Transport Physiology Flashcards
What is non-equilibrium steady state?
When a solute is in electrochemical dis-equilibrium across the cell membrane but unidirectional solute transport is equivalent. Na as an example in cells where the Na/K ATPase keeps concentrations constant inside the cell via equivalent unidirectional transport into and out of cell
Electrochemical potential
Partial molar free energy of solute in solution. Potential to do work when a difference in EP exists across cell membrane (concentration and voltage).
Considered a gradient of driving force acting on solute transport across membrane
Function of CONCENTRATION and CHARGE
Active transport
Movement of solute from LOW EP to a place of HIGHER EP on the opposite side. Requires input of energy.
Ability to generate and maintain electrochemical or chemical (concentration) potential differences (net transport against prevailing gradients acting as driving force)
*ion pumps, symporters, antiporters)
Passive transport
Movement of solute from HIGH EP to a place of LOW EP on the opposite side.
Channels and uniporters
Thermodynamic equilibrium
No net transport because of equal unidirectional solute transport in forward and reverse directions. Occurs in absence of electrical/chemical gradient (for electrolytes) and absence of solute concentration (chemical) gradient (for uncharged non-electrolytes)
Gradient (3Ds)
- Direction: “Up” is from low to high; “Down” is from high to low
- Difference in physical/chemical properties between two spaces
- Driving Force: source of PE, causes net movement from one side to the other
Primary Active Transport
Directly dependent on energy from ATP hydrolysis. Include ion-translocating ATPase (pumps)
Ex. Na/K ATPase, H/K-ATPase, H-ATPase, ect.
Secondary Active Transport
Indirectly dependent on energy from ATP hydrolysis (creates gradient the other depends on), but instead uses the energy stored in another EP gradient. Does this by coupling transport of 2+ solutes across the membrane in the same or opposite direction. Solute moving down gradient drives second solute up its gradient
2 Types of Diffusion
Simple: Passive, non-mediated transport - doesn’t use membrane transport proteins
Facilitated: passive, mediated transport - uses membrane transport protein (channel or carrier protein)
Mediated Transport
Uses membrane transport proteins that span the cell membrane.
Channels, carriers, pumps
Channel transport
Passive process, solutes travel down gradient, usually faster than carrier proteins. Can have voltage-gated or ligand-gated mechanisms
Carrier transport
Undergo conformational change to release solute on other side
- Facilitated Diffusion (uniport): passive - down gradient
- Cotransport (symport): active, energy from coupled transport of 2+ solutes (secondary AT) in the same direction
- Countertransport (antiport): active, energy from coupled transport of 2+ solutes moved in opposite directions
Pump transport
Primary active transport - ATP hydrolysis powers pump
Na/K ATPase
Important for intracellular homeostasis - Na gradient is the major driving force of essential metabolites into the cell. Outward K gradient generates inside negative voltage difference, which helps drive intracellular accumulation of solutes (along with Na current). Also each molecule “leaks” back to original side.
1 ATP hydrolyzed, 3 Na out, 2 K in
Transport Kinetics
Simple diffusion doesn’t demonstrate substrate “saturability” (slowing rate at higher concentrations), mediated transport demonstrates saturability.
Vmax: max rate of solute transport, determined by # of transporters present in membrane and time required for a cycle
