Biology:Unit membrane model

Robertson's unit membrane model' was given by J. David Robertson in 1959 after the Biomembrane model was proposed by Danielli and Davson in 1935. This Biomembrane model proposed by Robertson stated that Biomembranes are made up of four layers. Out of these four layers, two are protein layers and two are phospholipid layers. Phospholipid Bilayer is 25-35Å thick and Each of the two Protein layer is 20-25Å thick (Overall 65-85Å) Robertson used electron bombardment on membranes and observed two distinct zones. The two zones observed were the electron light zone (or electron transparent zone) and the electron dense zone. In this model Brown stated that there are two electron dense zones and one electron transparent zone at the centre. This arrangement of two electron dense and one electron transparent zone is called the Trilaminar Model or Tripartie Model. The electron dense zone is the sheet or layer of extended proteins (Beta Proteins).