Fluid Mosaic Model of Plasma Membrane

What is Fluid Mosaic Model of Plasma Membrane? – Fluid Mosaic Model Definition

The fluid mosaic model is one method to comprehend biological membranes, consistent with the majority of experimental findings. According to this hypothesis, membrane components such as proteins and glycolipids form a mobile mosaic in the fluid-like environment formed by a sea of phospholipids. There are limitations on lateral movement, and subdomains within the membrane have distinct functions.

• Fluid Mosaic Model is a generally accepted model for describing the structure of the plasma membrane that encircles cells. In 1972, S. J. Singer and Garret L. Nicolson proposed the model, which has since become a fundamental paradigm in cell biology.
• The term “fluid” refers to the fact that the plasma membrane is not a hard structure, but rather a constantly moving layer that is dynamic and flexible. The lipid bilayer comprises two layers of phospholipids organised with their hydrophilic (water-loving) heads facing outward and their hydrophobic (water-fearing) tails facing inward. Its configuration permits the construction of a stable barrier that divides the interior of the cell from the exterior environment.
• The “mosaic” part of the model alludes to the fact that the plasma membrane is formed of several molecules that are embedded within the lipid bilayer. They include integral membrane proteins, which span the entire membrane width, as well as peripheral membrane proteins and lipids that are only partially entrenched in the membrane. The arrangement of the molecules within the membrane is not random, giving it a mosaic-like look.
• The Fluid Mosaic Model explains how the plasma membrane regulates the flow of materials into and out of the cell, as well as how it communicates with neighbouring cells and responds to environmental changes. Also, the concept is essential for comprehending how medications and other compounds might interact with the plasma membrane and alter cellular activity.

Components of Plasma Membrane

The plasma membrane is a dynamic and complicated structure that works as a barrier between the cell and its surroundings. It consists of multiple distinct elements, including:

• Phospholipids: These are the fundamental structural components of the plasma membrane. They create a barrier between the inside of the cell and the outside environment by forming a lipid bilayer with the hydrophobic tails facing inward and the hydrophilic heads facing outward.
• Proteins: Proteins are buried within the lipid bilayer and provide a range of activities, including as transporting substances across the membrane, recognising other cells, and facilitating communication between cells.
• Cholesterol: Cholesterol is a lipid that is present in the plasma membrane. It aids in membrane stabilisation and prevents it from becoming overly fluid or stiff.
• Glycolipids: Glycolipids are lipids that include an attached carbohydrate group. They are positioned on the membrane’s outer surface and play a function in cell recognition.
• Glycoproteins: Glycoproteins are proteins that include an attached carbohydrate group. In addition to being placed on the outside surface of the membrane, they are involved in cell recognition and communication.
• Carbohydrates: Carbohydrates are often found linked to proteins or lipids on the plasma membrane’s outer surface. They contribute to the recognition and communication of cells.

Collectively, these components comprise the plasma membrane and enable it to fulfil its many vital duties, such as regulating the exchange of materials between the cell and its environment, communicating with other cells, and preserving the structural integrity of the cell.

The Fluid Mosaic Model of Plasma Membrane

• According to the fluid mosaic model, the cell membrane has a quasi-fluid structure in which lipids and proteins are mosaic-arranged.
• There are two types of globular proteins: extrinsic and intrinsic. The membrane-associated extrinsic protein is soluble and dissociates. The intrinsic protein is insoluble, partially entrenched on either the outer or inner surface of the bilayer, and participates in lipid bilayer lateral diffusion.
• The fluidity of the membrane’s lipid matrix allows membrane components to migrate laterally. This is the result of the hydrophobic interactions between lipids and proteins. Fluidity is essential for a variety of membrane functions. Phospholipids and numerous intrinsic proteins contain both hydrophilic and hydrophobic groups, making them amphipathic.
• Phospholipids are complex lipids composed of glycerol, two fatty acids, and a phosphate group bound to one of numerous chemical groups. They consist of both polar (hydrophilic) and non-polar (hydrophobic) areas. The polar portion is composed of a phosphate group and glycerol, whereas the nonpolar portion is made up of fatty acids.
• All nonpolar components of the phospholipid only make contact with the nonpolar components of nearby molecules. The polar region is located outside. This trait creates the impression of a bilayer. Yet, appropriate spacing is maintained between the fatty acid chains by interspersing unsaturated chains across the membrane. This configuration preserves the semi-fluidity of the plasma membrane.
• Each phospholipid molecule’s head is water-attractive, whereas its tail is water-repellent. The hydrophilic heads of both layers of the plasma membrane face the exterior, while the hydrophobic tails constitute the interior of the bilayer. Extracellular fluid is a watery solution in which cells reside, and cells themselves contain a watery solution (cytoplasm). The plasma membrane forms a ring around each cell to allow the water-loving heads to be in contact with the fluid while protecting the water-averse tails on the inside.
• Principal components of a plasma membrane include lipids (phospholipids and cholesterol), carbohydrates linked to certain lipids, and proteins. A phospholipid is a molecule composed of glycerol, two fatty acids, and a phosphate-linked head group. Cholesterol, a lipid composed of four fused carbon rings, is found alongside phospholipids in the membrane’s interior.

Development of the Fluid Mosaic Model

• This model was built over a long period of time by scientists from around the world. It began with the concept that the membrane was composed of a lipid bilayer, in which membrane phospholipid self-assembled into a dual layer with the hydrophobic, non-polar tails facing each other. The hydrophilic “head” sections are oriented towards the cytosol and extracellular area. This was confirmed by isolating cell membrane lipids and spreading them out in a single layer. This monolayer has double the surface area of the plasma membrane, confirming the notion that the lipids formed a bilayer.
• Yet, this was only the beginning, since it soon became clear that cell membranes required additional components to account for their diverse biophysical features. Unlike pure lipids, cell membranes are not as susceptible to freezing. The membrane’s permeability to large polar molecules could not be explained either.
• In the 1950s, more than 25 years after the lipid bilayer concept was suggested, cell membranes were first observed. Initial observations suggested that the lipid membrane was covered on both sides with thin protein sheets. Two scientists, Singer and Nicolson, developed this in 1972 to create the fluid mosaic model. In this instance, the phospholipid bilayer was claimed to be punctuated by several proteins that created a mosaic-like pattern in the lipid membrane. These proteins are capable of traversing the whole membrane or interacting with one of the two lipid layers. Some proteins were even able to bind to the membrane via a short lipid chain.
• The membrane is fluid, but has a cytoskeleton-anchored structure beneath it. The fluid nature of the lipid matrix constituting the membrane was originally demonstrated by artificially fusing membranes of different compositions. In less than an hour, the proteins of both cells redistributed themselves across the entire joined membrane.
• Certain membranes have been studied in great detail, with a resolution of less than a nanometer, using modern imaging techniques. These pictures can even disclose the relative positions of polypeptide chains and lipids within the membrane.

Factors Affecting Fluidity of Plasma Membrane

• Temperature: Temperature plays an important influence in determining the fluidity of the plasma membrane. The membrane becomes more fluid as the temperature rises, which can impact the stability and function of membrane proteins.
• Lipid composition: The makeup of the lipids that comprise the plasma membrane can also have an effect on its fluidity. The fluidity of lipids with unsaturated fatty acid tails is typically greater than that of lipids with saturated fatty acid tails.
• Cholesterol content: Cholesterol content is an additional element that affects the fluidity of the plasma membrane. At low temperatures, cholesterol works as a buffer and keeps the membrane from becoming excessively rigid, but at high temperatures, it stabilises the membrane and prevents it from becoming excessively fluid.
• Membrane protein content: The presence of membrane proteins can also influence the fluidity of the plasma membrane. The size and form of proteins can inhibit the fluidity of the surrounding lipids, resulting in a less fluid membrane.
• Cytoplasmic and extracellular environment: The chemical and physical features of the intracellular and extracellular environments can also have an effect on the fluidity of the plasma membrane. For instance, excessive salt concentrations might reduce membrane mobility, but a low pH can enhance it.

Fluid Mosaic Model Function

• They cause compartmentalization by separating the cells from their external environment. As organelle coverings, they enable the organelles to maintain their identity, internal environment, and functional diversity.
• The plasma membrane safeguards the cell from harm.
• The cell membrane permits the exchange of materials and information between organelles within the same cell as well as between cells.
• The selective permeability of cell membranes allows only certain chemicals to enter inward to varying degrees. Others are impenetrable to the membranes.
• On the surface of the plasma membrane are particular chemicals that serve as recognition centres and attachment sites. This allows WBCs to distinguish between germ and body cells.
• It offers a permeability barrier, preventing the exit of cellular material from the cell, while allowing the selective admission of organic and inorganic molecules. Hence, plasma membranes demonstrate selective permeability.

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References

• https://ib.bioninja.com.au/standard-level/topic-1-cell-biology/13-membrane-structure/fluid-mosaic-model.html
• https://www.jove.com/science-education/10698/the-fluid-mosaic-model
• https://www.biologyonline.com/dictionary/fluid-mosaic-model
• https://www.khanacademy.org/science/ap-biology/cell-structure-and-function/membrane-permeability/a/fluid-mosaic-model-cell-membranes-article