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1

Membrane Permeability: ions and polar molecules

cannot cross - impermeable
Na+, Cl-, sugars, a.a.

2

Membrane permeability: small, uncharged, somewhat polar

molecules can cross
glycerol, ethanol

3

Membrane permeability: hydrophobic molecules, gases

cross quickly
O2, CO2, N2, cholesterol based steroid hormones, hydrophobic (most drugs)

4

What is the permeability of morphine?

somewhat, polar therefore can cross the membrane

5

Heroin permeability across membrane?

crosses fast cuz its acetylated morphine and hydrophobic

6

Where and why are there ion concentration gradients?

- PM and organelle membranes
- ionic composition differs in cytosol and extracellular environment

7

Simple diffusion occurs in what direction?

From high to low concentration gradient
- spontaneously therefore delta G is negative when moving from high to low

8

The energy required to maintain the chemical gradient is delta G (+ve or -ve)

to maintain therefore +ve.
when moving down gradient = -ve

9

What is the equation for free energy?

delta g = RT ln c
c being c2/c1

10

If you were to transport hydrophillic solutes across the membrane without aid, how is this down and what is the velocity?

- slowly
- very few solutes have enough activation energy to overcome the barrier
- it must break the solvent-solute (h20) bonds first, pass, then reform

11

If you were to transport hydrophillic solutes across the membrane with a transporter, how fast would this be?

- with transporter
- reaches same equilibrium but
- FASTer
- lower activation energy needed

12

Membrane Channels vs Membrane Transporters? Difference in flux, saturation, gated?

Channels:
very fast
not saturable
gated open/close to stimuli
Transporters:
slow
saturable
no gate

13

Membrane Channels

- solutes flow through rapidly
- via diffusion
- not saturable (rate of transport is dependent on the concentration of the substrate) - down the gradient
- gated: open and close in response to stimuli
- highly selective - many types of channels

14

Passive Transporters

- facilitated diffusion
- down a concentration gradient
- highly selective - sterospecific (D vs L a.a.)
- transport one molecule at a time; saturable binding sites
- not a continuous pore, changes open/close

15

How do you increase the velocity of passive transporters?

- increase number of transporters since one transporter transports one set of molecules at a time

16

Are aquaporins channels or transporters?

Water channels
very fast

17

How do transporters work?

- substrate binds on one side
- conformational change
- other side opens
- substrate released
- conformational change to original side open

18

What are GLUT1, GLUT2, GLUT4 transporters and where are they expressed/roles, respectively?

- Glucose transporters
- GLUT1: ubiquitous - RBC and brain; basal glucose uptake (imports glucose)
- GLUT2: liver - removal of excess glucose in the blood; pancreas - regulation of insulin release; intestines; (exports glucose)
- GLUT4: muscle, fat, heart - activity increased by insulin; insulin sensitive and critical for diabetes to increase glucose uptake

19

Explain the GLUT4- total body glucose uptake graph with time.

- GLUT4 uptakes total body glucose
- insulin regulates GLUT4 uptake
- if normal, GLUT4 transporter will follow the concentration gradient and be selective to glucose
- as you give insulin (without resistance) with time, the glucose uptake increases significantly
- if you take away insulin, there is not a lot of glucose uptake (glut4 decreased) and you are left with lots of glucose in the blood

20

Active transporter

- transports agains concentration gradient
- pumps
- poweredby ATP hydrolysis
- ion gradients generated across membrane

21

How are cells kept from swelling?

Water association with Na+. 3 Na+ pumped out due to Na/K ATPase

22

Ion gradients: Na+, Cl-, K+, Ca2+. Which ones have high concentraton outside of the cell.

Na+, Cl-, Ca2+ - high outside
K+ high inside

23

What are the 3 classes of membrane transporters:

- Uniporter (one, one direction)
- Symporter (2 same direction, co transport)
- Antiporter (bidirectional, co transport)

24

Membrane potential is measured in what units? What is the definition?

- a charge imbalance as a result of a charged molecule moved across a membrane
- free energy is different on sides of membrane
- measured in volts

25

What is the inside of a plasma membrane at rest?

= - 60mV (between -50 to -70)

26

What is the equation for an electro chemical gradient?

delta G = RTlnc2/c1 + zFdeltaV

27

Na+K+ ATPase. What are the 4 points that it does?

- generates gradients of Na+ and K+
- controls cell volume (pump out water)
- drives active transport of other species (i.e. secondary active transport of Na+/Glucose)
- electrically excitable (nerve cells)

28

What is the tertiary structure of the Na+K+ ATPase?

tetramer of a2b2
- a performs the transport

29

What is the net charge generated by the Na+K+ ATPase?

- +ve net charge OUT
- membrane potential cuz -ve inside

30

What is the power stroke of this transporter?

- conformational change due to phosphorylation

31

ATPase transport cycle: Step One

bind 3 Na+ cytoplasmic to inside of cell

32

ATPase transport cycle: Step Two

Na+ binding stimulates the phosphorylation by ATP on the cytosolic side
- ATP adds a phosphate to the enzyme, ADP released

33

ATPase transport cycle: Step Three

Phosphorylation causes a conformational change
- release Na+ to extracellular outside
- affinity of Na+ decreased

34

ATPase transport cycle: Step Four

K+ binds the extracellular side
- this triggers the release of the phosphate group
- dephosphorylation

35

ATPase transport cycle: Step Five

- dephosphorylation causes conformational change and restore to original shape of the enzyme

36

ATPase transport cycle: Step Six

- 2 K+ released affinity decreases without P
- and cycle repeats
- this step works agains the concentration gradient but favours the electrical gradient cuz more negative interior and adding more positive

37

What is a secondary active transporter?

The transport of ion DOWN its GRADIENT can transport another isolute UP its gradient

38

What is an example of a secondary active transporter?

Na+ glucose symporter.
- Na+ is pumped out of the cell via Na+K+ ATPase
- Na+ is brought back into the cell by going DOWN the gradient
- glucose is brought into the cell in this symporter going UP the gradient
- going UP gradient so its an active transporter

39

Where is the Na+-glucose symporter located in the body?

- microvilli - intestinal lumen to epithelial cells (between intestine and blood)
- Na+ and glucose are brought into the epithelial cell

40

What drives the Na+- glucose symporter? How many molecules are needed?

high Na+ gradient outside, needs to go down gradient
- 2 Na+ are needed to drive glucose UP the gradient
(vs 3+ Na+ out for sodiumpotassium pump)

41

What mechanisms are involved with Na+/K+/Glucose between the epithelial cell and blood?

- NA+ K+ ATPase active transporter
- Na+ is brought out of the epithelial (opposing gradient)
- K+ is brought in
- glucose is brought out via glucose uniporter

42

GLUT2 is what type of transporter?

- passive transporter
- uniporter
- downhill efflux

43

How are ion channels gated?

- ligand gated
- voltage gated

44

How do ion gated channels affect neurons?

- presynaptic (ligand - Ach receptor ion channel)
- post synaptic - action potential response to change in voltage - Na+/K+ (de)polarize

45

Explain the first step to a nerve impulse?

1. Ach is the ligand that causes a small Na+ influx and slight depolarization
- this occurs in the cell body
- nerve impulse sends the Ach as a signal

46

In the graph, the second part to change in membrane potential is?

- depolarization
- occurs because of Na+ in = depolarize
- a full Na+ influx

47

In the graph the 3rd part of the change in membrane potential is?

- K+ efflux (out) = repolarization
- establish potential again

48

Where does an action potential occur and what does this mean?

- action potential occurs at the top +3-mV
- means the nerve has been fired = reaction
- action potentials are all or none, it must reach the threshold for a fire

49

What is the ionic composition/gradients in neurons?

high K+ cytosol
low Na+ cytosol

50

What transmits a nerve impulse?

- action potential
- neurotransmitter

51

What does the action potential do in a neurotransmitter?

- it carries the electrical signal down the axon

52

What does the neurotransmitter do in a neuron?

- it carries a signal molecule to the next cell

53

The Na+K+ ATPase in a neuron causes the _____. Ion voltage gated channels causes the _____.

- electro-chemical gradient
- action potential

54

What is the structure of the voltage gated K+ channel.

- tetramer
- each subunit with 2 transmembrane helices and a shorter helix - selectivity filter
- 2 outer helices in each subunit interacts with bilyayer
- inner helices contribute to inner pore

55

What causes the channel to be closed?

The +ve extracellular space interact with the +ve helix dipole from 4 Arg/Lys +ve residues
- electrostatic repulsion pushes down transmembrane helix to pinch off the channel

56

How is K+ stabilized in the channel/selective?

- K+ interacts with the carbonyl oxygens (O coordinates with unhydrated K+)
- it forms a cage precisely
- stabilizes it and replaces stabilizing interactions with O from water sphere and water molecules

57

How is K+ channel selective?

- size
- partial negative charges of C=O
- consensus sequence for K+ = gly-tyr-gly-val-thr

58

How does the K+ pass through?

the gate is opened when the membrane potential changes
- the depolarization causes the outside environment to become more negative and the +ve helices shift up
- open gate and release K+

59

What is the structure of voltage gated Na+ channels?

- 4 domains
- 6 transmembrane helices (S1-S6) each

60

Which helices of the voltage gated Na+ channel forms the central channel?

S5 and S6

61

What helix is the voltage sensor?

S4

62

What happens to the Na+ channel when the membrane is depolarized?

- voltage change induces conformational S4 (less +ve outside)
- S4 moves up.out of membrane
- channel opens

63

What helix is the activation/inactivation gate?

S6

64

How is the Na+ channel blocked?

- S4 voltage sensor helix has 4 Arg/Lys that repel the +ve exterior
- this pushes down on the channel/closed when at resting potential

65

At what membrane potential is the channel fully open?

-70 mV to +30 mV
- must pass threshold

66

What causes the S4 to pop up initially in order to open the channel and depolarize?

the neurotransmitter release
- depolarize at cell body to decrease repulsion
S4 pops up

67

What is the inactivation gate. Fast/slow?

- it occurs quickly
- if u increase the tether u increase the time to close and vice versa

68

What does the selectivity filter do?

- the part of the pore region that only allows Na+ in

69

How does the activation gate work?

S5 and S6 helices
form the channel
S6 is the activation gate and allows for the open/close

70

What are two defective ion channels and their result?

Na+ channels in muscle - causes paralysis
Na+ channels in neurons - stop action potentials i.e. terodotoxin binds Na+ channels of neurons
Na+ channels also inhibited by anaesthetics like lidocaine and cocain to dapen down CNS (anti-epileptic, anti-arrythmic drugs)