Cell physiology - Diffusion and resting membrane potentials Flashcards
Integral proteins that span the membrane and, when open, permit the passage of certain ions
Ion channels
Properties of ion channels and explanation
- Selective
- Selectivity is based on the size of the channel and the distribution of charges that line it
- May be open or closed as controlled by gates
- Ions can flow through only when the channel is open
- Conductance depends on the probability that the channel is open
- the higher the probability that a channel is open, the higher the conductance, or permeability
A small channel lined with negatively charged groups will be selective for what?
Small cations and exclude large solutes and anions
Note: In essence, it excludes large solutes because it is small and repels anions because it is lined with negatively charged groups
What controls the opening and closure of voltage-gated vs ligand-gated channel?
- Voltage-gated channel
- Changes in membrane potential
- Ligand-gated channels
- Hormones, second messengers, or neurotransmitters
An example of ligand-gated channel is the nicotinic receptor. What triggers it and what happens once it is open?
ACh binds to → channel at the motor end plate opens → permeable to Na+ and K+ → motor end plate to depolarization
Diffusion potential
- Definition
- Condition for it to be generated
- Size is determined by what
- Sign is determined by what
- Definition
- Potential difference generated across a membrane because of a concentration difference of ion
- Condition for it to be generated
- Can be generated only if the membrane is permeable to the ion
- Size of the diffusion potential depends on
- The size of the concentration gradient
- Sign of the diffusion potential depends on
- Whether the diffusing ion is positively or negatively charged
T/F. Diffusion potentials result in changes in the concentration of the diffusing ions
False
Diffusion potentials are created by the diffusion of very few ions and, therefore, do not result in changes in concentration of the diffusing ions
The diffusion potential that exactly balances (opposes) the tendency for diffusion caused by a concentration difference
Equilibrium potential
Diffusion potentials are created by the diffusion of very few ions; therefore ____
They do not result in changes in concentration of the diffusing ions
Two solutions of NaCl are separated by a membrane that is permeable to Na+ but not to Cl–. The NaCl concentration of solution 1 is higher than that of solution 2. What will happen?
- Because the membrane is permeable to Na+, Na+ will diffuse from solution 1 to solution 2 down its concentration gradient; Cl– is impermeable and therefore will not accompany Na+
- As a result, a diffusion potential will develop and solution 1 will become negative with respect to solution 2.
- Eventually, the potential difference will become large enough to oppose further net diffusion of Na+. The potential difference that exactly counterbalances the diffusion of Na+ down its concentration gradient is the Na+ equilibrium potential. At electrochemical equilibrium, the chemical and electrical driving forces on Na+ are equal and opposite, and there is no net diffusion of Na+.
Equation used to calculate equilibrium potential, what it tells us and the formula
Nernst equation
- tells us what potential would exactly balance the tendency for diffusion down the concentration gradient
- in other words, at what potential would the ion be at electrochemical equilibrium
Approximate values for equilibrium potentials in nerve and muscle of
- ENa+
- ECa2+
- EK+
- ECl–
- ENa+ = +65 mV
- ECa2+ = +120 mV
- EK+ = -85 mV
- ECl– = -85 mV
Measured potential difference across the cell membrane in millivolts (mV)
Resting membrane potential
By convention, RMP is expressed as and what it means
- The intracellular potential relative to the extracellular potential
- Approximate RMP of -70 mV which means 70 mV, cell negative
What ions will make the greatest contributions to the resting membrane potential?
- Ions with the highest permeabilities, or conductances, will make the greatest contributions to the resting membrane potential
- Ions with the lowest permeabilities will make little or no contribution
- For example: The resting membrane potential of nerve is –70 mV, which is close to the calculated K+ equilibrium potential of –85 mV, but far from the calculated Na+ equilibrium potential of +65 mV; at rest, the nerve membrane is far more permeable to K+ than to Na+*
- Note: The estimated values of equilibrium potential*
- ENa+ = +65 mV
- ECa2+ = +120 mV
- EK+ = -85 mV
- ECl– = -85 mV
