b. membrane structure
TRANSCRIPT
• Under the light microscope these are seen as a single layer
• the electron microscope shows two layers, 7nm wide
• Chemical Analysis shows the membrane is made of
Phospholipids andProteins (more than lipids by mass)
Phospholipids
Phosphate head – hydrophillic or “water loving”
Two fatty acid tails – hydrophobic or “water hating”
Phospholipids
In water phospholipids' tails will be repelled and the water loving heads will align next to the water molecules
This forms a micells
EM image of a membrane
This shows the membrane is made of what appears to be 3 layers
The total width is 7nm
Davson Danielli Hypothesis – which was wrong!
They suggested the outer layers were proteins and the inner layer was a phospholipid bilayer
proteins
Davson Danielli Hypothesis – which was wrong!
But this made the membrane too wide –
The width is the same as a phospholipid bilayer
7nm
Freeze etching is a technique which freezes a specimen and then cuts it along lines of weakness
Scanning em shows the membrane looks like thisThere are little bumps in itThey are proteins
• In 1972 Singer and Nicolson
proposed the Fluid Mosaic Model:• This described a mosaic of protein
molecules floating in a fluid lipid bilayer
A Simple Diagram of a Section of Membrane
1 = the phospholipid bilayerThe hydrophobic tails stop any polar substances moving across the membrane
2 = Transmembrane protein which has hydrophobic outer sides and a hydrophilic channelThis allows polar substances to cross the membrane by diffusion or active transport
Integral proteins do not cross the entire membrane. They may be fixed or move freelyThey may have metabolic functions –e.g. enzymes
3 = cholesterol with hydrophilic and hydrophobic portionsThese regulate fluidity ,mechanical stability, The hydrophobic regions prevent the passage of polar molecules
4 = glycolipid or glycoproteinCarbohydrate attached to a phospholipid or protein
These provide stability by projecting into the water medium and forming H-bonds with water molecules
They also form recognition sites for other cells, hormones and other molecules