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, 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.
Fluid mosaic model definition: 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.
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 summarises 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 neighbouring 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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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 centres 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 facilitate the selective entry of organic and inorganic substances inside. Hence, the plasma membranes show selective permeability.
Q 1: What are the Factors Affecting the Fluidity of the Plasma Membrane?
A: 3 main factors influence cell membrane fluidity:
Temperature: The temperature affects how 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 gases 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.
Q 2: Who Proposed the Fluid Mosaic Model of the Plasma Membrane?
A: Please have a look at the ‘Who proposed fluid mosaic model section’?
Q 3: What are the Molecules That can go Through the Cell Membrane?
A: 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.
Q 4: Explain the Fluid Mosaic Model of the Plasma Membrane.
A: Please have a look at the structure of the fluid mosaic model section.