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I've got kidney disease > Lecture 5: Active transport and membrane potentials > Flashcards

Lecture 5: Active transport and membrane potentials Flashcards

1
Q

The process of primary active transport is uphill/downhill

A

uphill

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2
Q

Describe the process of primary active transport

A

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

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3
Q

Give three examples of primary active transport

A
  • Na+/K+ ATPase
  • H+/K+ ATPase
  • Ca2+ ATPase
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4
Q

When is secondary active transport?

A

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

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5
Q

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

A

3 Na+ for 2 K+

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6
Q

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

A

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

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7
Q

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

A

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

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8
Q

Na+/K+ ATPase is blocked by what?

A

ouabain

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9
Q

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

A

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

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10
Q

What are the two forms of secondary transport?

A
  • cotransport

- counter transport

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11
Q

What is cotransport?

A

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

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12
Q

What is counte-transport?

A

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

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13
Q

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

A
  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
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14
Q

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

A

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

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15
Q

Summarise primary active transporters

  • energy?
  • what does it move and how?
  • saturation?
  • specificity?
  • competition?
  • inhibition?
A
  • requires metabolic energy (ATP)
  • able to move substrates against their electrochemical gradients
  • saturation kinetics
  • chemical specificity
  • competition
  • inhibition
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16
Q

Summarise secondary active transporters

  • energy?
  • what does it move and how?
  • saturation?
  • specificity?
  • competition?
  • inhibition?
A
  • does not require energy
  • downhill movement of one solute drives the uphill movement of a second solute
  • saturation kinetics
  • chemical specificity
  • competition
  • inhibition
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17
Q

Summarise passive transport

  • energy?
  • what does it move and how?
  • saturation?
  • specificity?
  • competition?
  • inhibition?
A
  • does not require energy
  • not able to move substrates against their electrochemical gradients
  • saturation kinetics
  • chemical specificity
  • competition
  • inhibition
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18
Q

What are the two 2 types of gradients?

A
  • concentration gradients

- electrical potential

19
Q

Why does a concentration gradient exist?

A

because ion gradients exist across the cell membrane

20
Q

Why does an electrical potential exist?

A

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

21
Q

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

A

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

22
Q

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

A

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

23
Q

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

A

the concentration gradient

24
Q

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

A

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

25
Q

Explain both the chemical and electrical gradients of Na+

A

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

26
Q

Explain both the chemical and electrical gradients of Cl-

A

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

27
Q

What is the equation for electrical potential?

What are the units?

A

Δφ = (φ2 - φ1)

units: joule per coulomb (volt)

28
Q

What is the electrochemical gradient?

A

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

29
Q

What is the equation for the electrochemical gradient?

What do the components mean?

A

Δμ = RT ln([C]2/[C]1) + zF(φ2 - φ1)

z = valence (usually 1 in this course)
F = Faradays constant (96500 coulombs per mole)
30
Q

All cells have a what?

A

membrane potential

31
Q

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

A

-60mV to -90mV

32
Q

What are other names for the resting membrane potential?

A

cell membrane potential
membrane potential
Vm, Em, φm

33
Q

The membrane potential can either be __________ or __________

A

static

dynamic

34
Q

What is a static membrane potential?

give an example of a cell with this

A

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+)

35
Q

What is a dynamic potential?

give an example of a cell with this

A

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)

36
Q

When are diffusion potentials created?

A
  • when you have a concentration gradient

- and a selective membrane

37
Q

Why do all cells have a potential?

A

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

38
Q

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

A

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.

39
Q

Describe diffusion potentials with reference to the experiment

A

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

40
Q

What would cause a larger equilibrium potential?

A

a larger concentration gradient between the sides of the cell

41
Q

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

A

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

42
Q

What is the Nernst Potential? What does it calculate?

What do the

A 
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
43
Q

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

A
  • 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
44
Q

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

A

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

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