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Fluid Mosaic Model of Cell Membrane Parts and Function

In this fluid mosaic model of cell membrane parts and function post we have briefly explained Phospholipids Cholesterol and Proteins of membrane and fluid mosaic model of plasma membrane.

S.J. Singer and Garth L. Nicolson first suggested the fluid mosaic model of plasma membrane in 1972 to describe the shape of the fluid mosaic model of plasma membrane. The fluid mosaic model of plasma membrane has changed in the years, but it is still the best way to explain the functions and structure of plasma membranes in the way we know them today.

Fluid mosaic model of plasma membrane explains the plasma membrane’s structure as a mosaic of constituents comprising cholesterol, phospholipids, carbohydrates, and proteins that provide the membrane with a fluid appearance. Plasma membranes vary from 5 to 10 nanometres in thickness. 

For example, as seen through light microscopy, the human blood red cells measure around 8 um in width, which is approximately 1,000 times larger than the plasma membrane. The percentages of lipids, proteins, and carbohydrates found in the plasma membrane differ according to the cell type. For example, myelin contains 18% protein and 76% lipid. The mitochondrial inner membrane is made up of 76% protein and 24% in lipid.

Fluid Mosaic Model of Plasma Membrane

Fluid mosaic model of plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins. Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane. Image Source: https://bio.libretexts.org/

Parts and Function

The cell membrane is primarily made up of three things: Phospholipids Cholesterol and Proteins.

Phospholipids

The principal membrane’s structure comprises amphiphilic, or dual-loving, phospholipids. The water-loving or hydrophilic regions of the molecules are in contact with the aqueous fluid both within and outside of the cell. The hydrophobic molecules, also known as water-hating, tend to be non-polar.
A phospholipid molecule comprises a glycerol backbone made of three carbons, having 2 fatty acid molecules bonded with carbons 1, 2 and a phosphate-containing molecule attached on the 3rd carbon.

Fluid Mosaic Model of Plasma Membrane

Phospholipid aggregation: In an aqueous solution, phospholipids tend to arrange themselves with their polar heads facing outward and their hydrophobic tails facing inward. Image Source: https://bio.libretexts.org/

This arrangement gives the molecules an area known as the head (containing phosphates) that is polar or has a negatively charged, as well as a second area known as the tail (the fatty acids) with no charge. 

They can interact with non-polar molecules in chemical reactions, but they generally don’t interact with polar molecules. In water, hydrophobic molecules are likely to form a ball or a cluster. 

The hydrophilic regions of phospholipids are prone to create hydrogen-bonded bonds to water and other polar molecules both on the inside and outside cells. So, the membrane surfaces facing the interior and outside of cells is hydrophilic. 

The centre of cell membranes is hydrophobic and does not react with water. Thus, phospholipids make an ideal lipid bilayer cell membrane, separating the cells’ fluid from fluid outside the cell.

Fluid Mosaic Model of Plasma Membrane

The structure of a phospholipid molecule: This phospholipid molecule is composed of a hydrophilic head and two hydrophobic tails. The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains. Image Source: https://bio.libretexts.org/

Protein

Proteins are the second largest element of fluid mosaic model of plasma membrane. Integral proteins are like their namesake. They are completely integrated into the structure of membranes, and their hydrophobic membrane-spanning regions are in contact with the hydrophobic portion of the bilayer of phospholipids.

Integral membrane proteins that are single-pass generally contain a transmembrane protein segment that is hydrophobically comprised of 20-25 amino acids. Some span only part of the membrane associating with a single layer while others stretch from one side of the membrane to the other and are exposed on either side.

Structure of integral membrane proteins in fluid mosaic model of plasma membrane: Integral membrane proteins may have one or more alpha-helices that span the membrane, or they may have beta-sheets that span the membrane. Image Source: https://bio.libretexts.org/

Fluid Mosaic Model of Plasma Membrane

Complex proteins can consist of 12 or more segments of one protein that are folded extensively and are encased within the membrane. The type of protein described has the hydrophilic regions or regions as well as one or more moderately hydrophobic regions.

This arrangement of regions of the protein tends to orient the protein alongside the phospholipids, with the hydrophobic region of the protein adjacent to the tails of the phospholipids and the hydrophilic region or regions of the protein protruding from the membrane and in contact with the cytosol or extracellular fluid.

Carbohydrates

Carbohydrates constitute the third main part of the plasma membrane. They are usually found on the outside of cells. They are linked by proteins (forming glycoproteins) or lipids (forming glycolipids). These chains of carbohydrates can be made up of two to 60 monosaccharide units and are either straight or branching.

Together with peripheral proteins, carbohydrates are specialized areas on the cell’s surface that allow cells to recognize one another. Recognition functions are vital to cells since it helps the immune system discern from cell types within the body and tissue or foreign cells. Similar glycolipids and glycoproteins can be present on the surface of viruses, and they can frequently alter, preventing immune cells from attacking and recognizing them.

The carbohydrates found on the outside cell’s surface the carbohydrates that make up glycoproteins and glycolipids are collectively known by glycocalyx (meaning “sugar coating”). The glycocalyx is extremely hydrophilic and draws large quantities of liquid to the cell’s surface. This assists in the interplay that the cell has with its surrounding water and the ability of cells to absorb substances that dissolve in water.

In Short

The principal layer of the membrane comprised of dual-loving, or amphiphilic and molecules of phospholipids.

Integral proteins, which are the second most important part of the plasma membrane, are completely integrated into the membrane structure by their membrane-spanning regions of hydrophobic membranes that interact with the hydrophobic area of the bilayer of phospholipids.

Carbohydrates, which are the third main element of plasma membranes, are usually found on the surface of cells. They are bound by protein (forming glycoproteins) or the lipids (forming glycolipids).

Further Readings

Fluid Mosaic Model of Cell Membrane Parts and Function