Plasma Membrane Structure

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The Plasma Membrane

The Plasma Membrane of a cell is composed of a Lipid Bilayer with Proteins embedded in it. The hydrophobic region of the lipid bilayer core excludes water, whereas the Hydrophilic regions both inside and outside of the cell allow water to be drawn into the structure through osmosis. In addition, there are various types of Protein structures associated with the Membrane, including integral Proteins, which are completely embedded in the lipid bilayer, and peripheral Proteins, which attach to the Membrane through their terminus region.


Lipid Bilayers

The cell's Plasma Membrane is composed primarily of lipids arranged into a lipid bilayer structure. Cholesterol is one component that contributes towards increasing the fluidity of the Membrane, as well as preventing the formation of lipid rafts which can compartmentalize the cell and restrict communication between different parts of the Membrane. The lipid bilayer is a highly stable structure that is resistant to changes in pH or temperature.


Proteins

The Proteins embedded in the Plasma Membrane play a variety of roles in the cell, including providing structural support for the Membrane to form a three-dimensional shape. In eukaryotic cells, Proteins are also involved in cellular signalling cascades involving hormones and neurotransmitters, which affect other molecules associated with the lipid bilayer structure.


Peripheral Proteins

The peripheral Proteins are attached to the lipid bilayer through their terminus region and can be found on the outside or inside of the cell. They play a variety of roles in the cell, including acting as receptors for signaling molecules, transporting ions or molecules across the Membrane, and participating in Protein-Protein interactions.


Integral Proteins

The integral Proteins are completely embedded in the lipid bilayer structure and are generally classified into two categories: alpha-helical or beta-barrel structures. The integral Proteins can be composed of either hydrophobic amino acids, which allows them to reside in the lipid bilayer core structure, or can contain both hydrophobic and hydrophilic regions in order to allow insertion into the Membrane. It is important to note that the Proteins embedded in the Plasma Membrane are not static and can move around within the lipid bilayer in a process known as lateral diffusion.


The Plasma Membrane

The Plasma Membrane is also known as the cell Membrane. It is a Membrane found in all living cells, and it demarcates between the inner part of the cell and 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 the Plasma Membrane are:

  • Phospholipids  

This forms the chemical composition of the 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 consist 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 these 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 nonpolar 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 nonpolar 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 these 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.

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FAQs (Frequently Asked Questions)

1. What is the Structure of a Plasma Membrane?

The 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?

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. 

3. What is the Plasma Membrane?

The Plasma Membrane is a lipid bilayer that separates the fluid inside the cell from the fluid outside of the cell. It is composed of various components, including Proteins, cholesterol, phospholipids, and carbohydrates. Proteins can be integral, meaning they are embedded in the Membrane, or they can be peripheral, meaning they are attached to the Membrane. Plasma Membrane components are dynamic, meaning they can move around and interact with one another. A Plasma Membrane is a semipermeable Membrane which means it allows a few things to pass through.

4. What shapes do cells have?

Cells can be round, flat, long and skinny, or any other shape depending on the type of cell. The Plasma Membrane helps to dictate the shape of the cell by allowing some things to pass through while keeping other things out. For example, the Plasma Membrane allows water and oxygen to pass through while keeping toxins and other harmful substances out. This is why cells have a specific shape that allows them to protect their internal environment while allowing in what they need to survive.

5. What is a lipid bilayer?

A lipid bilayer is a thin sheet composed of lipid molecules that form a barrier between two other solutions. It consists of hydrophobic and hydrophilic regions. In Plasma Membranes, the lipid part is the outer layer, while the hydrophilic part is on the inside. This allows for cell-to-cell communication and the transport of substances into and out of the cell. The lipid bilayer is a semipermeable Membrane which means it allows a few things to pass through and is permeable to small molecules and ions.

6. What is the fluid mosaic model?

The fluid mosaic model is an early model of the Plasma Membrane that was proposed by Singer and Nicholson. It states that there are lipids, Proteins, carbohydrates, and cholesterol in the Plasma Membrane that move around freely to form a barrier. Proteins can be found both embedded in the lipid bilayer as well as on the exterior. The fluid mosaic model is a dynamic model, which means that the components are in constant flux and can move around and interact with one another. This allows for cells to respond to their environment and change as needed.

7. What are integral Proteins?

Integral Proteins are Proteins that are completely embedded in the lipid bilayer. They have a hydrophobic transMembrane domain that allows them to pass through the Membrane and a hydrophilic CytoPlasmatic domain that is exposed to both sides of the Membrane. Integral Proteins are molecules that are embedded in the lipid bilayer. Proteins can be made up of amino acids, which are small organic molecules that form long chains together to create Proteins. Proteins are vital because they perform many different roles in cells, including transporting materials in and out of the cell, acting as enzymes or producing energy, and much more. Integral Proteins are important for the cell because they help to maintain the structure of the Plasma Membrane and allow for communication and transport to occur.

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