Structural Biochemistry/Lipids/Fluid Mosaic Model

Fluid Mosaic Model
The fluid mosaic model is used to describe the interactions of lipids and proteins in biological membranes. This model essentially proclaims the concept of lateral diffusion, stating that proteins can freely move about within a membrane and that such membranes are considered to effectively be two-dimensional. The fluid mosaic model of biological membranes are always fluctuating and adjusting. In 1972, the fluid mosaic model was introduced by S. Jonathan Singer and Garth Nicholson.

Fluid mosaic model of membranes states that membrane components are free to diffuse in the plane of the membrane. Some of the membrane proteins are restricted to specific regions of the membrane by interactions with cytoskeletal proteins. Also, although many phospholipids and membrane proteins can move laterally within a leaflet, they do not flip-flop from one leaflet of the bilayer to the other. Flip-flop of the phospholipids is very rare. The inner and the outer leaflets of the membrane may be made up of different phospholipids. Membrane fluidity refers to the movement of membrane phospholipids within the plane of the membrane. A decrease in fluidity is associated with decreased transport rates. The length of the fatty acid side chains also affects fluidity. The phospholipids with long hydrocarbon chains have increased hydrophobic interactions with neighboring lipids and thus decreased membrane fluidity. [Microbiology]

Some organisms can alter membrane fluidity in response to temperature stress by changing the length and degree of saturation of fatty acids present in membrane phospholipids. Cholesterol also influence membrane fluidity. The effects of cholesterol on membrane fluidity are complicated and depend on factors such as the ratio of saturated to unsaturated fatty acids in the membrane. The cholesterol also prevents packing of saturated fatty acids, thus increasing fluidity.

Proposed by S.J. Singer and Garth L. Nicholson in 1972, the fluid mosaic model provides a reasonable structure and image of the biological membranes in general. One of the most important features of this model is the idea that the phospholipid bilayer is fluid. The phospholipid molecule are free to move laterally. Relative to the lateral movement of the phospholipid molecules, there is very little exchange between the two halves of the bilayer. This minimal exchange, or flip flop action, allows asymmetric distribution of phospholipids. This asymmetry is an important feature of membranes. Membrane surfaces exhibit asymmetry. In other words, they have different characteristics on the two sides. These structural differences support the functional differences of the inner and outer sides of the membrane. For example, one of the most important functions of the outer surface of the membrane lies in its interaction and communication with other cells. This is often achieved by sugar molecules almost exclusively found on the outer surface that acts as distinguishing markers for the cell. The interior, on the other hand, serves different functions, and therefore has a different composition. In this model, the membrane is a mosaic of proteins embedded in a fluid phospholipid bilayer. The hydrophilic portions of the phospholipid and proteins are maximally exposed to aqueous interface. This feature ensures membrane stability. The fluidity of the molecule is affected by several factors. These include the type of lipid found in the membrane and the degree of unsaturation in the fatty acid chains of membrane lipids. The presence of a cis double bond introduces a kink into the fatty acid chain, which affects the packing of the phospholipid bilayer. The kink prevents the phospholipid molecules from being packed together too tightly, and thus contributes to the membrane fluidity. It is important to understand that in this model, both the membrane lipids and the embedded proteins are free to move. They may be mobile or fluid.

Proof of Fluidity
The fluid mosaic model obviously states that the lipid bilayer that surrounds the cell is fluid, flexible and always moving. In order to prove that an iconic experiment was done by taking a cell and saturating the lipid bilayer with fluorescence. After the cell was completed saturated with a green fluorescense, the cell was bleached in a single spot in the cell membrane. This created a very white spot among the green fluorescence coated cell membrane. After a short while, the bleach spot began to diminish in color and before you knew it, the area seem to be recoated with green fluorescence. The phenomenon behind this was the aspect of the cell membrane diffusing the bleached hydrophilic heads amongst the rest of the cell membrane. This diffusion allowed for the white bleach to diminish in color by being substituted by green fluorescenced hydrophilic heads. There was found to be two types of diffusion in the cell membrane, lateral diffusion and transverse diffusion. Lateral diffusion is the switching of positions in a side by side manner without any sort of flipping. This is the fastest mode of diffusion found in the cell. the second diffusion method transverse diffusion, is the flipping of the phospholipid heads to either side of the cell membrane. This method of diffusion however is less likely and happens significantly slower then lateral diffusion.



History & Development
Singer's studies of membranes started in the 1950s when scientists noticed that many water-soluble proteins (like those in cells) were also able to dissolve in nonpolar, nonaqueous solvents and that proteins adopted different shapes in hydrophilic vs. hydrophobic environments. Many proteins are found in environments also high in lipid content, and this prompted Singer to look into the relationship between proteins and lipid membranes.

Before Singer and Nicolson's fluid mosaic model of membranes, a triple-layered membrane model was proposed, the Davson-Danielli-Roberston (DDR) model. This model proposed a triple-layered membrane, with a lipid layer between two flat protein layers. However, when studied with respect to energetics of hydrophobic/hydrophilic interactions, this model is not feasible. Due to hydrophobic and hydrophilic interactions between amino acid residues of a protein, Singer therefore proposed that membrane proteins would assume folded conformations, not remain in a flat layer like the DDR model proposed. Also in contrast to the DDR model, Singer also proposed that logically and for maximum stability membrane proteins would not be separated from the lipid bilayer but rather incorporated as part of the membrane. These conclusions all came together in the fluid mosaic model, where the phospholipid bilayer is a fluid matrix and both lipids and proteins are capable of lateral and rotational movement (see membrane fluidity).