Cell Membranes

• 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

Phospholipids

Extend this micelle and you form a bilayer of phospholipids

The heads are in the aqueous environment or water, the tails are away from the water

EM image of a membrane

This shows the membrane is made of what appears to be 3 layers

The total width is 7nm

EM image of a membrane

Chemical analysis showed the membrane was made of PhospholipidsAndProteins

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

Tagging the proteins using radioactive isotopes shows they move around

• 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

Video