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BIOL 201 > Topic 4: Membranes and Transport > Flashcards

Topic 4: Membranes and Transport Flashcards

1
Q

What is the function of the biological membrane?

A
  1. Define boundaries; compartmentalize organelles
  2. Compartmentalization allows unique functions for organelles
  3. Regulate movement of molecules
  4. Cell-cell communication
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2
Q

What is the structure of the biological membrane?

A

composed of amphipathic molecules, contain both hydrophobic and hydrophilic domains

phospholipids are major constituent of the biological membrane

2 fatty acids attached to a glycerol backbone with a phosphate head

fatty acids, hydrocarbon chains 12-20 carbons long, can be saturated (all C-C) or unsaturated (C=C)

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3
Q

What is the basic structure of a phospholipid?

A

polar head group (phosphate head (-) charged)

C=C causes kink in the chain

two fatty acids attached to 3C glycerol backbone

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4
Q

What are three important properties of the biological membrane?

A

stable and self healing

provides sealed and closed compartments

hydrophobic core = major permeability barrier

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5
Q

What is the lipid composition of the biological membrane?

A

phosphoglycerides/phospholipids

sphingolipids

cholesterol

membrane glycolipids

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6
Q

What are phosphoglycerides and phospholipids?

A

derivatives of glycerol-3-phosphate with 2 fatty acyl chains esterified to glycerol backbone and polar head attached to a phosphate group

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7
Q

What are the four different head groups that can attach to the phosphate?

A

PE: small, NH3 containing head group

PS: amino acid head group

PC: largest head group, most abundant

sphingomyelin

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8
Q

What are sphingolipids?

A

polar head group is derived from sphingosine (amino alcohol with large HC tail)

amphipathic but not as cylindrical as phospholipids therefore don’t form bilayers

larger than phospholipids

most common type: sphingomyelin (forms myelin sheath)

sphingolipids form signaling centers called lipid rafts

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9
Q

What is cholesterol?

A

cholesterol derivatives make up the steroids

basic structure is at 4 ring hydrocarbon

has a single OH giving it amphipathicity

abundant in mammalian cells that acts to modulate membrane fluidity

up to 50% of the total membrane lipid in animal cells is cholesterol

usually found in both membranes of the bilayer

orientates itself in the layer using its single hydroxyl group (polar end) to interact with the polar head group of phospholipid (H-bond)

cholesterol is amphipathic but cannot form bilayers

modulates fluidity at high and low temperatures

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10
Q

What are membrane glycolipids?

A

formed by adding carbohydrate (sugar groups) to the lipids

some are glycerol based, but most are derivatives of sphingosine

glycosphingolipids: glycosylated in ER and Golgi, function in cell attachment and communication

prominent components of brain and nerve cell membranes

blood group antigens within the RBC membrane are glycosphingolipids called A antigen and B antigen

membrane glycolipids face the extracellular space (not cytoplasm)

functions: involved in cell recognition in nervous system, different sugars, different functions

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11
Q

Why is there a difference between fluidity and flexibility between membranes?

A

degree of fluidity and flexibility depends on lipid composition, temperature, structure of tails

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12
Q

What is membrane fluidity?

A

membrane fluidity is crucial for proper permeability and cell function

phospholipids are only held together by hydrophobic interactions therefore they are mobile within the bilayers

3D membrane mobility

lateral movement is rapid and frequent

phospholipids are held together by hydrophobic association

flip flop movement is rare without flippases

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13
Q

How is FRAP used to study membrane fluidity?

A

fluorescence recovery after photobleaching

laser can photobleach the fluor if exposed too long

measure degree of recovery, indicates mobility of components

measure time for recovery, indicates degree of fluidity

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14
Q

How does lipid composition influence fluidity?

A

sphingolipid rich membranes are less fluid therefore larger recovery time

PC is less fluid than PS and PE

cholesterol generally restricts phospholipid movement

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15
Q

How does the structure of phospholipid tails influence fluidity?

A

longer HC tails –> less fluid because more hydrophobic

more C=C –> more fluid because less tightly packed

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16
Q

How does temperature influence fluidity?

A

high temperature = more fluid, shorter recovery

low temperature = less fluid, longer recovery

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17
Q

What do the FRAP curves look like for biological and artificial membranes?

A

biological: slow and incomplete

artificial: fast and complete

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18
Q

What do the FRAP curves look like for membranes with cholesterol and without cholesterol?

A

with cholesterol: rapid

without cholesterol: slower but equally complete

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19
Q

What do the FRAP curves look like for membranes in high and low temperatures?

A

high temperature: fast and incomplete

low temperature: slow and incomplete

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20
Q

What is membrane lipid asymmetry?

A

all biological membranes exhibit an asymmetry across the bilayers

all types of phospholipids are present in both leaflets but they are more abundant on one side (leaflet) compared to the other

asymmetrical distribution of phospholipids

outer leaflet: contains sphingomyelin, PC, and glycosylation
inner leaflet: contains PS and PE

asymmetry may affect membrane curvature because exoplasmic face is less fluid

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21
Q

How is membrane asymmetry accomplished?

A

SER is site of phospholipid synthesis (inserted into bilayer at lumenal face)

flippases flip PE and PS to cytoplasmic side

cytoplasmic face always faces the cytoplasm

22
Q

What are other forms of asymmetry?

A

lipid rafts: microdomains within the bilayers called lipid rafts

rich in cholesterol and sphingolipids

thicker membrane

less fluid

likely signaling centers

23
Q

What are membrane proteins?

A

proteins are the functional unit of cell, therefore the functions of the membrane like cell-cell recognition, attachment and communication are mediated by membrane-associated proteins

24
Q

What are the three ways proteins can associate with the membrane?

A

integral membrane proteins

transmembrane proteins

peripheral membrane

25
Q

What are integral membrane proteins?

A

must be amphipathic in order to span the hydrophobic core of the membrane

cytosolic and exoplasmic space must contain hydrophilic surfaces in order to interact with the water in these domains

the membrane spanning domain must have hydrophobic amino acids exposed on the surface of the protein to span the hydrophobic core of the membrane

alpha helical membrane segments can be inserted: co-translationally at ER and post-translationally at the nucleus and mitochondria

26
Q

What are alpha-helix containing membrane spanning segments?

A

~22 hydrophobic amino acids are required to span bilayer

some pores formed by alpha helical transmembrane domains alternate polar and non polar amino acids

27
Q

What are beta-barrels are another type of membrane spanning domain?

A

contain alternating polar and non polar amino acid create polar pore

e.q. aquaporin beta barrel allows specific passage of H2O

28
Q

What are peripheral membrane proteins?

A

do not directly contact the hydrophobic core of the membrane but, rather, associate with integral or lipid-anchored proteins or direct interactions to lipid-head groups

can be cytoplasmic or exoplasmic but must associate with a membrane component

associate by both covalent and non-covalent interactions

interact with integral membrane proteins via non-covalent association (quaternary structures)

29
Q

What are lipid anchored membrane proteins?

A

bond covalently to one or more lipid molecules

hydrophobic segment of attached lipid is embedded in one of the leaflets of the membrane

anchors protein in the membrane

protein chain does not enter the bilayers

30
Q

What are outer leaflet proteins?

A

proteins that are linked to extracellular surface (GPI anchor, glycosylated inositol)

inositol (PI) is a form of phospholipid head that functions in membrane attachment and cell signalling

31
Q

What are inner leaflet proteins?

A

synthesized in cytoplasm

protein is covalently linked to a fatty acid tail

32
Q

What is trypsin?

A

trypsin digestion allows the identification of the non-transmembrane segments to discover the topology and orientation of membrane proteins

trypsin is a non-specific protease

33
Q

How does each transmembrane protein have a specific orientation?

A

orientation of membrane proteins is established during their synthesis and incorporation into the membrane

transmembrane proteins do not flip flop across the membrane

another level of asymmetry is the glycosylation of membrane proteins by the Golgi

the glyco- part of glycoproteins is only ever found on the exoplasmic face of the membrane

34
Q

How are transmembrane proteins predicted?

A

the topology of a particular protein can be predicted using hydropathy plots

hydrophobic amino acids are counted and given particular values in a computer

a strongly hydrophobic domain suggests a transmembrane segment

can predict whether it will be a C-terminal, N-terminal, exoplasmic, or multipass transmembrane segments

35
Q

What are N-terminal exoplasmic proteins?

A

N terminal signal sequence is highly hydrophobic and must also have a hydrophobic stop transfer sequence

36
Q

What are C-terminal exoplasmic proteins?

A

no N-terminal signal sequence but single strongly hydrophobic peak

37
Q

What are multipass proteins?

A

multiple hydrophobic domains

38
Q

What are hydropathy plots?

A

maps large stretches of non-polar amino acids to help predict alpha helical transmembrane segments, look for broad peaks

39
Q

What is the permeability of the bilayer?

A

permeable to small, uncharged or hydrophobic molecules

relatively impermeable to small uncharged polar molecules

most impermeable to large uncharged and polar molecules

impermeable to charged molecules (ions) because they form favorable interactions with water

40
Q

How do impermeable molecules cross the bilayer?

A

simple diffusion: movement across hydrophobic core, even impermeable can leak

facilitated diffusion: use of a transmembrane channel or carrier, can be gated –> open or closed during different conditions

active transport: movement of an impermeable molecule up its gradient, low concentrations to high concentrations, requires input of energy

41
Q

How are transporters involved in facilitated diffusion?

A

the role of transport proteins is to provide a path through the lipid bilayer, facilitating the “downhill” diffusion of the polar or charged solutes

42
Q

What are carrier proteins?

A

changes shape with transport

bind solutes on one side of the membrane

undergo a “conformational” change

transfers the solute to the other side of the membrane

believed to shield the polar or charged groups of the solute

example: GLUT1 carrier, moves glucose down gradient

43
Q

What are channel proteins?

A

do not change shape

form hydrophilic channels through the membrane

passage of solutes without a change in the proteins conformation

some are non-specific

most are highly selective (e.g. ion channels)

rapid transport relative to carrier proteins

can be beta barrels or composed of alpha helices

example: aquaporins, many beta sheet assemble into a barrel, create hydrophilic pore

44
Q

What is an ion channel?

A

permeable to specific ion like Ca2+

each formed by integral membrane proteins that surround an aqueous pore

highly effective

net flux depends on electrochemical gradient

45
Q

What are gated channels?

A

most ion channels exist in either an open or closed conformation

such channels are termed gated

46
Q

What are voltage-gated channels?

A

conformation state depends on charge difference in ionic charge on two sides of membrane

example: K+ channel opens with membrane depolarization

channels are very specific to solute

negative amino acids at opening repels negative ions

gating of this channel is accomplished by a voltage center

47
Q

How do you get ion selectivity?

A

by negative charge at surface

selectivity filter

pore size

48
Q

How does the pore open?

A

voltage gated

opens when inside of cell becomes more positive (depolarization)

normally inside of cell is more negative at depolarization (+50 mV)

at resting, positive charged S4 domain is attracted to inside of cell

at depolarization, positive charged S4 is attracted to outside of cell

shift = open channel

49
Q

What are chemical gated channels?

A

conformational state depends upon binding of a particular substance

example: acetylcholine acts on outer surface of certain cation channels

50
Q

What is active transport?

A

movement of a molecule up a gradient

allows passage of substance through membrane against electrochemical gradient for that substance

active transporter acts by undergoing conformational change upon binding substance

movement occurs only in one direction

three ways that energy can be supplied for active carriers: ATP hydrolysis, coupled transport, light driven

example: Na/K pump, establishes resting potential at -70 mV inside, cell constantly pumps 3 Na+ out and 2 K+ against concentration gradient for each ATP consumed

51
Q

What is coupled active transport of existing ion gradients?

A

movement of two molecules simultaneously, where one molecule moves using the energy gradient from the other molecule

symport: two solutes are moved in the same direction
antiport: two solutes are moved in opposite directions

can couple the resting potential with active transport of another molecule

example: use Na+ gradient as a source of energy for active transport of amino acids or sugar

BIOL 201 (13 decks)

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