Lecture 5: Active transport and membrane potentials

Question 1

The process of primary active transport is uphill/downhill

Answer 1

uphill

Question 2

Describe the process of primary active transport

Answer 2

this is coupled directly to a continuous supply of energy (ATP)
*the movement of one solute is NOT coupled to the downhill movement of another solute or water

Question 3

Give three examples of primary active transport

Answer 3

• Na+/K+ ATPase
• H+/K+ ATPase
• Ca2+ ATPase

Question 4

When is secondary active transport?

Answer 4

This is when the downhill movement of one solute is coupled to uphill movement of another solute. It does not use any energy (ATP)

Question 5

How many Na+ and K+ does Na+/K+ ATPase exchange?

Answer 5

3 Na+ for 2 K+

Question 6

Describe the exchange of Na+ and K+ by Na+/K+ ATPase

Answer 6

Initially, ATP is bound to the intracellular portion of Na+/K+ ATPase. This means that it is open to the ICF. 3 Na+ from the ICF bind to their 3 binding sites inside the Na+/K+ ATPase (the 2 K+ sites are blocked so no K+ binds). When this happens, ATP cleaves to form phosphate. This releases energy and the Na+/K+ ATPase changes shape to become open to the ECF. The 3 Na+ are released into the ECF and then become blocked off. The 2 Na+ binding sites are now available so 2 K+ bind from the ECF. The phosphate breaks off which causes the Na+/K+ ATPase to go back to its original shape (closed to ECF) and the K+ are released into the ICF

Question 7

What is the purpose of Na+/K+ ATPase?

Answer 7

it actively transports Na+ out of the cell and K+ into the cell to maintain the ion gradient across the cell membrane

Question 8

Na+/K+ ATPase is blocked by what?

Answer 8

ouabain

Question 9

What happens if the Na+/K+ ATPase does not maintain the ion gradient across the membrane?

Answer 9

the Na+ and K+ concentrations both reach equilibrium across the membrane so there is no longer a driving force

Question 10

What are the two forms of secondary transport?

Answer 10

• cotransport

- counter transport

Question 11

What is cotransport?

Answer 11

This is secondary active transport where both solutes go in the same direction

Question 12

What is counte-transport?

Answer 12

this is secondary active transport where the solutes go in opposite directions

Question 13

Give 2 examples of cotransporters and a brief overview of how they work

Answer 13

1. Na+ glucose transporter
2. Na+ amino acid cotransporters
   the movement of Na+ down its electrochemical gradient drives the movement of another solute up its electrochemical gradient in the same direction

Question 14

Give an example of counter-transport and a brief overview of how they work

Answer 14

Na+/H+ antiporter
The movement of Na+ down its electrochemical gradient drives the movement of another solute up its electrochemical gradient in the opposite direction

Question 15

Summarise primary active transporters

• energy?
• what does it move and how?
• saturation?
• specificity?
• competition?
• inhibition?

Answer 15

• requires metabolic energy (ATP)
• able to move substrates against their electrochemical gradients
• saturation kinetics
• chemical specificity
• competition
• inhibition

Question 16

Summarise secondary active transporters

• energy?
• what does it move and how?
• saturation?
• specificity?
• competition?
• inhibition?

Answer 16

• does not require energy
• downhill movement of one solute drives the uphill movement of a second solute
• saturation kinetics
• chemical specificity
• competition
• inhibition

Question 17

Summarise passive transport

• energy?
• what does it move and how?
• saturation?
• specificity?
• competition?
• inhibition?

Answer 17

• does not require energy
• not able to move substrates against their electrochemical gradients
• saturation kinetics
• chemical specificity
• competition
• inhibition

Question 18

What are the two 2 types of gradients?

Answer 18

• concentration gradients

- electrical potential

Question 19

Why does a concentration gradient exist?

Answer 19

because ion gradients exist across the cell membrane

Question 20

Why does an electrical potential exist?

Answer 20

because there is a potential difference (voltage) across the cell membrane

Question 21

How can we determine is movement of a charged species is active or passive?

Answer 21

we need to sum the chemical and electrical gradients to determine the overall electrochemical gradient

Question 22

What determines the movement of an uncharged solute across the membrane? Use glucose as the example

Answer 22

Because glucose is uncharged, it does not have an electrical potential and so it is only the concentration gradient that determines the movement of it across the membrane. For glucose because the concentration is higher outside the cell than inside (except in the gut where cotransport with Na+ is required), glucose will flow down its conc gradient into the cell through channels

Question 23

Which gradient is the only one which is important when considering the movement of uncharged solutes?

Answer 23

the concentration gradient

Question 24

The chemical potential (concentration gradient) between 2 solutions is given by:
What are the units?
What do each of the components mean?

Answer 24

Δμ = RT ln([C]2/[C]1)
the units are Joules/mole

where
R = universal gas constant
T = temperature in Kelvin
[C]x = concentration of solute x

Question 25

Explain both the chemical and electrical gradients of Na+

Answer 25

The concentration Na+ is higher in the ECF that in the ICF. Because ions flow from areas of high concentration to low concentration, the chemical gradient of Na+ pushes it into the cell. The inside of the cell is negatively charged compared to the outside of the cell. Because Na+ is positively charged it is attracted to the inside of the cells and therefore the electrical gradient pushes the Na+ into the cell

Question 26

Explain both the chemical and electrical gradients of Cl-

Answer 26

The concentration Cl-is higher in the ECF that in the ICF. Because ions flow from areas of high concentration to low concentration, the chemical gradient of Cl- pushes it into the cell. The inside of the cell is negatively charged compared to the outside of the cell. Because Cl- is negatively charged it is attracted to the outside of the cells and therefore the electrical gradient pushes the Cl- out of the cell

Question 27

What is the equation for electrical potential? | What are the units?

Answer 27

Δφ = (φ2 - φ1) | units: joule per coulomb (volt)

Question 28

What is the electrochemical gradient?

Answer 28

When you combine the electrical gradient and the chemical gradient. This is a measure of the driving force on a charged solute

Question 29

What is the equation for the electrochemical gradient? | What do the components mean?

Answer 29

Δμ = RT ln([C]2/[C]1) + zF(φ2 - φ1) ``` z = valence (usually 1 in this course) F = Faradays constant (96500 coulombs per mole) ```

Question 30

All cells have a what?

Answer 30

membrane potential

Question 31

What is the range of membrane potential for the inside of the cell?

Answer 31

-60mV to -90mV

Question 32

What are other names for the resting membrane potential?

Answer 32

cell membrane potential membrane potential Vm, Em, φm

Question 33

The membrane potential can either be __________ or __________

Answer 33

static | dynamic

Question 34

What is a static membrane potential? | give an example of a cell with this

Answer 34

epithelial cell when you move ions across the membrane, you change the potential which is why cells regulate the movement Na+ and K+ (mostly K+)

Question 35

What is a dynamic potential? | give an example of a cell with this

Answer 35

muscle cell or nerve cells open voltage gated Na+ channels, Na+ rushes in so you change the ratio of Na+ inside and outside and you generate an action potentials (the membrane potential goes from -60mV to +60mV)

Question 36

When are diffusion potentials created?

Answer 36

- when you have a concentration gradient | - and a selective membrane

Question 37

Why do all cells have a potential?

Answer 37

Because there are concentration gradients across cell membranes. Cell membranes are selective and have ion channels and carriers

Question 38

Explain how you can set up an experiment to show how diffusion potentials work

Answer 38

You can set up two compartments of a bath separated by a membrane. This membrane contains channels that are permeable to K+ but not to Cl-. There is a voltmeter which allows the measurement of any voltage that develops across the membrane. A high concentration of K+ and Cl- are added to the left and a low concentration is added to the right.

Question 39

Describe diffusion potentials with reference to the experiment

Answer 39

Initially an equal number of positive and negative ions are on each side of the membrane and therefore Em = 0mV. K+ diffuses down its concentration gradient but Cl- can't (the membrane is not permeable to Cl-). As a result, the right hand side develops an excess of positive charge and the left hand has an excess of negative charge. As charge separation occurs, an electrical gradient (voltage) develops. The electrical gradient opposes the concentration gradient (the electrical gradient develops until it balances the chemical gradient). At this point, there is no net movement of ions (Jnet = 0). The system is in equilibrium and we have an equilibrium potential

Question 40

What would cause a larger equilibrium potential?

Answer 40

a larger concentration gradient between the sides of the cell

Question 41

What is the value of the equilibrium potential? (ie. the electrochemical gradient equilibrium)

Answer 41

At equilibrium the chemical potential equals the electrical potential but they are operating in opposite directions. Therefore the electrochemical gradient is zero and there is no net movement of ions

Question 42

What is the Nernst Potential? What does it calculate? | What do the

Answer 42

``` This calculates the diffusion potential for an ion (ie. if diffusion can occur freely what will the membrane potential be when the electrical and chemical gradients are equal and opposite) Ex = (RT/zF)Ln([X]o/[X]i) where Ex = the equilibrium potential for ion x R = gas constant T = temperature in Kelvin z = valency F = Faraday's constant [X]o = concentration of ion outside [X]i = concentration of ion inside ```

Question 43

If there is an ion gradient across a membrane and the membrane is permeable to that ion then either what two things happen?

Answer 43

- the membrane potential will change to the equilibrium potential for that ion OR - if the potential is held at the equilibrium potential for that ion there will be no net movement of the ion across the membrane

Question 44

How would we know if the membrane potential (Vm) an equilibrium potential?

Answer 44

We could have to calculate the equilibrium potentials for the various ions and compare them to the membrane potential