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Cell Biology > The Plasma Membrane > Flashcards

The Plasma Membrane Flashcards

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1
Q

what are the role of compartments?

A
  • to get signals to meet
  • recruit signals with proteins in the membrane so the signalling molecules are ready next to each other
  • designate certain compartments for specific signalling pathways
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2
Q

how do lipids assemble?

A

spontaneously assembly

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3
Q

what are most lipids?

A

phospholipids

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4
Q

what is a micelle?

A
  • head groups are out
  • tails are pointing inwards
  • shape of a globe
  • not generally formed in biological systems
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5
Q

what is a bilayer?

A
  • heads outside and inside (they are hydrophilic)
  • tails are inside the layer (hydrophobic)
  • can form a liposome and form a flat bilayer
  • the shape of lipids can determine whether it becomes a flat bilayer or a round structure
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6
Q

what happens if the lipid is shaped like a cone?

A
  • the head has a much larger surafce area than the tail
  • lipids will form a circle (shape of a micelle)
  • same applies in a bilayer forms a liposome
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7
Q

what happens if the lipid is shaped like a cylinder?

A
  • same cross section as the tail

- forms a bilayer

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8
Q

why does the lipid bilayer need to close up?

A

cant be exposed to water at the edges

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9
Q

what is membrane curvature determined by?

A

shape of lipid

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10
Q

what lipid shapes do vesicles contain?

A

lipids that slightly cone shaped so they curve

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11
Q

is the membrane gel or fluid like?

A
  • gel-fluid transition of membranes is temperature dependent
  • transition temperature depends on membrane composition
  • organisms living at different temperatures need to control their membrane composition to maintain the correct fluidity
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12
Q

what is the fluid mosaic model?

A
  • things can move around but still keeps a barrier
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13
Q

how can you visualize a membrane with rabbit erythrocytes in an electron micrograph?

A
  • membranes stained with ricin
  • ricin binds to sugars on the surface of the plasma membrnae
  • doesn’t bind to inside components
  • can see both sides of the membrane
  • inside and outside are very different
  • proves that the membrane is asymmetrical
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14
Q

what does photobleaching show of a membrane?

A
  • fluorescence recovery after photo bleaching (FRAP) of a membrane bound dye
  • clustering of membrane components
  • shows free diffusion in the membrane
  • not every membrane component domain diffuse through
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15
Q

how is thickness determined?

A
  • by phospholipids
  • saturated acyl chains (non-fluid)
  • unsaturated (fluid)
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16
Q

what do steroids effect?

A
  • they increase lipid packing and thickness
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17
Q

why is membrane thickness important?

A
  • membrane proteins choose which membrane they go to based on thickness
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18
Q

what does fine tuning curvature, fluidity and thickness allow control over?

A
  • lipid and protein diffusion
  • membrane plasticity
  • protein conformational change for functions (transport, signalling and enzyme activity)
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19
Q

where are lipids initially made?

A

in the smooth ER

- modulates kind of phospholipid membrane composition and therefore tightly control how you make phospholipids

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20
Q

how are phospholipids made?

A
  • acyl transferase and phosphatase genes are the basic phospholipid building block of DAG
  • phospholipids are synthesised separately
  • different acyl transfereases can output different fatty acid chains
  • large diversity (many possible acyl chain and head group combinations)
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21
Q

what are the roles of transferases?

A

they can stick on head groups (this can be modified)

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22
Q

how is there lipid remodelling in the secretory pathway?

A
  • unsaturated fatty acid chains replaced with more saturated (ER to PM)
  • more cholesterol and spingolipids in late secretory pathway
  • carbohydrate head groups of glycolipids to glycans in the Gollgi
  • membrane becomes more asymmetrical and thick towards the plasma membrane
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23
Q

what is vesicle lipid trafficking?

A
  • some phospholipids packaged in vesicles in the ER

- changes membrane composition

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24
Q

what is free diffusion?

A
  • some arent as hydrophobic and so can move
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25
Q

what is an LTP?

A
  • lipid transfer protein

- can grab phospholipids from one membrane and move them to another

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26
Q

what are membrane contact sites?

A
  • allowed lipid transfer much more fficiently (important in maintaining composition)
  • membranes are close so LTP can sit between them and move back and forth
  • exist between compartments that may not necessarily otherwise communicate
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27
Q

what are the functions of MSC?

A
  • lipid transfer
  • coordinated Ca2+ release facilitate signalling and cytoskeletal dynamics
  • aid in organelle fission
  • generate a new membrane structure
  • aid in protein sorting between organelles
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28
Q

what is cholesterol?

A
  • essential component in membranes in humans
  • alters membrane fluidity
  • part of ‘membrane rafts’
  • has this effect because of its structure
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29
Q

what is the structure of cholesterol?

A
  • a small head group which is hydroxyl, the rest of it is hydrophobic
  • hydrophobic part is very rigid
  • a ring structure
  • wedges itself into the membrane and stops the membrane moving around
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30
Q

how is cholesterol made and transported?

A
  • not made by host human cells
  • transported to all tissues by LDL
  • binds to LDL receptor
  • delivers cholesterol into the cell through endocytosis
  • if theres too much then the LDL receptor isnt made
  • LDL just sits in the body waiting to be used, it will sit and deposit in the arteries (clogging)
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31
Q

what is HDL?

A

transport cholesterol from the cells to the liver to be degraded

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32
Q

where are glycolipids made?

A

made in the ER and Golgi by the addition of monosaccharides

33
Q

how are glycolipids made?

A
  • addition of monosaccharides including ceramide

- first addition are on the cytosolic or luminal faces of the Golgi and ER

34
Q

what additions happen in the Golgi?

A

most additions, often branched oligosaccharides

35
Q

where are glycolipids important?

A
  • important function of lipid rafts
  • important in immunity (blood groups)
  • gangliosides (components of neuronal membranes)
36
Q

why are membrane proteins important?

A
  • synaptic vesicle is full of proteins
  • lipid bilayer, lipids and lots of protein
  • proteins alter the structure and function of membranes too
37
Q

what can membranes be covered in?

A

glycoproteins and glycolipids

38
Q

what are peripheral membrane proteins?

A
  • proteins attached to the surface
  • or lipid binding
  • they bind the membrane surface and do not cross the membrane
  • head group of phospholipid can bind to a groove in the protein (polar pocket)
  • or a groove that recognises a specific ligand
  • hydrophobic protrusion that penetrates into the membrane
  • clusters of basic residues that bind anionic phospholipid head groups
39
Q

what are the clusters of basic residues that bind anionic phospholipid head groups?

A
  • BAR domain has a positive charged domain

- has 2 effects: can shape the membrane sometimes and attach

40
Q

how can amphipathic helices cause membrane deformation?

A
  • one side hydrophilic and one side hydrophobic
  • polar and hydrophobic amino acids on the opposite sides of the helix
  • sits in the outer leaflet
  • wedges membrane and initiates curvature
41
Q

how can loop insertion cause membrane deformation?

A
  • inserts deeper

- loops hydrophobic amino acids

42
Q

how can curved lattices cause membrane deformation?

A
  • lattices bind cargo proteins but the lattice forms a curved polymer
43
Q

how can BAR domain proteins cause membrane deformation?

A
  • proteins bind the bilayer via a curved surface eg BAR domain containing proteins
44
Q

what is the association of peripheral proteins with membranes described as?

A

dynamic - goes on and off the membrane

45
Q

what does the association of peripheral protein with membranes depend upon?

A
  • type of membrane
  • Ca2+ concentration
  • availability of lipid species
  • shape of membrane
46
Q

how are peripheral proteins anchored in the membrane?

A
  • have a lipid anchor, a lipid tail is added to the protein which inserts into the membrane
  • lipid anchors can be tucked away when the protein is in the cytosol
47
Q

is lipidation reversible?

A

yes

48
Q

what is a GPI anchor?

A

essentially a phospholipid and then a glycan

  • associated with lipid rafts
  • on the outside
49
Q

what is a transmembrane protein?

A
  • an integral membrane protein
  • they cross the membrane so theres proteins on both sides of the membranes
  • they are embedded
50
Q

what is type IV transmembrane protein?

A
  • polytopic (like GPCR)
  • cross the membrane several times
  • membrane proteins with more than one TM domain
51
Q

what is type III transmembrane protein?

A
  • crosses the membrane once
  • majority is in the cytosol
  • tail anchored proteins
  • no class signal peptide
  • put into the membrane through special machinery
52
Q

what is type II transmembrane protein?

A
  • carboxyl terminus is the lumen side, NH3+ is in the cytosol
  • signal peptide targets it to membrane but stays on cystolic side
53
Q

what is type I transmembrane protein?

A
  • carboxyl terminus on to cytosol side

- signal peptide guides N terminus across the membrane

54
Q

what does the alpha helix in the transmembrane protein allow?

A
  • helices allow them to cross the membrane
  • the alpha helix spans the membrane
  • the helix has a peptide backbone
  • carbonyl binds with hydrogen bonds to amino groups
  • out from the helix there are side chains
  • as long as the amino acids have a hydrophobic side chain you can cross the membrane
55
Q

how long are the helices that cross the membrane?

A

helices of around 20 hydrophobic amino acids spans the membrane

56
Q

what do aromatic amino acids often contact?

A

lipid head groups

57
Q

what are non-hydrophobic amino acids in membrane helices?

A

they have functional roles

- eg positive charges in the voltage sensor of some channels

58
Q

how can you identify a TM domain?

A
  • look for a stretch of 20 amino acid stretches of hydrophobic amino acids
59
Q

how long is a transmembrane domain?

A

matches membrane thickness

60
Q

what can make membranes more soluble?

A

detergents

61
Q

how do detergents effect the membrane?

A
  • a little bit inserts into the bilayer
  • more detergent means the membrane breaks up
  • a high amount removes lipid into micelles
62
Q

what components of the membrane can resist solubilzation in some detergents?

A

lipid rafts

63
Q

what does decreased membrane fluidity lead to?

A
  • lipid rafts
64
Q

what are the features of lipid rafts?

A
  • contain a lot of cholesterol
  • low fluidity = less diffusion
  • have specific interactions (have TM proteins which has cholesterol binding site)
  • fatty acid modified lipidated proteins
  • GPI anchors prominently found in the rafts
65
Q

when do lipid rafts occur?

A
  • happens when there is strong association between lipids

- proteins bind lipids via their transmembrane domain or can be lipid anchored to become raft associated

66
Q

what are the functions of lipid rafts?

A
  • signalling (platform for assembling signalling proteins)
  • protein function (rigidity alters protein conformation)
  • host pathogen interactions
67
Q

how is the plasma membrane a barrier?

A

limiting membrane separates the outside from the inside and therefore needs transport

68
Q

how do water soluble molecules get through the membrane?

A
  • need proteins to help them through

- water itself needs a protein channel

69
Q

what do transport proteins do?

A
  • actively transport to increase concentration
70
Q

why do lipids need to be transported?

A

they need special transporters to generate lipid asymmetry

71
Q

how can gradients be generated?

A

selective membrane transport

72
Q

what are pumps?

A
  • active transport
  • fuelled by energy (ATP)
  • high affinity, used to accumulate solutes against large gradients
73
Q

what are carriers?

A
  • not directly energized
  • transporting along a concentration gradient or using a secondary solute
  • faster than pumps, slower than channels
74
Q

what are channels?

A
  • diffusion, pores, passive very fast transport along a concentration gradient regulating open or shut state
  • often specific for an ion
  • transports water
75
Q

what is lipid asymmetry?

A
  • need this for the functional working of the lipid bilayer
  • synthesis of many phospholipids occurs on the cytoplasmic of the ER
  • hence PE and PS are on the correct leaflet but not PC
76
Q

what are flippases?

A

transport out to in

77
Q

what are floppases?

A

transport in to out

78
Q

what do flippases and floppases do?

A

both pumps use ATP to get the phospholipid headgroup across the membrane against its concentration gradient

79
Q

how is water transported across the membrane?

A
  • aquaporin is a water channel

Cell Biology (10 decks)

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