[Biology Class Notes] on Fluid Mosaic Model Theory Pdf for EXAM

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.

What is the Fluid Mosaic Model?

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.

Who Proposed a Fluid Mosaic Model?

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.

Structure of Fluid Mosaic Model / Mosaic Model of Membrane Structure

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. 

Fluid Mosaic Model Components

Lipids

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

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

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.

Fluid Mosaic Model Function

• 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.

Conclusion:

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.