The cell membrane is made up of a phospholipid bilayer only 7nm wide. Phospholipids have a hydrophilic phosphate head and hydrophobic fatty acid tails which allow them to spontaneously form a bilayer in water with heads facing out and tails inside. The membrane also contains proteins, which early models incorrectly thought layered on top of the phospholipids but are now known to float within the fluid bilayer. The fluid mosaic model describes the current understanding of the membrane with proteins and phospholipids forming a flexible yet selective barrier.

1. Cell Membranes

2. • 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 and
Proteins (more than lipids by
mass)

3. Phospholipids
Phosphate head – hydrophillic or
“water loving”
Two fatty acid tails –
hydrophobic or “water hating”

4. Phospholipids
In water phospholipids'
tails will be repelled and
the water loving heads
will align next to the
water molecules
This forms a micells

5. Phospholipids
Extend this micelle and
you form a bilayer of
phospholipids

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

7. EM image of a membrane
This shows the
membrane is made
of what appears to
be 3 layers
The total width is
7nm

8. EM image of a membrane
Chemical analysis
showed the
membrane was
made of
Phospholipids
And
Proteins

9. Davson Danielli Hypothesis –
which was wrong!
They suggested the
outer layers were
proteins and the inner
layer was a
phospholipid bilayer
proteins

10. Davson Danielli Hypothesis –
which was wrong!
But this made the
membrane too wide –
The width is the same as a
phospholipid bilayer
7nm

11. Freeze etching is a technique which freezes a specimen
and then cuts it along lines of weakness
Scanning em shows the membrane looks like this
There are little bumps in it
They are proteins

12. Tagging the proteins using radioactive isotopes
shows they move around

13. • In 1972 Singer and Nicolson proposed
the Fluid Mosaic Model:
• This described a mosaic of protein
molecules floating in a fluid lipid
bilayer

14. A Simple Diagram of a Section
of Membrane
1 = the phospholipid bilayer
The hydrophobic tails stop any polar substances
moving across the membrane

15. 2 = Transmembrane protein which has hydrophobic
outer sides and a hydrophilic channel
This allows polar substances to cross the membrane by
diffusion or active transport

16. Integral proteins do not cross the entire membrane.
They may be fixed or move freely
They may have metabolic functions –e.g. enzymes

17. 3 = cholesterol with hydrophilic and hydrophobic
portions
These regulate fluidity ,mechanical stability, The
hydrophobic regions prevent the passage of polar
molecules

18. 4 = glycolipid or glycoprotein
Carbohydrate 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

20. Video