Module 2: Bio-membranes

Question 1

Effects of carbon chain length

Answer 1

The longer the chain length, the greater the melting point but the lower the solubility in H2O.

Question 2

Effects of double bonds

Answer 2

the greater the number of double bonds, the lower the melting point

lead to unsaturated carbon chains and kinked

they stop close packing when unsaturated

Question 3

Glycerophospholipids

Answer 3

2 fatty acids - Glycerol - phosphate -alcohol

fatty acids are often unsaturated
alcohol is the head group

Question 4

Glycerophospholipid head groups

Answer 4

Serine, Ethanolamine, Choline, Glycerol, Inositol

Question 5

Sphingolipids

Answer 5

Major membrane components

Derivatives of the amino alcohol sphingosine

N-acyl fatty-acyl derivatives of
sphingosine are called ceramides

Question 6

Steroids

Answer 6

Mostly of eukaryotic origin

Most common is cholesterol (also known as sterol)

Cholesterol is a major component of the plasma membrane

Question 7

Similar molecules in the
different kingdoms

Answer 7

sterols:
cholesterol - animal
ergesterol - fungal
stigmasterol - plant

Question 8

Biological membranes

Answer 8

Define external boundaries of cells/
intracellular compartments (eukaryotic cells)
– Regulate traffic across this boundary

Functions
– Signal transduction
– Cell communication
– Complex reaction sequences
– Energy transduction

Special properties
– Flexible
– Self-sealing/can fuse
– Selectively permeable
– Two-dimensional

Question 9

Membrane: fluid mosaic model

Answer 9

Lipid bilayer (~30-40 Å thick)

Lipids are in constant motion
* Free lateral diffusion
* Almost no unassisted flipping

Membrane proteins also diffuse
laterally

Question 10

Lipid aggregates

Answer 10

in bilayers, the individual units are cylindrical (head = side-chain)

in micelles, individual units are wedge-shaped (head > side-chain)

also forms vesicles or liposome

Which structure forms is determined by
* Size of the fatty acyl chains
* Degree of saturation of the fatty acyl chains
* Size of hydrophilic head group
* Temperature

Question 11

Stabilisation of bilayers (bonding)

Answer 11

ionic bonds between head groups
hydrogen bonds with water

van der waals interactions between fatty tails

Question 12

Lipid mobility in bilayers

Answer 12

lipids have two main types of motion:
1. spinning without changing location
- rotation around their long axis
2. lateral diffusion - movement with the same leaflet
- phospholipids exchange positions with its neighbouring molecules
- VERY FAST!

this gives them a viscosity similar to olive oil (100x water)

Question 13

transverse diffusion “flip flop”

Answer 13

very rare and takes a lot of energy - has a high energy barrier

Question 14

affect of heat on membranes

Answer 14

heat disorders the interactions between the fatty tails to change the membrane from a gel to a fluid state

gel + heat = fluid

Question 15

effects of fatty acids on membrane fluidity

Answer 15

long chain fatty acids - aggregate extensively to give low fluidity (gel-like state)

short chain fatty acids - have less surface area to aggregate and increase fluidity

unsaturated fatty acids - also aggregate less extensively and increase fluidity

Question 16

Sphingomyelin and cholesterol

Answer 16

SM associates into a thicker, more gel-like bilayer than phospholipids

cholesterol increases thickness by ordering fatty acid tails, and stabilises head group interactions

Question 17

what aspects of membrane function require curvature

Answer 17

• viral budding
• formation of vesicles e.g. at plasma membrane
• stability of curved structure e.g. microvilli in small intestine

proteins help stabilise curved membranes!

Question 18

how does asymmetry arise?

Answer 18

lipids do not spontaneously flip from one leaflet to the other

specific enzymes can catalyse translocations
- lipids that are synthesised in one leaflet stay in that certain leaflet when moved e.g. in the cytosolic side of golgi moves to stay in cytosolic side of cell (plasma) membrane

BUT pc arrives at the plasma membrane on the cytosolic side but is transported to the other leaflet by “flippase” enzymes which require energy from ATP hydrolysis

Question 19

Catalysed transbilayer translocations

Answer 19

Flippase:
moves PE + PS from outer to inner leaflets

Floppase:
moves phospholipids from inner to outer leaflets

Scramblase:
moves phospholipids in either direction toward equilibrium

note: all use ATP hydrolysis

Question 20

membrane microdomains

Answer 20

micro-domains control lateral diffusion
e.g. stable associations of sphingolipids and cholesterol : lipid rafts

• can be disrupted by methyl-3-b-cyclodextrin (removes cholesterol from membranes) or antibiotic filipin (sequesters cholesterol)
• specific signalling proteins associated with lipid rafts

Question 21

membrane proteins

Answer 21

different membranes have different compositions in terms of proteins:lipids ratio. some have more protein than lipid. e.g. bacterial, mitochondrial, chloroplast

Question 22

membrane protein functions

Answer 22

transporters
receptors
adhesion molecules
lipid synthesis
energy transduction

Question 23

types of membrane proteins

Answer 23

integral (transmembrane)
- firmly associated with membrane (need detergents etc to remove)
- span membrane (hydrophilic and hydrophobic interactions)

lipid-anchored
- protein covalently linked to one or more lipid molecules
- lipid embedded in membrane leaflet to anchor the protein
- polypeptide does not enter the bilayer

peripheral
- can be released with milder treatment
- does not contact hydrophobic core of bilayer
- forms hydrophilic interactions with membrane surface or other membrane proteins