Cell Membrane

Key Concepts

• Plasma membrane
• Fluid mosaic model
• Osmosis
• Diffusion

Introduction: 

We know that a cell membrane is also called a plasma membrane or cytoplasmic membrane. It is a biological membrane that protects internal organelles from its outside atmosphere. The main function of a plasma membrane is to protect the cell. It is consists of a phospholipid bilayer with proteins enclosed in it. The cell membrane is selectively permeable to ions and some organic molecules. Cell membrane is very accommodating in order to permit certain molecules or cells such as red blood cells and white blood cells to pass through it. 

The complete structure of the cell membrane was studied after the discovery of electron microscope. The understanding of the structure of the cell membrane is outside the resolution of a light microscope. Thus, all the experiments performed before the discovery of electron microscope could only provide indirect proof for the presence of a membrane-like structure all over the cell. 

The presence of cell membrane can be shown or understood by the points given below:  

• The presence of cell membrane in plant cells can be confirmed by the plasmolysis of plant cells in hypertonic solution. 
• Animal cells occur as a physical entity even without a cell wall. This is possible only due to the existence of some form of membrane encircling it. 
• When the cell’s exterior part is punctured, protoplasm comes out. From this, it can be assumed that the seepage is due to the rupture of the surrounding membrane. 

Models of Cell membrane 

Many authors have given different models and ideas for the presence and structure of cell membrane. But Fluid mosaic model suggested by Singer and Nicolson is the most acknowledged of them all. In 1902, Overton suggested a fundamental model for the transfer of small neutral solutes. He tried on numerous cells both from plants and animals and decided that these unique osmotic properties of living protoplasts are due to the selective solubility method of the membrane. He also speculated that outer cover of the membrane comprised of fats and sterols. Overton hypothesized that the cell membrane comprised of a thin layer of lipid. 

In 1926, Gorter and Grendel performed experiments on hemolyzed RBC and decided that the RBCs were covered by two layers of lipid molecules over the complete cell surface.  

In 1931, Danielle and Harvey examined the surface tension of the cells. Usually, surface tension at the water-oil interface is about 0.01-0.015 Newton/meter, whereas the surface tension for cells is zero. This low surface tension for the cell shows the presence of proteins in the cell membrane. 

The following are the molecular models of the cell membrane suggested by different biologists. 

Lipid – Lipid Bilayer Model: 

In 1902, Overton noted that substances soluble in lipid could selectively pass through the membranes. On the basis of this, he said that the cell membrane is composed of a thin layer of lipid. 

Gorter and Grendel (1925) were the first scientists to propose a possible structure of the cell membrane. On the basis of studies of plasma membranes of red blood cells, they showed that the lipid obtained from the red cell spread in area two times that of a simple molecular film. So, it was believed that the membrane comprised of two layers of lipid molecules, the polar hydrophilic groups of the molecules positioned on the outside and hydrophobic ends placing at right angles to the surface positioned on the inside. These models of Gorter and Grendel could not describe the appropriate structure of plasma membrane, but they laid the groundwork for future models of membrane structure. 

Sandwich Model (Davson –Danielli Model) 

In 1935, Davson and Danielli suggested sandwich or trilamellar model for plasma/ cell membrane. According to this model, the plasma membrane is a sheath-like structure that consists of two lipid layers sandwiched between uninterrupted or continuous layers of proteins.  In this model, the arrangement of phospholipid molecules is so that hydrophilic heads of the phospholipid molecules face outside and hydrophobic non-polar lipid tail face in the inner region of the membrane. The model also suggested that the polar ends of lipid molecules are associated with a single molecular layer of globular proteins. So, the cell membrane thus, consists of a double layer of phospholipid molecules sandwiched between two uninterrupted layers of protein. This model is also called a lipo-protein sandwich model because the lipid layer is sandwiched between two protein layers. 

Unit Membrane Model: 

In 1959, Robertson suggested the unit membrane model, which says that all cellular membranes have the same membrane structure. They named this same membrane structure as the unit membrane. As per this model, the unit membrane comprises of bimolecular lipid present in between outer and inner layers of protein. In the unit membrane model, the protein coatings are asymmetrical. Mucoprotein is present on the outer surface, while on the inner surface, non-mucoid protein is present.

 

Fluid mosaic model 

The fluid mosaic model was first proposed in 1972 by Singer and Nicolson to describe the composition of the cell membrane. The fluid mosaic model explains the structure of the cell membrane as a mosaic of components that contains phospholipids, cholesterol, proteins, and carbohydrates. It provides the cell membrane with a fluid character. The thickness of cell membranes ranges from 5 to 10 nm. The amount of proteins, lipids, and carbohydrates in the cell membrane differs with cell type. For example, myelin has 18% protein and 76% lipid whereas the mitochondrial internal membrane has 76% protein and 24% lipid. 

Fluid part – the phospholipid bilayer is viscous, and single phospholipids can move position. 

Mosaic part – the phospholipid bilayer is inserted with proteins, developing in a mosaic of components. 

Every phospholipid molecule has a head that is hydrophilic (attracted to water, hydro = water; philic = loving), and a tail that is hydrophobic (repulses water, hydro = water; phobic = fearing). Both layers of the cell membrane have the hydrophilic heads facing outside and the hydrophobic tails facing inside of the bilayer. Because cells live in an extracellular fluid (watery solution), and they contain a cytoplasm (liquid) inside them, the cell membrane creates a circle all-around every cell so that the water-loving heads are in contact with the extracellular fluid, and the water-fearing tails are safe on the internal side. In other words, the proteins are inserted in the lipid bilayer in such a way that proteins float in the lipid sea. The surface of the lipid layers is disrupted by haphazardly scattered protein molecules. These proteins may either stick to the polar surface of the lipids or partly penetrate the lipid bilayer. Some proteins are also found to be connected with the sugar chains of glycoprotein. In simple words, as per this model, proteins were embedded within the lipid bilayer rather than existing as separate layers. 

Functions of cell membrane (plasma membrane)  

• We know that the plasma membrane allows the transport of selective substances inside the cell and outside the cell. Hence, the plasma membrane is called a selectively permeable membrane.  
• Plasma membrane helps to keep the adjacent cells in contact.  
• The cell membrane allows the transport of small molecules like water, mineral ions, glucose, amino acids, oxygen, CO₂ etc.). The movement of these small molecules across the plasma membrane is done by any one of the following methods:  

Osmosis: In this method, the water molecules move from the area of higher concentration (where the solute is less and water is more in the solution) to the area of lower concentration (where the solute is more and water is less) through a semipermeable membrane. In this process, energy is not used. In osmosis, the only movement of water molecules takes place by diffusion. Here, the movement of solute does not occur. 

Diffusion: In this method, the solute and solvent molecules move freely from the higher concentration area to the lower concentration area to reach an equilibrium. In this method, energy is not spent. For example, the diffusion of perfume spray in the air, diffusion of insecticide spray in the air. Diffusion and osmosis are passive transport mechanisms as they do not require energy for the process to take place.  

Summary:

1. Cell membrane is also called a plasma membrane.
2. It consists of a phospholipid bilayer with proteins enclosed in it.
3. The cell membrane is selectively permeable to ions and some organic molecules.
4. In 1902, Overton noted that substances soluble in lipid could selectively pass across the membranes.
5. Gorter and Grendel (1925) were the first scientists to propose a possible structure of the cell membrane.
6. In 1935, Davison and Danielli suggested sandwich or trilamellar model for plasma/ cell membrane structure.
7. In 1959, Robertson suggested the unit membrane model, which says that all cellular membranes have the same membrane structure.
8. The fluid mosaic model first proposed in 1972 by Singer and Nicolson explains the structure of the cell membrane as a mosaic of components that contains phospholipids, cholesterol, proteins, and carbohydrates.
9. The movement of small molecules across the plasma membrane is done by osmosis and diffusion.