Defining the fluid mosaic model
The fluid mosaic model is a way of understanding biological membranes, which is consistent with most experimental observations. This model states that the components of a membrane, such as proteins or glycolipids, form a mobile mosaic in the fluid environment created by a sea of phospholipids. There are restrictions on lateral movement and subdomains within the membrane have specific functions.
Development of the Fluid Mosaic Model
This model has been developed over many years through the meticulous work of scientists around the world. It began with the hypothesis that the membrane consisted of a lipid bilayer in which the phospholipid of the membrane self-assembled into a bilayer with the nonpolar and hydrophobic ends facing each other. The hydrophilic "head" regions face the cytosol and the extracellular region. This was verified by extracting cell membranes and spreading the lipids in a single layer. The total surface area of this monolayer was twice that of the plasma membrane, supporting the idea that lipids formed a bilayer.
However, that was just the beginning, as it turned out that cell membranes needed to be composed of more components to accommodate their diverse biophysical properties. For example, unlike pure lipids, cell membranes do not freeze very easily. Membrane permeability to bulky polar molecules cannot be explained either.
More than 25 years after the lipid bilayer model was proposed, cell membranes were first visualized in the 1950s. Initial observations seemed to indicate that the lipid membrane was coated with thin layers of protein on both sides. However, in 1972, two scientists, Singer and Nicolson, refined this to create the fluid mosaic model. The phospholipid bilayer is said to be interrupted by several proteins that form a mosaic-like pattern on the lipid membrane. These proteins can traverse the entire membrane or interact with either of the two lipid layers. Some proteins may even be membrane bound only through a short lipid chain.
The membrane is fluid, but it also has an underlying structure anchored to the cytoskeleton. The liquid nature of the lipid matrix that forms the membrane was first illustrated by an experiment in which membranes of different compositions were artificially fused together. Proteins from both cells were redistributed across the fused membrane in less than an hour.
The image illustrates this model and shows the lipid bilayer with different types of integral membrane proteins as well as cholesterol, glycoproteins and glycolipids. The image also shows the anchorage of the membrane to the cytoskeleton.
Now, some membranes have been studied in detail using advanced imaging techniques, with a resolution of less than a nanometer. These images can even show the relative locations of different polypeptide and lipid chains within the membrane.
Functions and components of biological membranes
The main function of cell membranes is to delineate the inner and outer regions of the cell. Inside the cell, organelle membranes perform the same function for subcellular structures.
This function comes with a caveat – the cell must actively communicate with the external environment, exchanging materials while maintaining essential nutrients and warding off harmful substances. The components and structure of biological membranes help accomplish these tasks and maintain their selective permeability.
Biological membranes, especially cell membranes, are made up of phospholipids, cholesterol, and proteins.
The first is the phospholipid bilayer itself, which forms a hydrophobic layer that separates the aqueous environments on both sides. Phospholipids are amphipathic molecules composed of a polar and hydrophilic "head" region composed of a phosphate group and the nonpolar hydrophobic "tail" composed of two long-chain fatty acids. These two segments are covalently linked to a glycerol molecule.
The figure shows a schematic representation of the chemical structure of a phospholipid, where R1 and R2 refer to the two fatty acid chains. Usually one of the two fatty acids is unsaturated, with at least one double bond between two carbon atoms.
As can be seen in the figure, an unsaturated fatty acid has a fold in its structure. This is an important feature that affects the fluidity, tensile strength and permeability of the membrane.
In addition, the membrane contains three types of proteins. Integral membrane proteins span the entire membrane, usually with alpha helices forming the transmembrane region. These proteins form channels and pores that allow the movement of large or polar molecules across the hydrophobic segment of the membrane.
Furthermore, proteins can be incorporated into a single membrane sheet. These proteins are commonly used in signaling cascades and can act as carrier molecules, carrying a signal from one segment of the membrane and relaying it to another region. These membranes are called peripheral membrane proteins. Finally, some proteins bind very loosely to the membrane, with only a small lipid tail introduced into the hydrophobic region.
Membrane proteins can be covalently linked to carbohydrates, forming glycoproteins. These can interact with water molecules and stabilize the membrane, in addition to serving as important tools for intercellular communication. They form receptors for hormones and neurotransmitters.
The other important role of glycoproteins is to create a kind of "cell signature". When immune cells recognize these glycoproteins, they are able to distinguish the body's cells from those of pathogens. For example, the classification of blood into types A, B and O depends on the type of glycoprotein present on the surface of red blood cells.
The presence of cholesterol in the phospholipid layer allows the membrane to maintain its permeability and integrity even at different temperatures. It seems to be embedded in the middle of phospholipids. Cholesterol prevents compaction of hydrophobic tails at low temperatures and membrane expansion with heat. In this way, small molecules such as carbon dioxide and oxygen can always move freely across the membrane, while the cell retains its selective permeability to larger molecules.
Other models for membrane structure
The fluid mosaic model was refined in the early 1980s by two scientists named Mouritsen and Bloom to create the "mattress model" for the membrane's structure. They revealed the fact that although previous experiments suggested that the entire membrane is fluid and allows free diffusion of proteins, there are actually subdomains within each membrane.
For example, if a transmembrane protein has a hydrophobic region slightly larger than the average width of a cell membrane, the lipid bilayer will deform to accommodate that protein. If there are several proteins whose hydrophobic regions do not exactly match the width of the membrane, the lipid bilayer would end up looking like a mattress, with thicker and thinner regions in between.
Thicker regions would likely create a slope that would slide the proteins "downwards", leading to protein aggregation in some regions. Likewise, these deformations can lead to the accumulation of specific lipids around these proteins. These observations join previous experimental data suggesting the presence of lipid rafts and the preferential association of proteins with lipids supporting the "mattress model".
Modern models of membrane structure also take into account the effect of lipid composition. Cell membranes are made up of many hundreds of phospholipids, each of which is made up of different fatty acid side chains. These fatty acids can have different lengths and different degrees of saturation. There are also thermodynamic considerations in studying membrane properties, as even at physiological temperatures the thickness of cell membranes and the distribution of various lipids can change.
Finally, membranes also have structures called lipid rafts, which are composed of specialized lipids, cholesterol, and proteins bound to the membrane by glycolipids. Lipid rafts are important subdomains, especially for signal transduction.
- Amphipathic Molecules- Molecules containing polar hydrophilic regions and non-polar hydrophobic regions.
- Antigen– Any molecule capable of eliciting an immune response.
- signal transduction– Transmission of information in the form of electrical or chemical signals from outside the cell to the inside.
- sphingolipid– Fatty acid derivatives of a molecule called sphingosine. Commonly seen in membrane lipid rafts.
- thermodynamics– The branch of science that deals with the relationship between heat and fundamental quantities such as energy, work, entropy, and temperature.
1. Which of these statements about the structure of membranes is correct?
A.Consists mainly of cholesterol molecules
B.Glycoproteins on the cell surface are necessary for immune recognition
C.Lipid rafts were predicted by early models of cell membrane structure
D.all of the above
Answer to question #1
Bit is true. For example, ABO blood typing depends on the glycoproteins present in the cell membrane. Those with type A and B blood have the A and B antigen, respectively, and those without either of these glycoproteins have type O blood. The other antigen relevant to blood typing is the highly immunogenic region of the D antigen called Rhesus factor. Those with the Rhesus factor antigen are referred to as "positive". Combining these two families of antigens results in blood types such as AB positive, O negative, B negative, A positive, and so on. As immune cells mature, they are exposed to different cells in the body and their glycoprotein compositions, leading to their ability to distinguish "self" antigens from "foreign" antigens. If this maturation does not occur properly, autoimmune diseases can occur. These reactions are artificially attenuated during some surgical procedures (organ transplants) and some pregnancies.
2. Which of these are features of the fluid mosaic model of cell membranes?
A.Lipid bilayer composed of amphipathic phospholipid molecules
B.A mosaic of proteins, cholesterol and other membrane components
C.The lateral diffusion capacity of the membrane components
D.all of the above
Answer to question #2
Dit is true. The fluid mosaic model builds on earlier hypotheses about membrane structure, which postulated that biological membranes consist of a lipid bilayer. While proteins were originally thought to form thin layers on both sides of the membrane bilayer, the fluid mosaic model considered the presence of globular proteins to be an integral part of the membrane's structure. This model also implied that there was complete and free diffusion of all "mosaic" components along the phospholipid bilayer.
3. Which of these ideas represent a refinement of the fluid mosaic model?
A.Varying cell membrane thickness in different regions depending on the composition of integral membrane proteins
B.Presence of lipid rafts for signal transduction
C.Melting of lipids at physiological temperatures and changes in lipid composition in different membrane subdomains
D.all of the above
Answer to question #3
Dit is true. When an integral membrane protein has a longer or shorter hydrophobic transmembrane region, the cell membrane appears to deform to maintain full stretch within its hydrophobic lipid region. Lipid rafts, composed of specialized fats, cholesterol, and sphingolipids, can preferentially concentrate or exclude some proteins in subdomains in the membrane, allowing rapid signal transduction.