When does potential difference occur?
Potential difference occurs when there is a separation of electrical charge
Why is it called potential difference?
- The different electrical charges are attracted to each other, i.e., there is a force between them
- This force has the potential to do work
- There is potential energy due to the difference in the distribution of electrical charges
What is the potential difference that exists across all the cell membranes?
Membrane potential
What’s the potential diff measured in?
Volts
- The greater the amount of charge separation, the greater the amount of volts
Because the potential difference is very tiny we use units of millivolts (1/1000 of a volt) - If there is no voltage that means you’re dead
Why is there a small charge separation compared to the inside of a cell to the outside?
- Across the plasma membrane of cells, there are different concentrations of ions (charged particles like the cations Na+, K+, Ca2+ and the anion Cl- ) as well as charges on proteins
- There are a few more negative ions on the inside of the plasma membrane than on the outside
Is the cytoplasm more negative or the extracellular space?
The cytoplasm is more negative
Which direction do potassium Ions want to go?
K+ is always trying to leave the cell (EFFLUX) b/c K+ wants to go down its concentration gradient because it is in high concentration inside of the cell
- When K+ leaves the cell, the potential difference becomes greater
Why care about potential difference?
They can be signals (APs)
Efflux meaning
Ions trying to LEAVE the cell
Cytoplasm to ECS
Are there more K+ in the cell or outside?
Higher [K+] in cytoplasm
Lower [K+] in ECS
Which direction do sodium Ions want to go?
Sodium ions are always trying to enter the cell (INFLUX) b/c
- Na+ “wants” to go down its concentration gradient
sodium is less concentrated inside of the cell than outside so sodium attempts to enter the cell
- Na+ wants to go down its charge gradient
the inside of the cell is negative so the positively charged sodium ions attempt to enter the cell
When Na+ enter the cell, is the potential difference greater or lessened?
When Na+ enters the cell, the potential difference is lessened (the voltage moves towards zero) because the positive charge on the sodium ions decreases the net amount of negative charge in the cell
What is resting membrane potential?
The resting potential is what is in a cell at rest
- Every cell has a RMP
- Inside of a neuron at rest it is about -70mV other cells are less but none are less than -40mV
What are the one or two forces pushing on the ions that make the RMP?
- The first force is that which pushes ions down their concentration gradient (i.e., ions diffuse from areas where they are in high concentrations to areas where they are in low concentration)
- The second force is that which pushes ions down their charge gradient (i.e., positive charges diffuse towards areas of negative charges and negative charges diffuse towards areas with positive charges)
Inside vs outside of a cell (+/- and what ions)
Inside (cytoplasm)
- More (-) protein
- More K+
- More negative
Outside (ECS)
- More Na+
- More positive
What causes RMP?
Uneven distribution of ions
- The extracellular fluid is rich in Na+ and Cl-
- The cytosol is rich in K+ and protein
- The protein molecules within the cell have a negative charge but cannot move (they help keep the inside of the cell negative)
If you add up all the charges you get a few more (just a few) negatives on the inside thus they line up on the membrane
What are the 2 ways to maintain the RMP?
- Differentially permeable membrane
- Sodium-potassium pump (Na+/K+ ATPase)
How does a differentially permeable membrane work?
- The membrane is much less permeable to sodium ions than potassium ions
- Sodium leaks into the cell much more slowly than potassium leaks out of the cell
- As a result of the different movement of positive ions more positive charge leaks out than positive charge leaking in the cell a net negative charge is left in the cell
How does sodium-potassium pump (Na+/K+ ATPase) work?
- The membrane is somewhat leaky (i.e., Na+ comes in and K+ goes out) so the ions have to be returned whence they came or the RMP runs down
- Three sodium ions (3 Na+) are pumped out for every two potassium ions (2 K+) that are pumped into the cell (net negative charge is left) this is mildly electrogenic (that means it causes a wee bit of the RMP)
What format is the changes in membrane potential in?
As electrically-coded messages
What are the 2 types of membrane potential changes?
Graded potentials
Action potentials
How are potential changes generated?
By opening channels (eg. Na+ pump)
How are messages sent quickly?
They are electrically encoded
How do voltage-gated ion channels work?
- Voltage change
- Voltage-gated ion channel gate opens (b/c of voltage change)
- Cations enter (positive ions)
- Potential decreases b/c of the entry of cations
- Voltage-gated ion channel gate close
