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Cell Membrane: A Fluid Mosaic Model

The cell is considered a structural and functional unit of life. Each and every cell present in the body is enclosed by a bubble-like structure called a cell membrane. It is also called the plasma membrane. Its main function is to separate the interior and exterior of the cell. 

While discussing the structure of the cell membrane it is important to know what cell membrane is made up of? The cell membrane or plasma membrane not only helps to differentiate the borders of the cell but also allows the cell to interact with its surrounding environment in a controlled and disciplined way. Cells should be able to exclude or take in or excrete certain substances in a specific amount. In addition to this, they are able to communicate with other cells, by identifying the other cells and sharing the relevant information.

Here, we’ll have a closer look at the different components of the plasma membrane. The cell membrane function, the diversity of the membranes. How these membranes work together to make a sensitive, flexible, and secure boundary around the cell.


Plasma Membrane Structure

The accepted and currently used model for representing the structure of the plasma membrane is called the fluid mosaic model. It was first proposed by S. J. Singer and G. L. Nicolson in the year 1972. The model was found to be evolved over time, but it can still provide a good basic description for the structure of the membranes in many of the cells.

According to the structure of the fluid mosaic model, the plasma membrane looks like a mosaic that has primary components as phospholipids, cholesterol, and proteins. These components are found to be moving freely and fluidly in the plane of the membrane. Interestingly this fluidity can be explained as if you insert or prick the membrane with a very fine needle. The membrane will simply part to flow around the needle and once the needle is removed, it gets back to the flow together seamlessly.


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Plasma Membrane Components

The cell membrane is composed of lipids, proteins, and carbohydrate groups that get attached to some of the lipids and proteins. The lipids include phospholipids and cholesterol.

  • A phospholipid is a kind of lipid that is made of glycerol, it has two fatty acid tails and a head group linked to the phosphate. Biological membranes usually consist of two layers of phospholipids where their tails point inward. This type of arrangement is called a phospholipid bilayer.

  • The cholesterol layer is another lipid layer that is composed of four fused carbon rings. It is found along the sides of the phospholipid layer in the core of the membrane.

  • Membrane proteins can extend the way inside the plasma membrane, where these proteins can cross the membrane entirely or can be attached loosely to its inner or outer face.

  • Carbohydrate groups are found only on the outer surface of the plasma membrane. It is attached to the proteins to form glycoproteins, or to the lipids to form glycolipids.

The proportions of the lipids, proteins, and carbohydrates present in the plasma membrane may vary depending on the different types of cells. For a typical cell, the proteins can contribute about 50 percent of the composition by mass. Whereas, lipids of all types contribute about 40 percent. The remaining 10 percent is contributed by carbohydrates.


Cell Membrane Function

Cell membranes act as a barrier between two different regions and gatekeepers. They are semi-permeable in nature, which means that few molecules are allowed to diffuse across the lipid bilayer. Small hydrophobic molecules and gases such as oxygen and carbon dioxide can cross the membranes rapidly. Smaller polar molecules, like water and ethanol, can pass through the membranes, but these can enter slowly. The cell membranes can restrict the process of diffusion of highly charged molecules and large molecules. The highly charged molecules include ions and large molecules include sugars and amino acids. The passage of these molecules completely depends on the transport of specific proteins that are embedded in the membrane.

The transport proteins in the membrane are very specific and selective they can move across the membrane and they often use the energy to catalyze passage. Also, the proteins transport certain nutrients against the concentration gradients. This also puts an extra requirement of additional energy. The ability of the membranes to maintain the concentration gradient and move the materials against them is crucial to cell health and maintenance. 

Other types of transmembrane proteins are involved in communication-related jobs. These types of proteins can bind the signals, like hormones or immune mediators. The binding of these proteins can change the protein that can transmit the signals to other intracellular messenger molecules. Like transport proteins, the receptor proteins are also function-specific and are selective in nature.


Diversity of Cell Membrane

In contrast to the single-celled or prokaryotes, eukaryotic cells along with plasma membrane, also have intracellular membranes that can surround various organelles. In these cells, the plasma membrane is part of an extensive endomembrane system. This system includes the (ER) endoplasmic reticulum, nuclear membrane, Golgi apparatus, and lysosomes.

Membrane components are exchanged in an organized manner throughout the endomembrane system. For instance, the ER and the Golgi apparatus membranes have different compositions. The proteins found in these membranes include sorting signals, they act as molecular zip codes that can specify their final destination.

Mitochondria and chloroplasts are also surrounded by membranes, but they have unusual membrane structures. Each of these organelles instead of having one membrane they has two surrounding membranes. The outer membrane of these organelles has pores that allow the passage of small molecules. The inner membrane is filled with the proteins that form the electron transport chain. These double membrane layers of mitochondria and chloroplasts are similar to some of the modern-day prokaryotes.

Membranes are composed of lipids and proteins, where they serve as a barrier for certain functions of cells and intracellular organelles. These membranes can keep the outer molecules "out" and the inner molecules "inside". This allows certain molecules and relays messages to cross through a chain of molecular events.

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