Plasma Membrane
or the
Plasmalemma.
Year 12
Places found :• Around animal cells and plant cells.
• Vesicles for transport of substances in / out
of cells =Endo/Exocytosis.
• Endoplasmic Reticulum (ER)
Functions
• To control the movement of substances
into and out of cells
• To keep the cells contents together and
separate from other cells
• To allow communication between cells
Structure.
• A phospholipid bilayer about 7-10nm thick.
• Under the light microscope it appears as a thin
line and until the advent of the electron
microscope the structure could not be studied.
Phospholipids
• A special type of lipid
• 2 fatty acid chains
making a hydrophobic
tail.
• Chains have no charge
and are non-polar /
insoluble.
• Phosphate group is
hydrophilic = has a
charge so is soluble.
History of
Packing :1930-40 Danielli and Davson studied
triglyceride lipid bi-layers over water surfaces.
Found they arranged themselves with polar heads
facing outwards = forming droplets
However, if they add proteins they formed sheets
over water as this decreased the surface
tension.
NB. The polar pore.
1950 Robertson Model
• Did not see the spaces for pores on electron
micrographs.
• He said that the rail road track was due to the
binding of Osmium tetroxide to proteins and
polar groups on the surfaces of the membrane.
• 1966 Lenard and Singer
• Noted over 30% of proteins were twisted
into an alpha helix (secondary structure)
• More likely the proteins were spherical than
flat.
• Major turning point :- Freeze Fracturing
technique
• Clearly showed bumps in membrane as
proteins.
• Proteins are embedded in the bilayer.
• Transmembrane proteins pass through the bilayer
• Others may be inserted at the cytoplasmic or
exterior face = Extrinsic / Intrinsic
Singer and Nicholson 1972
• Aided by electron microscope Singer and
Nicholson proposed the fluid mosaic model
• Fluid = moving ; mosaic = made of pieces
• A bilipid layer with proteins + Cholesterol
molecules between the phospholipids and
polysaccharides attached to the proteins or
lipids and constantly moving.
The Fluid Mosaic Model
Cholesterol
Cholestrol helps to keep the lipid layer free to
move.
Membrane proteins
• Create pores through which water and water
soluble chemicals can pass
• Act as carriers in active transport
• Form receptor sites for hormones
• Are important in cell recognition
Glycoproteins and Glycolipids.
• All proteins with sugar side chains are
involved in cell –cell signalling and self
recognition.
• This is important in the immune system
response and recognising bacterial invasion