Membranes

Question 1

What are micelles?

Answer 1

Micelles are closed monolayers with a fatty acid core and polar surface. Fatty acids form micelles.

Question 2

What are bilayers?

Answer 2

They are effectively 2 monolayers held in close proximity and they are formed spontaneously by phospholipids. Hydrophobic core and hydrophilic outer layer - polar heads and non-polar body.

Question 3

What are vesicles/liposomes?

Answer 3

Liposomes are composed of a lipid bilayer separating an aqueous internal compartment from the bulk aqueous phase. They contain an aqueous cavity.

Question 4

What is an issue that bilayers have?

Answer 4

The edge effect -> the phospholipids on the edge are exposed to water and that is energetically unfavourable. This is managed by the formation of liposomes (has the head groups of one of the monolayers facing inwards - cavity is filled with water) and the head groups of the the other monolayers is facing the outside (occurs without ATP - is a spontaneous process)

Question 5

What are liposomes used for?

Answer 5

They are used in drug delivery:

• nanoparticles delivery of drugs
• place drugs within the aqueous cavity
• can insert hydrophobic drugs into the hydrophobic core
• allows for efficient delivery of drugs to target sites within the body

Question 6

What are the modifiers of membrane fluidity?

Answer 6

Composition

Temperature

Question 7

How does composition modify membrane fluidity?

Answer 7

• Membranes have to be fluid at physiological pH
• Fatty acid composition will define the packing
• Presence of double-bonds -> poorer packing of adjacent chains and higher degree of mobility
• Saturated hydrocarbon chains -> firm packing, stable structures, less movement and less fluidity
• Cholesterol can modulate the fluidity

Question 8

How does temperature modify membrane fluidity?

Answer 8

• At 37 degrees all biological membranes are fluid
• Temperature < physiological -> a semi-solid state and all motions of the individual lipids are highly constrained
• Higher temperature -> lipid molecules become dynamic, interact with neighbours very transiently, mobility at that temperature is defined by the hydrophobic tails

Question 9

How do lipids move in membranes?

Answer 9

• Lateral diffusion of lipids in membranes is rapid
• Transverse diffusion (flip-flop) is very slow -> unfavourable because the polar head has to pass through the hydrophobic core
• Two monolayers can have different lipid compositions

Question 10

What can lateral diffusion be measured by?

Answer 10

Fluorescence Recovery After Photobleaching experiments (FRAP)

• Take an intact cell
• Label the polar head with fluorescent dye
• Photobleaching -> shine an intense beam of light in that region
• Looks like a hole -> they are no longer fluorescing -> incubate them at 37 degrees
• If movement has occurred, then the hole will be filled up and the bleached lipids will diffuse out
• The rate at which this happens can be used to calculate the diffusion rate

Question 11

How do sterols and sphingolipids contribute to the membrane?

Answer 11

Sterols and sphingolipids cluster together to form lipid rafts.

• Form stable entities when compared to the rest of the membrane
• Lateral diffusion is more slower
• Recruit proteins - platform to mediate particular functions

Question 12

What bonding is involved with peripheral membrane proteins?

Answer 12

Ionic interactions and H bonding

Question 13

What bonding is involved with integral membrane proteins?

Answer 13

Hydrophobic interactions

Question 14

How can peripheral membrane and integral membrane proteins by released from membranes?

Answer 14

Peripheral:

• High salt
• Change of pH
• Chelating agents

Integral:
- Detergents -> sodium dodecyl sulphate

Question 15

How can we determine orientation/arrangement of membrane proteins?

Answer 15

Example -> glycophorin

• The carbohydrate component will be on the extracellular domain
• Add a reagent that is not able to cross the membrane (protease will only be able to digest the portions of the protein exposed on the surface)

Example -> trypsin (is able to cleave the carbonyl side of lysine and arginine but only has access to the outside part of the protein of an intact cell n

Question 16

Based on sequence can we predict transmembrane protein domains?

Answer 16

Genome sequencing projects allows the sequences of all proteins to be predicted

• Sequence will consist of hydrophobic amino acids
• Conformation will be an alpha helix
• Span equal to width of the membrane
• 20 hydrophobic residues to span a 3 nm membrane

Question 17

What are hydropathy plots?

Answer 17

• Look for possible transmembrane domains
• Y axis (positive values - very hydrophobic, negative values - hydrophilic)
• X axis -> residue number
• We are looking for a peak above the x-axis that will stretch
• It is possible that a molecule has more than one transmembrane domain

Question 18

What is passive and active transport?

Answer 18

Passive transport:

• No energy used (no ATP)
• From high concentration to low concentration
• Carriers may be used for facilitated diffusion or channels

Active transport:

• Requires energy in the form of ATP
• From low concentration to high concentration
• Carriers are used

Question 19

What type of transporter is GLUT-1?

Answer 19

Passive transport (facilitated diffusion) 
T1 and T2 
Has 12 transmembrane domains 
There is an occasional blue circle that is polar and charged side chain -> how does this occur in a hydrophobic region? 
- the polar side chains point inwards and create a niche - arrange themselves which allow the glucose to pass through