In 1972, Singer and Nicholas put forward the “Fluid Mosaic Model” of membrane structure in which a mosaic protein molecules floats in a fluid lipid bilayer. This model is proposed that membrane is made up of lipid and protein but the protein does not form a continuous layer covering both sides of the membrane as proposed by Danielli and Davson.
The phospholipids are arranged in a bilayer, with their polar, hydrophilic phosphate heads facing outwards, and their non-polar, hydrophobic fatty acid tails facing each other in the middle of the bilayer. This hydrophobic layer acts as a barrier to all but the smallest molecules, effectively isolating the two sides of the membrane. Different kinds of membranes can contain phospholipids with different fatty acids, affecting the strength and flexibility of the membrane, and animal cell membranes also contain cholesterol linking the fatty acids together and so stabilising and strengthening the membrane.
In mosaic model the protein molecules are either partially (peripheral protein) or wholly embedded (integral protein). Some of these proteins that float, consist of pores that allow the passage of particular molecules or ions through the membrane. In absence of these pores, the polar molecules could be difficult to cross the membrane.
According to this model, the membrane structure is not static, the lipid molecule linked to one another only by weak bond.