2.2.3. Membrane I

Question 1

What is bulk flow?

Answer 1

The movement of a solutionfrom one compartment to another due to a pressure gradient

Question 2

What is simple diffusion?

Answer 2

The movement of a solute through a membrane due to an electrochemical gradient. No transport assistance is involved.

Question 3

In simple diffusion, how do we know what direction the solute will move?

Answer 3

Whatever gradient exists will push the solute from High to Low (electric: High potential to low potential, concentration: high concentration to low)

Question 4

What is facilitated diffusion?

Answer 4

The same process as general diffusion, but transport proteins of some kind are needed to help the solute move across the membrane.

Question 5

What is are some key differences between simple diffusion and simple facilitated diffusion?

(There are three)

Answer 5

1) Specificity: The solute has to fit into the transport protein
2) Competition: Different solutes may fit into the same protein, so they may compete for that space
3) Saturability: Once all protein carriers are engaged, the max rate of diffusion is achieved. Simple diffusion has no max diffusion rate

Question 6

Does facilitated diffusion require energy?

(Is it an active or passive process?

Answer 6

Facilitated diffusion is still a passive process. No energy, in terms of ATP, are used to enable its action

Question 7

What is Primary Active Transport?

Answer 7

A transport protein pumps a solute against its energy gradient.

Question 8

How is primary active transport different than facilitated diffusion?

Answer 8

1) PAT requires energy
2) PAT pumps solutes against their gradient, facilitated diffusion allows for their movement down their gradient

Question 9

What is Secondary Active Transport?

Answer 9

A combination of facilitated diffusion and active transport.

One solute moves down its gradient using a transport protein, and this same protein uses the energy released from that process to pump another solute against its gradient.

Question 10

What are the relative concentrations of the following solutes inside and outside of the typical cell?

K⁺

Na⁺

Cl^-

Organic Anions (A^-)

Answer 10

K: Low outside the cell, high in

Na: High outside the cell, low in

Cl: High outside the cell, low in

A: Almost none outside the cell, a LOT in the cell

Question 11

How permeable is the typical cell to:

K

Na

Cl

Organic Anions (A)

Answer 11

K: High permeability

Na: Low permeability

Cl: Very high permeability

A: Impermeable

Question 12

How does water and water solubles solutes move into the cell if its bilipid layer is impermeable to water?

Answer 12

There are channels or pores in the cell that are filled with water, allowing for solute and water movement into and out of the cell

Question 13

Is an actual cell (in general) permeable, impermeable, or semipermeable to ions?

Answer 13

Semipermeable

Question 14

What are ion leakage channels, and what important role do they play in cells like neurons?

Answer 14

Ion leakage channels are pores in the cell membrane that exhibit selective permeability, allowing ONLY specific ions to pass.

These channels are responsible for the setting the resting potential of neurons

Question 15

Are leakage channels gated?

Answer 15

No, they are always open

Question 16

What types of gated channels are there?

Answer 16

Voltage gated

Ligand gated (or transmitter gated)

Question 17

In the cell, how is permeability related to an ion’s conductance?

Answer 17

The permeability of a cell to an ion is what establishes that ion’s conductance

Question 18

Define the term equilibrium potential.

Answer 18

The equilibrium potential refers to the electrochemical potential at equilibrium of a particular ion, as calculated by the Nernst equation

Question 19

Define the term resting potential.

Answer 19

The resting potential refers to the weighted average based upon membrane permeabilities of all the equilibrium potentials of the various ions in a given cell, as calculated by the Goldman equation.

Question 20

Compare the states of equilibrium and steady state in a cell.

Answer 20

The cell is in equilibrium once ion concentration and the electrochemical gradient is balanced inside and outside of the cell. However, the sodium potassium pump prevents the cell from going into complete equilibrium and maintains the Na+ and K+ ion concentration such that the inside of the cell is -90 mV.

The resting potential of cells changes based upon which cell you are observing

Question 21

What is the membrane potential of a resting cell?

Answer 21

It will almost always be negative, though there are a few exceptions.

Question 22

Why is the Nernst equation important for measuring membrane potential?

Answer 22

The Nernst equation takes an ion concentration and converts it to electric potential, allowing us to measure the membrane potential through ion concentrations.

Question 23

How do we know which direction an ion will move if given a chance across a cell membrane?

Answer 23

Ions will move across a membrane in order to move the cell’s membrane potential towards the ion’s equilibrium potential

Ex: K⁺ eq. potential = -97 mV, so it leaves the cell to make the cell more negative

Na⁺ eq. potential = +66 mV, so it enters the cell to make it more positive

Question 24

What effect will increasing the conductance of an ion have on the membrane potential?

Answer 24

Increasing the conductance of an ion will give it more weight in determining the cell’s membrane potential, so the cell’s membrane potential will move towards that ion’s equilibrium potential.

Question 25

What is the equation that gives us the current (movement) of an ion across a membrane?

Answer 25

I_x = g_x(Em-E_x)

I_x = current of ion X

g = conductance

E_m = membrane potential

E_x = ion potential

(Em-Ex) is known as the “driving force”

Question 26

What is cell hyperpolarization?

Answer 26

Making the cell’s membrane potential more negative.

Question 27

What is depolarization?

Answer 27

Making the cell’s membrane potential more positive

Question 28

How may we change a cell’s membrane potential?

Answer 28

We may hyperpolarize or depolarize a cell by changing its permeability to an ion(s).

Question 29

How does the concentration gradient change during a cell’s depolarization or hyperpolarization?

Answer 29

Typically, the concentration gradient does not change much at all, since the movement of so few ions are needed to change the electric potential.

Question 30

What are some ways we can artificially change a membrane’s potential?

Answer 30

1) Change ion concentrations
2) injecting current into a cell via an electrode
3) Using drugs to alter the membrane’s permeability

Question 31

How depolarized and hyperpolarized can a cell become (if ion conductance were to become infinite)?

Answer 31

The max is the ion’s equilibrium potential for each side, so:

E_K = -97 mV (max hyperpolarization)

E_Na = +66 mV (max depolarization)

Question 32

What is the importance of the Goldman Equation?

Answer 32

It calculates the membrane potential based on the permeability of the cell to various ions (typically, Na, K, and Cl)

Question 33

What is the role of the Na/K ATPase pump in maintining membrane potential?

Answer 33

Because of the presence of Na and K leak channels, Na and K would attempt to reach their own equilibrium potentials rather than the potentials they reach in a normal cell. The Na/K ATPase pump pumps Na out of the cell as it leaks in, and recaptures K as it leaks out of the cell.

Question 34

How much Na and K does the Na/K ATPase pump pump for each ATP?

Answer 34

3 Na

2 K

Question 35

The Na/K ATPase pump is an example of which kind of transport?

Answer 35

Primary Active Transport

Question 36

Bonus: What is an example of Secondary Active Transport?

Answer 36

Na/Glucose Transporter