Cells are the structural and functional basis of all plants and animals. These cells control all the physiological activities in the body of plants and animals. Although the cell is the smallest unit, it plays a major role in the body of the organism. Prokaryotic cells are composed of capsules, cell walls, cell membranes, cytoplasm, genetic material, pili, and flagella. The presence of cell membranes in both cells can be observed. The topic of the discussion below is this cell membrane structure (3) & (5).
A cell membrane or plasma membrane is a delicate, living, elastic, bilayer and the differentiated membrane made up of lipoproteins on the outside of each animal cell and below the cell wall of a plant cell. It is also called plasmalemma, cytomembrane, and biomembrane.
The cell membrane is an important component of the cell. It protects the cell from various external damages. An example of a cell membrane visible to the naked eye is a thin membrane attached to a boiled egg. The cell membrane gives the cell a specific shape.
This membrane separates one cell from another cell. Cell membranes help to form and protect the endoplasmic reticulum, the Golgi bodies, nuclear membrane, and the cell organelles inside the cell (2) & (1).
It is the delicate and elastic membrane on the outside of the protoplasm of the eukaryotic cell. The cell membrane is located in the next part of the cell wall of the eukaryotic plant cell. On the other hand, the outer covering of the animal cell is the cell membrane. The location of the cell membrane just below the cell wall of the prokaryotic cell (6).
The word “cell membrane” was first used in 1855 by C. Nageli and C. Cramer. In 1913, a scientist J. Q. Plowe used the term plasmalemma instead of a cell membrane. Then various cytologists said that plasma membrane is another name for cell membrane (1).
There are some properties of the cell membrane.
1. The cell membrane is a living part of the cell.
2. Cell membranes are present in all plant and animal cells.
3. The cell membrane is a semi-permeable membrane.
4. Between the two protein layers of the cell membrane there is a lipid layer.
5. Carbohydrates layer in the cell membrane is called the glycocalyx.
6. The cell membrane preserves the cell organs and regulates the movement of various objects.
7. This membrane is a membrane that is very delicate and resilient.
8. Many times the cell membrane folds to form microvilli for absorption and the cells form desmosomes to stay attached (1) & (7).
Structure of cell membrane
Thousands of substances interact in cells to form a life. Structure of the cell membrane is complex. The cell membrane performs all activities by its specific structure and properties. It is about 75Å thick and is made up of three layers.
1. The first layer is the outer dense layer about 20Å and is made up of proteins. The thickness of this layer is the highest.
2. The middle layer is light-colored with a thickness of about 35Å. This layer is the next layer of the outer dense protein layer and made up of lipids. The middle layer is located between the outer and inner thick protein layers.
3. The last layer is the inner layer is also 20Å. Its thickness is very high like the outer layer, it is also made up of proteins.
The plasma membrane or cell membrane contains some amount of water along with proteins and lipids. However, the amount of water is not high here. Cell membranes contain about 30% of water.
There are many tiny holes in the cell membrane with 8 to 50 Å in diameter. Different scientists have described the structure of cell membranes in different ways. There are different structures of cell membranes based on this discussion.
Like- Danielli and Davson model, Robertson’s unit membrane model, Hilleir and Hoffman’s micellar theory, Singer and Nicholson’s fluid-mosaic model, etc.
Danielli and Davson model is called the sandwich model. Danielli and Davson said plasma membranes have a bi-layered phospholipid layer. There are protein layers inside and outside the phospholipid layer.
According to Robertson, the plasma membrane is three-layered. The outer two layers of these three layers are made up of proteins and each of them is 20- 25Å thick. They are aquatic. Between these two layers lies a 30- 35Å thick lipid layer. Although all plasma membranes are made up of phospholipids and proteins, they are not present in equal amounts in all membranes.
Fluid mosaic models are the most common of the cell membrane structure models. Scientists Singer and Nicholson discover this model. Singer and Nicholson presented an exaggerated design of the plasma or cell membrane. According to them, the plasma membrane contains a two-layer liquid and protein particle (3) & (8).
The cell membrane is an important and necessary part of the cell. This membrane provides the distinction between intracellular and extracellular environments. These are made up of lipids, proteins, and carbohydrates. The cell membrane is composed of about 40% lipids and about 60% of proteins.
There are also some amounts of carbohydrates (5 to 10%) in the cell membrane. 40% of lipids contain phospholipids and cholesterol molecules. And 60% of proteins include lipoproteins, glycoproteins, enzymatic proteins, carrier proteins, structural proteins, etc. (1) & (6).
The function of cell membrane
1. The main function of the cell membrane is to give the cell a specific shape.
2. It surrounds all the objects in the cell.
3. This membrane protects the inner object from external adverse conditions.
4. The transfer of different objects is through the cell membrane.
5. It acts as a structure that contains enzymes.
6. The cell membrane also has a role in interconnection, growth, and movement.
7. It protects the protoplasm of the cell.
8. The cell membrane plays a major role in the diffusion and osmosis of cells.
9. The cell membrane helps in the entering of liquid and solid food into the cell in the process of pinocytosis and phagocytosis.
10. The cell membrane present in the cell controls the transport of various substances.
11. The function of the cell membrane is to produce and protect different cell organelles in the cell.
Written by: Manisha Bharati