Cell membrane which is also called the plasma membrane is a thin membrane that surrounds every living cell. It delimits the cell from the environment around it. Within the cell are its components, often large, water-soluble, highly charged molecules such as nucleic acids, proteins, carbohydrates, and substances that are involved in cellular metabolism. Outside the cell are nutrients that the cell must absorb to live and grow as well as ions, acids, and alkalis that are toxic to the cell. Hence, the cell membrane has two functions:
It acts as a barrier keeping the constituents of the cell in and unwanted substances out.
It acts as a gate allowing transport into the cell of essential nutrients and movement from the cell of waste products.
It describes the structure of cell membranes where a flexible lipid layer is spread with large protein molecules that act as channels through which other molecules enter and exit any cell.
According to this model, the components of a membrane such as proteins or glycolipids, form a mobile mosaic in the fluid-like environment created by a sea of phospholipids. There are restrictions to lateral movements, and subdomains within the cell membrane have distinct functions.
The fluid mosaic model of cell membrane was first proposed by S.J. Singer and Garth L. Nicolson in 1972. The model has evolved, but it still accurately summarizes the structure and functions of the plasma membrane. The mosaic model of membrane structure describes the structure of the plasma membrane as a mosaic of components including phospholipids, proteins, carbohydrates, cholesterol, and proteins that gives the membrane a fluid character.
Plasma membranes range from 5 -10 nm in thickness. The proportions of proteins, lipids, and carbohydrates in the plasma membrane are different from cell types. For example, myelin constitutes 18% of protein and 76% of lipid. The mitochondrial inner membrane has 76% of protein and 24% of lipid.
According to the fluid mosaic model of the cell membrane, it has a quasi fluid structure in which lipids and proteins are arranged in a mosaic manner.
The globular proteins are of two types: extrinsic and intrinsic proteins. The extrinsic protein is soluble and it dissociates from the membrane. The intrinsic protein is insoluble and is partially embedded either on the outer surface or on the inner surface of the bilayer and takes part in lateral diffusion in the lipid bilayer.
The lipid matrix of the membrane has a fluidity that permits the membrane components to move laterally. It is due to the hydrophobic interactions of lipids and proteins. The fluidity is important for a number of membrane functions. Phospholipids and many intrinsic proteins are amphipathic, that is they possess both hydrophilic and hydrophobic groups.
Phospholipids are the complex lipids that are made up of glycerol, two fatty acids and, in place of third fatty acid, a phosphate group bonded to one of several organic groups. They have polar (hydrophilic) as well as non-polar (hydrophobic) regions. The polar portion consists of a phosphate group and glycerol, while the nonpolar portion consists of fatty acids.
All nonpolar parts of the phospholipid contact only with the nonpolar part of the neighboring molecules. The polar portion occurs outside. This characteristic feature gives the appearance of a bilayer. However, between the fatty acid chains, proper spacing is maintained by interspersing unsaturated chains throughout the membrane. This type of arrangement maintains the semi-fluidity of the plasma membrane.
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A head of each phospholipid molecule is attracted to water, whereas its tail repels water. Both layers of the plasma membrane have the hydrophilic heads pointing toward the outside, whereas the hydrophobic tails form the inside of the bilayer. Cells bide in a watery solution called extracellular fluid, and they contain a watery solution inside of them (cytoplasm). The plasma membrane builds a circle around each cell in order to enable water-loving heads to be in contact with the fluid, and the water-fearing tails to be protected on the inside.
Lipids (phospholipids and cholesterol), carbohydrates attached to some of the lipids, and some proteins are the principal components of a plasma membrane. A molecule consisting of glycerol, two fatty acids, and a phosphate-linked head group is called a phospholipid. Cholesterol, another lipid made up of 4 fused carbon rings, is found alongside the phospholipids in the core of the membrane.
Lipids are referred to as the most abundant component of the fluid mosaic model. Lipids comprise both phospholipids and cholesterols. Phospholipids are amphipathic, i.e. they have both hydrophobic and hydrophilic parts. They form a lipid bilayer that splits up the inside of the cell from the outside. This lipid bilayer consists of the hydrophilic heads facing the aqueous environment inside and outside the cell, and the hydrophobic tails facing inward. Cholesterols, a type of steroids, are responsible for regulating membrane fluidity and flexibility. Membrane fluidity enables the movement of specific molecules and ions across the plasma membrane.
Proteins are the second major component of the mosaic. Proteins can distinctly associate with the lipid bilayer. For example, some are entirely integrated into the membrane, like integrins that serve as transmembrane receptors, and transport proteins that shuttle molecules across membranes. Such integrated proteins are called integral proteins. Other proteins can be found only on the surface of the cell or in the cytosol, as is the case with estrogen receptors, called peripheral proteins.
Carbohydrates is the last component of the fluid mosaic model. They are situated on the exterior surface of the membrane where they are bound to proteins for the formation of glycoproteins or to phospholipids for the formation of glycolipids. These carbohydrate complexes are called the glycocalyx i.e. the sugar coating of the cell. Some carbohydrates in the mosaic also play essential roles as markers allowing cells to distinguish between self i.e. cells of the same organism and non-self i.e. intruding foreign cells or particles.
Simultaneously, these components create a cell’s plasma membrane, with a thickness ranging between five to ten nanometers. Plasma membranes interact with their surroundings to carry out many essential processes to maintain cellular function and homeostasis.
The cell membranes cause compartmentalisation, they separate the cells from their external environment. As organelle coverings, they allow the cell organelles to maintain their identity, internal environment and functional individuality.
The plasma membrane protects the cell from any injury.
The cell membrane allows the flow of materials and information between different organelles of the same cell as well as between one cell and another.
Cell membranes have selective permeability, which means they allow only selected substances to pass inwardly to selected degrees. The membranes are impermeable to others.
The plasma membrane possesses specific substances at its surface which function as recognition centers and points of attachment. Because of this, WBCs can differentiate between germ and body cells
It provides a permeability barrier and thus prevents the escape of cellular materials outside the cell, and facilitates the selective entry of organic and inorganic substances inside. Hence, the plasma membranes show selective permeability.
A fluid mosaic model is demonstrated for the gross organization, structure of the proteins and lipids of biological membranes. The model is consistent with the restrictions urged by thermodynamics. The proteins that are integral to the membrane are a heterogeneous set of globular molecules in this model. Each molecule is arranged in an amphipathic structure in such a way that the nonpolar groups largely buried in the hydrophobic interior of the membrane, and the ionic and highly polar groups protrude from the membrane into the aqueous phase.
1. What are the Factors Affecting the Fluidity of the Plasma Membrane?
Three main factors influence cell membrane fluidity:
Temperature: The temperature affects phospholipids. When it’s cold the phospholipid molecules are found closer together and when it’s hot they move away from each other.
Cholesterol: The cholesterol molecules are randomly distributed across the PLP bilayer, helping the bilayer stay fluid in different environmental conditions. The cholesterol holds phospholipids together so that they don’t separate too far.
Also, without cholesterol, the phospholipids in cells will start to get closer together when exposed to cold. This would make it difficult for small molecules such as gasses to enter in between the phospholipids like they normally do.
Saturated and Unsaturated Fatty Acids: Fatty acids are what make up the phospholipid tails. Saturated fatty acids are chains of carbon atoms that have only single bonds between them. As a result, the chains are straight and easy to pack tightly. Unsaturated fats are chains of carbon atoms that have double bonds between some of the carbons. The double bonds create kinks in the chains, making it harder for the chains to pack tightly.
2. Who Proposed the Fluid Mosaic Model of the Plasma Membrane?
Plasma membrane is the cell membrane that separates the interior and exterior components of the cell from the surroundings. The cell wall is on the outer side of the plasma membrane. According to the Fluid mosaic model, there is a lipid bilayer having proteins embedded in them. This bilayer provides elasticity and fluidity to the plasma membrane. It limits the lateral diffusion of membrane components because it has the two-dimensional liquid, and there is little carbohydrate in this layer.
Please have a look at the ‘Who proposed fluid mosaic model section’?
3. What are the Molecules That can go Through the Cell Membrane?
There are 5 major categories of molecules found in the cellular environment. These can travel across the cell membrane:
Small, nonpolar molecules such as oxygen and carbon dioxide can travel across the lipid bilayer and do so by squeezing through the phospholipid bilayers.
Small, polar molecules such as water molecules cross without the help of proteins. This is a slow process.
Large, nonpolar molecules such as carbon rings also travel through but it is again a slow process.
Large, polar molecules such as glucose.
4. Explain the Fluid Mosaic Model of the Plasma Membrane.
The Fluid mosaic model was proposed by Singer and Nicolson in 1972. As stated in this model, the quasi-fluid nature of lipids allows lateral movement of proteins within the overall bilayer. The ability of proteins to move within the membrane is measured as its fluidity. The plasma membrane is made up of lipids, in a bilayer and within the cell membrane. The polar head is situated towards the outer sides, whereas the hydrophilic tails towards the inner part. Please have a look at the structure of the fluid mosaic model section.