Membrane lipids

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  • Created by: lridgeway
  • Created on: 30-10-20 10:46

Types of lipids in membranes

Main membrane lipids are phospholipids (aka phosphoglycerides) which are unsaturated and conatin cis double bonds. Other major ones are sphingolipids (NH instead of O and often often saturated or contain trans double bonds) and sterols (eg cholesterol).

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Fluidity of membrane

Lipids in phospholipid (eg phosphatidylcholine (PC)) are less linear and more disordered due to cis double bond forcing the chain off at an angle. Trans double bonds can fit within a linear exteneded chain and are more solid. 

Means bilayers of cis bonds are fairly disordered, known as fluid phase, and bilayers of trans bonds are more ordered, known as gel phase. PC layers tend to thinner due to being less ordered.

Cells normally want thier membranes to be fluid because this allows thinsg to move around within bilayer. Any real or artificial bilayer can usually be induced to change from one phase to the other by heating. 

Cholesterol is at a very high conc in some membranes. It is flat so packs against trans (flat) lipids and makes them event flatter and thus longer. 

All this taken into account real biological membranes have different thicknesses depending on their composition. 

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Shapes of lipids and membranes

Different lipids have different cross sectional areas which affect the overall shape of the membrane. Phosphatidylcholine is basicallly cylindrical so packs nicely into flat bilayers. Phosphatidylethanolamine has a head group smaller width than the lipid tail so this curves the bilayer inwards. The cell can control the shape of the membrane by controlling where lipids are positioned. 

There are other ways to fix the curvature of the membrane:

  • membrane proteins ( shape and oligomerization) 
  • Helix insertion eg caveolin
  • Scaffolding molecules 
  • Cytoskeleton - cytoskeletal proteins push or pull the membrane about 
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Lipid concentration and distribution

Lipid concentrations vary a lot between different membranes. For example the proportion of cholesterol increases as you move out to plasma membrane and the proportion of phosphoglycerides decreases. This is organised by the cell to generate different properties. 

Lipids are also distributed differently between the two leaflets of the membrane which is very asymmetric. For example there is significantly more sphingomyelin inn the outer leaflet. Proteins are also 100% in one orientation: GPI-anchored proteins are all on the outside and lipid-anchored on the inside. 

Cells have an ATP-dependent enzyme called a flippase which flips lipids between bilayer leaflets as this cannot happen spontaneously 

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Phase separation and membrane rafts

Lipid composistions across membranes vary to encourage phase seperation of different lipid components (like oil/water seperation). This creates different regions within the membrane and is the part that the fluid mosaic model of the membrane fails to describe fully. 

The thicker more rigid regions, which have high levels of sphingolipids and cholesterol are called membrane rafts (aka lipid rafts). 

Proteins also separate into the different regions. Proteins with long TM helices, GPI-anchors and palmitoyl anchors go into membrane rafts and those with prenyl anchors prefer not to. 

Formation of rafts is controlled by the cell and is an important mechanism for altering the location of membrane proteins e.g bringing signalling proteins together, organising the start of endocytosis. Small membrane rafts upon stimulation can form bigger more stable rafts. The proteins is these may form a functional unit. This is especially important in signalling for dimerisation of proteins as this stimulates bigger rafts to form and more proteins to come togther. 

Attachment of GPI anchor, prenylation etc is a covalent modification and can easily be altered allowing many proteins to move in and out of rafts. The cell controls these processes. 

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Atomic force microscopy

Membrane rafts can be observed using this technique. 

Your sample sits on a flat surface (a piezoelectric scanner - moves is a current goes through it) with a cantilever arm above it. The arm is held at one end but the other is free to move over the sample. A laser is set up to bouce off the arm and be detected. If you can move the sample the arm moves over it and via the laser detections you can work out the height of the membrane. 

Thicker layers of membrane are higher and collect into patches which tend to be red and contain proteins which are spikes on a image. These are likely membrane rafts. 

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