L6-membrane potentials and electrical gradients

Question 1

What are excitable cells? Examples?

Answer 1

cells which have the ability to be electrically excited resulting in the generation of ACTION POTENTIALS (AP)
ex: neurons, muscle cells (skeletal, cardiac and smooth), and some endocrine cells (ex: insulin-releasing pancreatic B cells) are excitable cells.

Question 2

Typical ion distribution across cell

Answer 2

• high Na+ outside, high K+ inside
• More free Ca2+ outside than inside
• Giant negatively charged proteins (anions) inside cell

Question 3

Why is potassium ion concentration higher within the cell as opposed to sodium?

Answer 3

• Because K+ dissolves through membrane easier than Na+.
• K+ has a smaller HYDRATION SHELL. Easier for it to shed its shell and bind to a neg protein
• membrane has more K+ leaks than Na+ leak channels

Question 4

Resting membrane potential in most neurons

Answer 4

-70mV

Question 5

Are lipids good conductors?

Answer 5

No.

Lipids contain few charged particles, which are immobile (poor conductors)

Question 6

What is the measure of stored charge called?

Answer 6

capacitance

Question 7

Why is the lipid bilayer a good capacitor?

Answer 7

Because it can maintain the separation of charged ions across a relatively narrow space

Question 8

Lipids have a ___ head and ____tail

Answer 8

Lipids have a POLAR head and NONPOLAR tail

Question 9

What neutralizes some of the potential created by K+ alone?

Answer 9

small net diffusion of Na+ inward

Question 10

When net movement of K+ stops, it is at _______________

Answer 10

Electrochemical equilibrium

Question 11

What is “equilibrium potential?”

Answer 11

• The energy required to stop movement of ions

- it is the potential generated when an ion is at electrochemical equilibrium

Question 12

Concentration of K+ and Na+ ions inside/outside of cell is maintained 2 different ways:

Answer 12

1. Na+/K+ pump:
   - uses ATP
   - 3 Na+ out, 2 K+ in
   - contributes negligible potential
2. Different solubilities in cell water and affinity for cell protein:
   - K+ is more soluble in internal water than Na+ and therefore preferentially enters cell
   - K+ has a smaller hydration shell than Na+. It can lose it more readily and bind to proteins

Question 13

How does a voltage develop across a membrane?

Answer 13

1. the concentration gradient for K+ tends to move this ion outside of the cell
2. s a result, the outside of the cell becomes more positive
3. Anions are unable to move across membrane. Therefore, the inside of the cell becomes more negative as K+ ions leave and large negative proteins stay.
4. The resulting ELECTRICAL gradient tend to move K+ into the cell.
5. No further net movement of K+ ions occurs when the inward electrical gradient exactly counterbalances the outward concentration gradient. The membrane potential at this point is -90mV (equilibrium potential for K+)