<Li> Peripheral proteins: More hydrophilic, and thus are non-covalently linked to the polar heads of phospholipids and other hydrophilic parts of other membrane proteins on the surface of the membrane . </Li> <Li> Lipid anchored proteins: Essentially hydrophilic, so, are also located on the surface of the membrane, and are covalently attached to lipid molecules embedded in the layer . </Li> <P> As for the fluid nature of the membrane, the lipid components are capable of moving parallel to the membrane surface and are in constant motion . Many proteins are also capable of that motion within the membrane . However, some are restricted in their mobility due to them being anchored to structural elements such as the cytoskeleton on either side of the membrane . </P> <P> In general, this model explains most of the criticisms of the Davson--Danielli model . It eliminated the need to accommodate membrane proteins in thin surface layers, proposed that the variability in the protein / lipid ratios of different membranes simply means that different membranes vary in the amount of protein they contain, and showed how the exposure of lipid - head groups at the membrane surface is compatible with their sensitivity to phospholipase digestion . Also, the fluidity of the lipid bi-layers and the intermingling of their components within the membrane make it easy to visualize the mobility of both lipids and proteins . </P>

Which of the following types of models could be used to represent a cell