What is bulk flow?
A net difference in pressure that drives solvent and solute across a semipermeable membrane. Like edema from hypertension.
For many physiologic activities, we must use a Na/K gradient. How does our body maintain this gradient?
Primary active transport by the Na/K ATP pump. 3 Na are expelled for every 2 K brought into the cell. This is how the outside stays more positive than the inside.
How do secondary active transporters move solute against their electrochemical gradient?
They use the down hill solute's gradient to move the uphill solute across the membrane.
What size of axons transmit signals most quickly?
Hyperpolarization, Depolarization and Resting potential
How does resting potential in cells compare to the ECF?
30mV to 90mV depending on the tissue you're in
What is a measure of how easily ions can cross a membrane?
Px and gx. (Permeability and conductance)
What makes up a non-gated potassium channel?
Potassium leakage channel = a hole in the membrane filled with water that only allows passage of potassium ions.
What type of channels are selectively permeable when open and inaccessible when closed?
How do concentrations of K, NA, Cl and A- (big - charged ions that are trapped in the cell) differ inside and outside of a cell? How does permeability differ between these solutes?
K=high inside, Na=low inside, Cl=high outside, A-=high inside
What will happen with this solution?
There is a large concentration gradient going from left to right. K+ will move down the gradient to the other side. A- cannot pass through and will sit near the membrane as it is attracted to the side it cannot pass to. Eventually enough K+ will pass through that the K+ electrical gradient pushing toward the left will negate the K+ concentration gradient and you will be in equilibrium.
What does the equilibrium membrane potential equal in a system with only one permeable ion?
How would the whole tub of water diagram look for this kind of membrane?
The sodium will experience the same electrical gradient as potassium because they are both positively charged. However, the sodium concentration gradient will be opposite potassium, both gradients for sodium point in the same direction and sodium goes inside the cell. Sodium going inside the cell makes the electrical gradient less for potassium and it begins to dribble out of the cell is out of equilibrium.
What does the tub of water look like in a real world cell?
Sodium and potassium are far from equilibrium, but chloride is in equilibrium
What drives ions across membranes?
Membrane Potential (Em) - Equilibrium Potential (Ex) = Driving force…i.e. where you are - where the ion wants you to be
How do you figure the amount of current across a membrane?
What decides how close the membrane potential will be to the equilibrium potential?
Membrane permeability of the ion. Higher permeability = closer membrane potential to equilibrium potential.
How does the resting potential relate to permeability of ions to the cell?
It is a weighted average of ion concentrations of all permeable ions in the cell
Is the net current for potassium small or large?
Small. Driving force is small but conductance is large.
Is the net current for sodium small or large?
Small. The driving force is large but the conductance is small.
What ways can you change membrane potential?
Change permeability, ionic concentrations, inject current into cells or give drugs that change permeability.
Why can you consider Na and K gradients relatively constant in most cells during stimulation?
So few ions need to cross the membrane to create an action potential
What happens when you increase permeability/conductance of an ion?
The membrane potential will move toward the equilibrium potential for that particular ion
What is the most depolarized/hyperpolarized you can get?
The highest (depolarized) and lowest (hyper polarized) equilibrium potentials of the ions in your cell.
What happens when you increase extracellular potassium concentration?
The concentration gradient decreases, making the electrical gradient larger and potassium moves into the cell and the membrane potential depolarizes. Eventually the cell will have a new membrane potential and equilibrium potential and K+ will again begin moving out of the cell.
What happens when you increase extracellular sodium concentration?
The concentration gradient increases and the driving force gets bigger. A little more Na will flow into the cell and you will produce a very slight depolarization.
Why is depolarization so small when Na is added to the extracellular space?
Resting permeability is already very low