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MBB266 > Membranes (Williamson) > Flashcards

Membranes (Williamson) Flashcards

1
Q

From an evolutionary perspective, why did membranes arise?

A

To define a barrier between ‘inside’ and ‘out’.

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2
Q

What is the key function of membranes that all cells perform?

A

Transport of chemicals in/out (nutrients/waste)

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3
Q

What functions of membranes evolved later?

A

Conversion of membrane potential to energy. Cellular recognition. Signalling from outside to inside. Molecule trafficking (internal).

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4
Q

Why do Eukaryotic cells need to compartmentalise?

A

They are so much larger than Prokaryotic cells. Different functions must be enclosed, molecules must be transported to different places.

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5
Q

Membrane lipids have hydrophobic and hydrophilic ends. What does ths cause?

A

Spontaneous aggregation.

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6
Q

In vitro, what structures can membrane lipids spontaneously aggregate into?

A

Lipid bilayer. Liposome. Vesicle.

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7
Q

What does FRAP stand for?

A

Fluorescence Recovery after Photobleaching.

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8
Q

What did using FRAP on membrane proteins demonstrate?

A

That membrane proteins are laterally mobile.

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9
Q

What does Atomic Force Microscopy of a membrane show?

A

That the bilayer is tightly packed with proteins.

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10
Q

AFM showed membranes to be tightly packed with proteins. What does this imply?

A

That membranes may not be as fluid as the the Fluid Mosaic Model implies.

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11
Q

What type of lipids make up most of the membrane?

A

Phospholipids (phosphoglycerides)

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12
Q

Phospholipids bind the tails with Os. What do sphingolipds use?

A

NH

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13
Q

What double bond configurations are often found in sphingolipids?

A

Trans

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14
Q

How do cells alter membrane fluidity?

A

My changing the lipid composition of the membrane

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15
Q

How does lipid composition affect fluidity?

A

Cis double bonds take the lipid chains off at angles, whereas trans double bonds allow the chain to extend linearly.

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16
Q

What lipids does cholesterol pack against?

A

Trans double bond lipds.

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17
Q

What affect does cholesterol packing have on the lipid bilayer?

A

It flattens the lipids making them longer (increasing the bilayer length)

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18
Q

PE (Phosphatidylethanolamine) has a smaller head group than PC (Phosphatidylcholine). What effect does this have on the bilayer?

A

PE in the bilayer caues it to curve.

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19
Q

Describe 5 ways in which lipids are used to control membrane curvature.

A

1) Lipid composition - head group and acyl chain 2) Membrane proteins - shape and oligomerisation 3) Cytoskeleton - Internal actin control/external motor control 4) Scaffolding - Indirect/Direct +ve, -ve 5) Amphipathic helix insertion

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20
Q

Which orientation are GPI anchored proteins in?

A

Outside

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21
Q

Which orientation are lipid anchored protein in?

A

Inside

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22
Q

What are flippases and what do they do?

A

Flippases are ATP dependant enzymes that flip lipids in the bilayer.

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23
Q

What is phase separation in membranes?

A

Groups of similar lipds coming together to form clusters.

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24
Q

What is a membrane raft?

A

Thicker membrane regions with more cholesterol and sphingolipids.

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25
Q

Proteins also form groups based on similarities. Describe some group types.

A

Proteins with TM helices. Proteins with GPI anchors. Proteins with palmitoyl anchors. Proteins with prenyl anchors.

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26
Q

Is membrane raft formation spontaneous?

A

No.

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27
Q

Why is membrane raft formation controlled by the cell?

A

As this can form a control mechanism for various systems. eg Bringing signalling systems together. Starting endocytosis. T Cell activation.

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28
Q

Membrane rafts can be used to bring proteins together or to move them apart. What kind of proteins can be added/removed from these rafts?

A

GPI linked, prenylated etc - any covalently linked protein.

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29
Q

What major proteins are involved in ligand-mediated endocytosis?

A

Caveolins

30
Q

How do caveolins lead endocytosis?

A

They insert halfway into the membrane, causing it to curve inwards.

31
Q

What is patch clamping used for?

A

Measuring conductance across a membrane (ion channels)

32
Q

What does the patch clamping data show? (There are two ion channels)

A
  • Ion channels have the same current when open
  • Opening/closing is random
33
Q

What types of ion transporters are found in an axon?

A
  • Voltage-gated Na+ channel
  • Voltage-gated K+ channel
  • Na+/K+ pump
  • K+ leak channel
34
Q

What is the typical resting membrane potential?

A

-60mV

35
Q

At what membrane potential does the Na+ channel begin to open?

A

-40mV

36
Q

What is the Na+ channel plug, and when is it ‘used’?

A

It plugs the open channel after it has been open for ~1ms. It remains closed until normal resting potential has been re-established.

37
Q

What is the main difference between the K+ channel and the Na+ channel?

A

The K+ channel opens and closes slower than the Na+ channel.

38
Q

What does the Na+/K+ channel do?

A

It pumps 3Na+ out for every 2K+ in. It is ATP dependant.

39
Q

What to K+ leak channels do?

A

They are always open, allowing small amounts of K+ to leak out, giving the cell membrane a negative potential.

40
Q

What are nerve impulses?

A

Transient changes in membrane potential.

41
Q

Since nerve impulses are ‘all or nothing’, how is a stronger signal created?

A

More rapid impulse firing.

42
Q

An action potential initally depolarises the membrane to the threshold potential of -40mV. What happens at this point?

A

Na+ channels open, causing Na+ to rush in to the cell. This causes the membrane potential to rush up to around +35mV.

43
Q

After the peak of the action potential (+35mV), what happens?

A

The delayed K+ channels start to open. Potassium leaves the cell reducing the membrane potential. It becomes hyperpolarised ( > -70mV)

44
Q

What is a useful consequence of membrane hyperpolarisation in action potentials?

A

Action potentials can only travel forwards.

45
Q

How many sodium and potassium ions are moved in/out of the cell, per μm2 of membrane during an AP?

A

105 of each ion.

46
Q

During an AP, what percentage of total cell ions are used?

A

1% of all cell sodium/potassium.

47
Q

What is the consequence of the percentage of ions used in an AP?

A

APs are very energy efficient.

48
Q

What is the speed of a action potential down an unsheathed axon?

A

1ms-1

49
Q

Why is the myelin sheath of vital importance?

A

It speeds up nerve impulses 100x. 1ms-1 is far too slow for any useful kind of nerve signalling.

50
Q

How long are the myelin sheath nodes, and how often do they occur?

A

They are 1μm long and occur every 100μm of nerve.

51
Q

What do myelin sheath nodes do?

A

They allow the nerve impulse to jump, speeding the impulse up.

52
Q

What are the four major pathways of signal transduction into the cell?

A
  1. Hydrophobic molecule diffusion
  2. Ion Channel
  3. GPC Ligand Receptor
  4. Ligand Enzyme Receptor
53
Q

What are G-proteins?

A

Molecular switches

54
Q

How do G-proteins change between on/off states?

A

They are on when bound to GTP and off when bound to GDP

55
Q

How does GTP being bound maintain the G-protein in an on state?

A

The extra phosphate in GTP (compared to GDP) forms H bonds with Thr35 and Gly60 in the ‘spring sections’.

56
Q

In the cell, which is higher - [GTP] or [GDP], and by how many times?

A

[GTP] is 10x higher than [GDP]

57
Q

What are the two factors that aid G-proteins, and what do they do?

A
  1. GEF - Guanine Exchange Factors - it swaps GDP for GTP
  2. GAP - GTPase-activating protein - helps the G-protein to hydrolyse GTP
58
Q

What mechanism do receptor-linked kinases use to transduce signals?

A

Dimerisation

59
Q

When a ligand binds to a R-l kinase, and dimerisation occurs, what happens next?

A

The R-l kinase autophosphorylates.

60
Q

What are Grb2 and Sos proteins?

A

Modular signalling-adaptors

61
Q

What domains does Grb2 contain, and what do they do?

A
  • SH2 - binds phosphorylated R-l kinases
  • SH3 - recognises polyproline helices
62
Q

What does Sos bind to?

A

The polyproline helix in Sos binds to SH3.

63
Q

Once Sos is complexed, what does it activate>

A

It brings GEF to the cell surface to activate a cell surface linked Ras (G-protein)

64
Q

Once Ras becomes active, what does it activate?

A

A kinase called Raf

65
Q

What is the general structure of GPCRs?

A

7 transmembrane helices

66
Q

GPCR intracellular loop 3 binds to what?

A

A heterotrimeric G protein (Gα, Gß, Gγ)

67
Q

In the trimeric G-protein, which subunit bind GDP/GTP?

A

Gα

68
Q

Once Gα is activated, what does it do?

A

Moves along the membrane looking for a target to activate.

69
Q

What is a typical Gα target?

A

Adenylyl cyclase

70
Q

What does adenylyl cyclase do?

A

It converts ATP to cAMP, which acts as a secondary messenger.

71
Q
A

MBB266 (4 decks)

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