Plasma Membrane Structure

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The Plasma Membrane

The plasma membrane is also known as the cell membrane. It is a membrane found in all the living cells, and it demarcates between the inner part of the cell from the outer part. In the case of plant cells, a cell wall is observed before the plasma membrane in the exterior part of the plant cells. The same is the case for some bacterial cells as well. The plasma membrane is mainly composed of a layer of lipid molecules. This layer is semipermeable and is responsible for the regulation and transportation of materials. Also, the movement of molecules, both large and small, in and out of the cell is controlled by the plasma membrane.

Components of Plasma Membrane

When asked what is the composition of the plasma membrane the composition can be divided into lipids and proteins. The main constituents that form the composition of plasma membrane are:

  • Phospholipids – 

This forms the chemical composition of plasma membrane. These are lipid molecules with the phosphorus head groups forming the main fabric of the membrane

  • Peripheral Proteins – 

They occur on the outer or inner surface of the phospholipid bilayer, not in the middle

  • Cholesterol – 

The hydrophobic tails of the phospholipid bilayer consists of these molecules.

  • Carbohydrates –

Usually found on the outside of the membrane which also forms glycoproteins and glycolipids

  • Integral Proteins – 

They are embedded inside or in-between the bilipid layer. 

Plasma Membrane Structure and Function

The plasma membrane is fluid in nature and as mentioned is made up of proteins, lipids, and carbohydrates. It does not allow the ions and other water-soluble molecules to pass through. Whenever required they can only pass through with the help of carriers or through the transmembrane channels and transmembrane pumps. The proteins present in the membrane are also responsible for creating and maintaining a potential difference and gradient. Disease like cystic fibrosis occurs due to defects in this function of the membrane. The structural composition ratio includes lipids making up 20% - 80% of the plasma membrane-based upon where it is present. 

When asked the plasma membrane is made up of which two components the answer is the lipids and the proteins. The plasma membrane is composed of a phospholipid bilayer, i.e. two layers set up back-to-back. When asked what is the chemical composition of the plasma membrane the general answer is the phospholipid bilayer because the phospholipids as described above have one head and two tails where the head is polar and hydrophilic. Tails on the other hand are nonpolar and hydrophobic and hence they form the interior portion of the membrane. There are also proteins embedded in the membrane.

The Functions of the Plasma Membrane in Points are Given below:

  • The proteins that are embedded in the membrane perform specialized functions such as cell-cell recognition and the transport of molecules selectively. 

  • It provides protection to the cell along with a fixed environment in the interior of the cell. It is responsible for performing different functions. 

  • In the case of white and red blood cells, it is flexible as it aids in the movement through the blood capillaries.

  • In addition, it also holds the cytoskeletal structure that provides shape to a cell and associates with the extracellular matrix and other cells to form a tissue. 

  • The plasma membrane is the primarily responsible component for interacting with adjacent cells. 

  • The membrane also helps the proteins to monitor and maintain the chemical climate inside the cell, and also provides aids in shifting the molecules in the membrane.

The Fluid Mosaic Model of the Plasma Membrane

When asked to describe the structure of the plasma membrane, the structure is usually provided by the fluid mosaic model. This is because the structure of the plasma membrane is best represented through the fluid mosaic model as a collection of cholesterol, carbohydrates, proteins, and phospholipids. It was first proposed by Garth L. Nicolson and S.J. Singer. This model best explains the structure of the plasma membrane. The model depicts the plasma membrane structure as a mosaic of various components such as proteins, cholesterol, phospholipids, and carbohydrates and it also includes the fluid character of the membrane.

It is approximately 5nm - 10 nm thick. The components of the plasma membrane, i.e., carbohydrates, lipids, and proteins differ from cell to cell. For example, the inner membrane of the mitochondria consists of 24% lipid and 76% protein, in myelin it is 76% lipid and 18% protein.

The main fabric of this membrane consists of phospholipid molecules that are amphiphilic i.e. both hydrophobic and hydrophilic. The hydrophilic regions of the lipid molecules are in touch with the outside and inside environment of the cell. The hydrophobic molecules on the other hand are non-polar in nature and are present in the middle. One phospholipid molecule comprises a three-carbon glycerol backbone along with 2 fatty acid molecules associated with carbons 1 and 2, and one phosphate-containing group connected to the third carbon.

In this structure,  the head, which is a phosphate-containing group, possesses a negative charge while the tail, a region containing fatty acids, does not contain any charge. They usually interact with the non-polar molecules in a chemical reaction but, do not typically interact with the polar molecules.

When introduced to the water, the hydrophobic molecules show a tendency to form a cluster. The hydrophilic areas, unlike the hydrophobic ones, have the tendency to form hydrogen bonds with water and with other charged molecules inside and outside the cell. Therefore, the membrane surface present on the exterior and interior of cells is hydrophilic whereas the middle layer does not have any interaction with water. Hence, phospholipids form a lipid bilayer cell membrane and separate the fluid present inside the cell from the fluid present in the exterior of the cell.

The second major component is the proteins present in the plasma membrane. Integral proteins or known as integrins, integrate completely into the membrane. Typically, single-pass integrins have a hydrophobic transmembrane portion which consists of 20-25 amino acids. Few of this pass-through only a portion of the membrane being linked with one layer whereas others span from one side of the membrane to another side.

Some complex proteins contain 12 segments of one protein, in a highly convoluted form while being implanted into the membrane. Such types of proteins contain a hydrophilic region along with one or more hydrophobic areas. Owing to this nature, they typically align along with the phospholipids.

The third most important component of the membrane is carbohydrates. Generally found outside the cells they are attached to either the lipids or proteins forming glycolipids or glycoproteins respectively.  When present on the exterior surface of the cells, these carbohydrates and their components of both glycolipids and glycoproteins together are known as glycocalyx, which is extremely hydrophilic in nature and thus attracts huge quantities of water towards the cell surface. This helps the cell to be in constant interaction with its fluid-like environment and also in the ability of the cell to acquire any substance dissolved in water.

FAQ (Frequently Asked Questions)

1. What is the Structure of a Plasma Membrane?

Ans: Plasma membrane is the membrane that separates the inside and outside portions of the cell. It is a bilipid layer with the tails of the lipids present in the center and the head being at the ends. The membrane is quite fluid and is represented best by the fluid mosaic model.

2. What are the main Functions of the Plasma Membrane?

Ans: The main functions of the plasma membrane include:
(i) Protection of the cell against toxic substances.
(ii) Specific movement of molecules such as the nutrients, ions, etc for proper functioning.
(iii) Maintaining cellular integrity and inter-cell communication.