Plasma Membraneor 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