrophobic barrier
• selective and regulated passage of ions and nonelectrolytes across the cell membrane important for homeostasis
• transport proteins may help

Question 2

Electrochemical potential

Answer 2

• partial moral free energy of the solute or potential to do work when a difference in electrochemical potential exists across the cell membrane
• the potential of a solute on either side of the cell membrane is a function of solute activity, the solute charge and valence and electrical potential difference across the membrane
• for neutral solutes without charge such a non-electrolytes only the chemical potential matters

Question 3

Solute Electrochemical Potential Gradient

Answer 3

-the electrochemical or chemical potential difference of a solute across the cell membrane may be considered a gradient of driving force acting on solute transport across cell membrane

Question 4

-Active transport

Answer 4

• movement of solute from low electrochemical potential to high on other side of cell membrane
• ability to generate and maintain an electrochemical or chemical potential difference for ions and nonelectrolytes across the membrane

Question 5

Passive transport

Answer 5

-movement of solute from a place of high electrochemical potential on one side of cell membrane to a place of lower electrochemical potential on the opposite side

Question 6

Thermodynamic Equilibrium

Answer 6

• in absence of solute electrochemical potential difference across the membrane, and a driving force for solute transport, transport mechanisms that are passive mediate equal unidirectional solute transport in the forward and reverse direction across the membrane resulting in no net transport
• for anions and cations this happens when the chemical and electrical driving forces acting on solute transport are equal and opposite

Question 7

Unidirectional and net Solute Transport

Answer 7

-for electrolytes and nonelectrolyes where unidirectional solute transport is equal and opposite across the cell membrane, no net transport occurs and the charged or uncharged solutes are in transmembrane electrochemical equilibrium with respect to the driving forces acting on the transported solutes

Question 8

Non-equlibrium steady state

Answer 8

• solute is in electrochemical disequilibrium across the cell membrane but unidirectional solute transport is equivalent
• exists for Na across the cell membrane where active transport of Na by the Na/K ATPase maintains a relatively constant, low intracellular Na concentration and an inward Na electrochemical potential gradient is maintained by equivalent unidirectional Na transport into and out of the cell

Question 9

Gradient

Answer 9

• difference: in physical or chemical properties of a defined space or place relative to a comparable space or place
• direction: up/against is from low to high, down is from high to low
• driving force: source of potential energy acting on the movement or change in the physical and/or chemical properties of a defined space or place relative to a comparable space or place- high BP (arterial) and low BP (venous) is driving force for blood flow in arterial to venous direction

Question 10

Passive Transport Mechanisms

Answer 10

• 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
• never can go low to high (up)

Question 11

Primary/Secondary Active transport

Answer 11

• mediate net solute transport across cell membrane in a direction from lower solute electrochemical potential to higher electrochemical potential or up the solute electrochemical potential gradient
• the source of required energy arises from the hydrolysis of phosphate bonds of of ATP in primary active transport
• arises from a coupling to a second solute, moving down its electrochemical potential gradient in secondary active transport

Question 12

Primary active transport

Answer 12

• include ion-translocating ATPase or pumps which are directly dependent upon and transduce the energy released from ATP hydrolysis into the potential energy stored in the formation and maintenance of an ion electrochemical potential gradient
• Na/K ATPase, H/K-ATPase, H-ATPase, Ca-ATPase, Mg-ATPase, Organic anion-ATPase and Organic cation-ATPase

Question 13

Secondary active transport

Answer 13

• symporters and antiporters indirectly dependent upon the energy released from ATP hydrolysis
• transduce the energy stored in one solute electrochemical potential gradient into the formation and maintenance of a second solute electrochemical potential gradient by coupling the transport of two or more different solutes across the membrane in the same symport or opposite gradient antiport
• mediate concentrative, intracellular accumulation of solutes such as glucose, amino acids, vitamins, etc

Question 14

Non-mediated transport

Answer 14

• simple diffusion, passive
• transfer of solutes across the membrane withour interaction with or mediation by the presence of membrane transport proteins

Question 15

Mediated transport

Answer 15

-channels, carriers and pumps- distinguished by differences in functional properties and ability to mediate active or passive transport

Question 16

Channel mediated transport

Answer 16

• passive
• mediates transport of solutes down the gradient
• can be opened or closed depending on the position of a gating mechanism
• the conformation change opening and closing the gating mechanism is channel protein specific and may be induced by ligand binding or changes in voltage across the membrane

Question 17

Carrier mediated transport

Answer 17

• Faciliated Diffusion (Uniporter- passive, goes down the gradient, carrier proteins associate with solute undergo conformational change and release solute on the other side of membrane
• Cotransport- symporter- active and goes against gradient, source from coupled transporter of two or more solutes across the membrane where one acts of driving forces moving down electrochemical potential gradient and is coupled to active transport of a second solute moving against its electrochemical potential gradient
• Countertransport- antiport- active and does against gradient, the transport of the driving and driven solute are in opposite direction across the membrane

Question 18

Pump mediated transport

Answer 18

• active, against gradient
• arises directly from ATP hydrolysis and the effective pump mediated transduction of the energy released upon ATP hydrolysis into the energy stored in formation of a solute electrochemical potential difference across the cell membrane

Question 19

Ion-Translocating ATPase

Answer 19

• primary active transport is mediated by ATPases
• transduce energy of ATP hydrolysis into the energy sotred in the formation of transmembrane ion gradients
• Na/K ATPase-Na out, K into the cell
• Na concentration gradient inward driving force for secondary active transport of essential organic and inorganic metabolites
• K outward gradient maintained by Na/K ATPase generates inside negative voltage difference across the cell membrane
• 1 molecule of ATP hydrolyzes, 3 Na are extruded and 2K are accumulated into cell
• steady state- leak of of Na into the cell and K out of the cell with active transport of Na and K across the cell membrane

Question 20

Carrier Mediated Secondary Active Transport

Answer 20

• up or against the electrochemical potential gradient
• can be primary or secondary energy source
• carrier mediated secondary transport may couple transport of driving and driven solute in the same or opposite direction

Question 21

Kinetics of Diffusion

Answer 21

-Simple Diffusion- linear, does not show saturability
-Carrier-Mediated- at low solute concentrates the rate of solute transport increases linearly with solute concentration, and as solute concentration increases further, the rate of solute transport increases less and less as it approaches a maximum value at high solute concentrations
-Km-substrate affinity
Vmax- quantitation of maximal transport
-maximal velocity is only as fast as the rate limiting step, which is conformational change of transporter

Question 22

Substrate Selectivity/Specificity

Answer 22

• may be further classified by which substrates are transported by a particular transport protein
• Na channel will not transport K

Question 23

Competitive or Noncompetitive Transport Inhibition

Answer 23

• one may inhibit the transport of a second substrate B without the necessary transport of A
• Amoriloride inhibits epitheial Na channel but it doesnt do others
• differences in transport inhibition arise from the presence (competitive) or absence (noncompetitive) of an interaction between the transported substrate and the inhibitor for occupancy at the same (competitive) or different (non competitive) sites in the transport protein

Question 24

Stoichiometry

Answer 24

• the number of substrate molecules transported in one complete cycle of molecular events mediated by the transport protein and resulting in transfer of substrate across the membrane
• Na-Glucose 1Na in:1 glucose in
• 3Naout : 2K in

Question 25

Electrogenicity

Answer 25

• confers membrane potential difference (voltage) as we as substrate concentration difference as an additional driving force favoring or opposing transport
• inside negative membrane potential in addition to the inwardly directed Na concentration gradient are both thermodynamic forces driving electrogenic Na-Glucose cotransport across the membrane

Question 26

Transporter Mediated Homeostasis of Acid-Base Balance

Answer 26

• homeostasis of intracellular pH depends on the steady state balance of transport function mediating acid efflux and acid influx
• the efflux of the base equivalent HCO3 from the cell effectively acid loads the cell by leaving behind inside the cell, a H equivalent for every HCO3 transported out of the cell