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Cell Biology (BY1CB1) > Week 5 (lipid Membranes) > Flashcards

Week 5 (lipid Membranes) Flashcards

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1
Q

What is the main functions of membranes?

A

Enclosed reaction compartment
Maintain concentration gradients
Determines what enters and leaves the compartment
Displays an interaction surface with the environment

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2
Q

Prokaryotic membranes

A
  • some have a single membrane
  • outer membrane for protection
    (Fairly permeable to small molecules)
  • inner membrane- permeability barrier
  • region between membranes is teriplasm periplasm
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3
Q

Eukaryotic membranes

A
  • single lipid bilayer cell membrane
  • internal compartments surrounded by specialised membranes
    E.g. mitochondria, nucleus, endoplasmic reticulum
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4
Q

Are charged molecules highly soluble?

A

Yes

Interactions with water dipoles

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5
Q

Are apolar molecules highly soluble?

A

No

No interaction with the water dipoles

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6
Q

Define amphilic (amphipathic)

A

Amphiphilic: Molecules that have polar and non polar parts
- Surfactants/ detergents are amphiphilic molecules as they contain polar and non polar parts
Amphipathic: Molecules orient themselves at the interphase between organic and water phase
(polar heads facing water phase, non polar tails facing organic phase)

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7
Q

Why is phase separation observed?

A

Due to insolubility of water in hexane and vice versa

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8
Q

What is the purpose of Micell formation?

A

Minimises exosure of lipophilic residues to aqueous surrounding

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9
Q

How do detergents work?

A
  1. Soap or detergent dissolves in water
  2. Surfactant ions orientate themselves in grease and water
  3. Agitation begins to separate grease from surface
  4. Process continues
  5. cleaning complete
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10
Q

Describe the structure of phospholipids

A
  • two fatty acyl groups
  • phosphate groups
  • head group
  • glycerol backbone
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11
Q

Describe the structure and function of cholesterol

A
  • intercalates with acyl chains and reduces mobility (fluidity decreases at high temperatures)
  • disrupts orderly packing of extended acyl chains
    (Increase fluidity at low temperature and decreases fluidity at high temperatures)
  • maintains fairly constant fluidity across temperature ranges
  • most of the molecule is hydrophobic except for the OH polar head
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12
Q

Describe the spontaneous formation of membranes

A
  • lipid bilayer form spontaneously
  • growth of bimolecular sheet is spontaneous and rapid in water
  • driven by hydrophobic interactions
  • leads to the creation of cell membranes
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13
Q

Explain the structure of the bilayer membranes

A
  • two lipid sheets
  • polar heads on outside
  • hydrophobic tails inside
  • typically 4-6nm thick
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14
Q

Explain the structure of liposomes/ vesicles

What are they useful for?

A
  • aqueous environment enclosed by lipid bilayer
  • can be formed from planar bilayers
  • bilayer that vary in size from 50nm to > 10 micrometer
  • very useful experimental tools (can form drugs as they can can fuse to the plasma membrane and deliver it’s contents inside)
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15
Q

Explain the movement of lipids in membranes: lateral diffusion

A
  • movement within a leaflet (same side of the bilayer)
  • 2D lateral diffusion
  • rapid e.g. a lipid can diffuse the length of a bacterial cell in 1 second at 37 degrees
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16
Q

what allows membranes to be asymmetric?

A
  • the slow rate of transverse diffusion allows membranes to be asymmetric
  • in bacteria, lipids are added to the inside of the membrane so they need enzymes to flop them to the outer leaflet
  • thus only, one of the monolayers is added to
  • since transverse diffusion is slow, these lipids rarely flip between leaflets
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17
Q

Which model was produced by Singer and Nicholson in 1972

A

Fluid Mosic model:

  • Membranes are 2D solutions of oriental lipids and globular proteins
  • lipid bilayer is solvent for membrane proteins and permeability barrier
  • membrane proteins undergo free diffusion in a membrane
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18
Q

Does temperature affects fluidity of the membrane?

A

Yes

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19
Q

What can freely diffuse through the membrane?

A

Membrane proteins

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20
Q

What does the composition of the membrane determine?

A

The fluidity of the membrane

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21
Q

What is is the effect of temperature on phase transition?

A
  • membranes below the phase transition temperature exist as a gel (solid)
  • above the transition temperature they exist as a fluid
  • changes affect activity of membrane proteins e.g. transport and catalytic activity
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22
Q

What are integral membrane proteins?

A

They are embedded in the membrane (they are ampiphilic)

23
Q

What are the most commonly found phospholipids in biological membranes?

A 
(Phosphatidl)
1. ethanolamine
2. Choline 
3. Serine
4l inositol
24
Q

Is cholesterol found in prokaryotic/ fungal membranes?

A

No

Just eukaryotes

25
Q

Explain lipid structure in aqueous environment

A
  • planar bilayer
  • micelles (polar head group forms outside, hydrophobic tails inside
  • vesicles/ liposomes (formed from planar bilayer)
26
Q

Why do some lipids form micelles and other form bilayer?

A

Phospholipids have 2 hydrophobic tails ( too cramped for just one layer) prefer to form liposomes/micells
Detergents only have 1 hydrophobic tails so it’s more favourable to form a bilayer

27
Q

Explain the movement of lipids in membrane: what is transverse diffusion?

A
  • Movement between leaflets
  • polar head has large solvation shell must be shed as it moves through the hydrophobic later to the other side of the membrane
  • massive energy requirement
  • catalysed by flippases
  • about one billionth the rate of lateral diffusion
  • doesn’t happen very often
28
Q

What can asymmetry be generated and maintained by?

A

Enzymes:

Flippases and floppases use ATP to move specific phospholipids from one leaflet to another

29
Q

Where do eukaryotic cells establish asymmetry?

A

In the ER or Golgi and membrane fragments flow from these organelles to other membranes

30
Q

Can there be a charge difference between the two leaflets?

A

Yes

31
Q

What is the state (solid or liquid) of the lipid determined by?

A

The composition of the hydrocarbon chains

More saturated= can be more tightly packed= more solid

32
Q

Does temperature affect the structure of the membrane?

A

Yes
But cholesterol is there to buffer this effect and help the membrane stay in the same state across a range of temperatures

33
Q

What is the main function of a membrane?

A

-Form external boundaries of cells
(Impermeable to ions and most polar molecules)
- separate compartments in cells

34
Q

What can freely diffuse through the membrane

A

Gasses

Allows efficient gas exchange

35
Q

What is the composition of most of the membrane?

A

-Lipids: 25-50% by mass
Phospholipids, glycophingolipids, cholesterol
- carbohydrates: glycolipids and glycoproteins
- proteins: 50-75% by mass

36
Q

What are the functions of membrane proteins?

A
  • membranes are relatively impermeable so protein channels/transporters need to allow ions to pass through
  • extracellular signals need to be detected
  • structural proteins, connective proteins
37
Q

What are integral membrane proteins?

A

They are imbedded in the membrane

38
Q

What are some of the formations of integral proteins?

A
  • alpha helix —> recognition, receptors
  • helical bundle —> enzymes, transporters, receptors
  • beta barrel—> transporters (channel proteins)
39
Q

What is the main characteristic of peripheral membrane proteins?

A

They do not interact with the hydrophobic core of the membrane

40
Q

What are the different hydrophobic parts of peripheral membrane proteins?

A
  • amphiphathic alpha helix
  • hydrophobic loop
  • lipidation
  • electrostatics
41
Q

What is the glycocalyx?

A

Surface of cells is covered with a thick layer of poly-sugar side chains
Involved in cell to cell recognition

42
Q

What extra support does the membrane have?

A

A cytoskeleton which forms network to support the membrane thus making it resistant to stress

43
Q

How thick are membrane typically?

A

7.5 - 10 nm

44
Q

What is the periplasm?

A

Space between the outer and inner membranes in prokaryotes

45
Q

Which bacteria have double membranes?

A

Gram negative

46
Q

What do gram positive bacteria have instead of a double membrane?

A

A single membrane + a thick layer of peptidoglycan

47
Q

What would happen to ampiphatic (ampiphillic) molecules in an organic solvent?

A

They would form inverse micelles

48
Q

Which kind of lipids are most commonly found within cell membranes?

A

Glycerophospholipids

49
Q

What are the 4 main phospholipids in the membrane?

A

(Phosphadtidyl) ethanolamine
(Phosphadtidyl)Choline
(Phosphadtidyl)serine
(Phosphatidyl) inositol

50
Q

Which phospholipids are negatively charged?

A

*

51
Q

Do you find cholesterol in prokaryote membranes?

A

No

52
Q

What state will a substance be in if it’s acyl chain has lots of C-C double bonds?

A

Liquid

More unsaturation= harder to pack together= lower melting point

53
Q

What makes a membrane protein integral?

A
  • it passes through the lipid bilayer at least once (interacts with the hydrophobic core

Cell Biology (BY1CB1) (16 decks)

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