Cell membranes, also known as biomembranes, play a vital role in living cells. They are thin, delicate, elastic, and selectively permeable boundaries that enclose the cell.
Various names, such as cell membrane by Nageli and Kramer, plasma lemma by J.Q. Plower, or bio-membrane or plasma membrane have referred to these membranes.
The concept of the “unit membrane” was proposed by Robertson.
Structure of Cell membranes:
There have been different theories and models proposed to explain the structure of cell membranes over the years. Such as-
- Lipoidal theory
- Sandwich model
- Unit membrane model
- Fluid mosaic model
The lipoidal theory, put forward by Overton in 1902, suggested that the plasma membrane consists of a single layer of lipids, based on its permeability characteristics.
However, this theory was later found to be insufficient in explaining the complexity of the membrane.
In 1935, Davson and Danielli proposed the sandwich or tri-lamellar model. According to this model, the plasma membrane is composed of three layers: two layers of proteins surrounding a bimolecular layer of lipids.
Each protein layer is approximately 20Å thick, while the phospholipid bilayer measures around 35Å. Therefore, the total thickness of the membrane is approximately 75Å (with an average range of 75-100Å).
Phospholipid molecules are referred to as amphipathic because they possess hydrophilic head and hydrophobic tail regions. The hydrophilic heads of the phospholipids interact with the protein layers through hydrogen and ionic bonds, while the hydrophobic tails are attracted to each other through van der Waals forces.
Unit Membrane Model
Robertson proposed the unit membrane model in 1959, suggesting that all cellular and organelle membranes share a similar structure and function, with differences primarily in chemical composition and size. However, these models were eventually rejected as they failed to adequately explain the elasticity and selective permeability observed in plasma membranes.
Fluid Mosaic Model,
The most widely accepted model for the structure of the plasma membrane is the fluid mosaic model, proposed by Singer and Nicolson in 1973.
According to this model, proteins are arranged in a mosaic pattern within the phospholipid layer. The membrane can be envisioned as a protein iceberg floating in a sea of phospholipids or a gulab jamun (a popular Indian sweet) suspended in a concentrated sugar solution (representing the proteins and phospholipids, respectively).
Phospholipids are the main components of cell membranes, constituting approximately 40% of the membrane composition. They play a crucial role in forming the continuous structural framework of the cell membrane.
The main types of phospholipids found in biomembranes include phosphatidyl serine, phosphatidylcholine (lecithin), and P-ethanolamine (cephalin).
The fluidity of the plasma membrane is a crucial characteristic that allows for the proper functioning of cells. This fluidity is primarily attributed to the presence of phospholipids, which are rich in unsaturated fatty acids. U
nsaturated fatty acids have double bonds in their carbon chains, which introduce kinks and prevent tight packing of the phospholipids. As a result, the phospholipid layer becomes more fluid as these unsaturated fatty acids are liquid in nature.
A derivative of lipid, cholesterol, is also present in the plasma membrane. Cholesterol molecules have a more rigid structure compared to phospholipids. Consequently, they play a role in maintaining the stability of the membrane structure.
Molecules of cholesterol insert themselves between the phospholipids, reducing their mobility and preventing excessive fluidity. By interacting with the phospholipids, cholesterol helps regulate the fluidity of the plasma membrane, ensuring optimal membrane function.
It is worth noting that the presence of cholesterol differs in prokaryotic and eukaryotic membranes. Cholesterol is absent in prokaryotic cell membranes. Instead, hopanoids, specifically pentacyclic sterols, provide stability to the prokaryotic cell membrane.
Hopanoids share similar structural characteristics with cholesterol and serve a comparable function in stabilizing the membrane structure of prokaryotes.
Proteins, comprising approximately 57% of the plasma membrane, are another essential component. Two types of proteins can be found in the membrane-
- Integral (or intrinsic) proteins
- Peripheral (or extrinsic) proteins
Integral (or intrinsic) proteins
The outer surface of plasma membranes contains oligosaccharides found in glycolipids and glycoproteins, which play a significant role in cell-to-cell recognition. Fertilization, such as the recognition between sperm and egg, and blood antigen interactions are prime examples of cell recognition mechanisms. Furthermore, the plasma membrane houses around 30 types of enzymes, including the vital ATPase enzyme responsible for hydrolyzing ATP. ATPase plays a crucial role in active transport processes, facilitating the movement of materials across the membrane.
Integral proteins tightly bind to the phospholipids and are not easily released from the membrane. Some of these proteins are confined within the lipid bilayer, providing stability to the membrane structure.
Others traverse the entire thickness of the membrane, acting as tunnel proteins that facilitate the passage of water-soluble materials across the membrane.
Transmembrane proteins, such as glycophorin and porins, extend from the outer to the inner side of the membrane. Porins are found in the outer mitochondrial membrane and bacterial membranes.
Peripheral (or extrinsic) proteins
peripheral proteins are superficially arranged on the outer side of the membrane and can be easily separated. These proteins often possess enzymatic activity and can freely move within the membrane structure.
Some proteins, such as permeases and translocases, function as carriers for the transport of materials across the membrane.
Spectrins, which are helical extrinsic proteins, are found on the cytosolic face of the membrane and are attached to intrinsic proteins. Spectrins are part of the cytoskeleton, providing structural support to the membrane.
The plasma membrane exhibits an asymmetrical structure. Carbohydrates are predominantly present on the outer surface, forming glycolipids or glycoproteins.
In contrast, spectrin proteins are found exclusively on the inner surface of the plasma membrane. This asymmetry contributes to the unique functions and interactions of the membrane with its surroundings.
The outer surface of plasma membranes contains oligosaccharides found in glycolipids and glycoproteins, which play a significant role in cell-to-cell recognition.
Fertilization, such as the recognition between sperm and egg, and blood antigen interactions are prime examples of cell recognition mechanisms.
Furthermore, the plasma membrane houses around 30 types of enzymes, including the vital ATPase enzyme responsible for hydrolyzing ATP. ATPase plays a crucial role in active transport processes, facilitating the movement of materials across the membrane.