What Is the Plasma Membrane and Its Function?

The plasma membrane is a component of the cell that delimits the cell's organelles from the extracellular space outside. In plant cells, it is the most exterior part of the cell except for the cell wall. It is the most exterior part of animal cells since they lack a cell wall. While the cell wall is a rigid structure, the cell membrane is much more flexible and is semi-permeable. The characteristics of the plasma membrane include the following:

• Selective permeability: the plasma membrane mediates what can enter and leave the cell. It also uses selective proteins to prevent the passage of compounds that should not be transported. For example, nutrients go in, but metabolic waste goes out. The pH and ionic balance are also regulated. These processes are regulated by osmosis.
  
  
• Flexibility: this membrane has fluidity that allows it to give movement to the cell. Despite its flexibility, it contains everything within the cell and allows it to keep structure.

In addition to being known as the plasma membrane or cell membrane, it is sometimes referred to as the cytoplasmic membrane. This is due to the fact it holds in the cell's cytoplasm. You can learn more about this important part of cells that is not an organelle with our article on what is cytoplasm and its function?

The below diagram shows the bilipid layer of the cytoplasmic membrane.

The functions of the plasma membrane vary depending on the composition and type of cell. However, we can say that all cell membranes allow the following activities:

• Barrier function: the membrane is a permeable barrier that divides the aqueous intracellular medium containing the dispersed organelles with the outside of the cell. This helps protect the inside of the cell.
  
  
• Maintaining the electrochemical gradient: this is produced by the difference in ions inside and outside the cell. It is the differences in ionic charge that produce the electrical and chemical gradient. An example of this gradient is the sodium-potassium pump which produces energy.
  
  
• Transport of small molecules: the plasma membrane allows the passage of certain molecules, molecules necessary to carry out various processes. This transport can be passive or active. It is passive when the molecules move in favor of the electrochemical gradient without requiring energy. When the transport is against the electrochemical gradient, it is active.
  
  
• Communication: at the same time that it functions as a barrier, the selectivity of the membrane allows cells to communicate with each other. It does so by allowing the passage of metabolites that are needed for different processes. This communication is made both between cells and between a cell and its environment, with the help of signals that the cell interprets through receptors and molecules. Furthermore, the signals can be electrical when detected with neurotransmitters or ion channels.
  
  
• Cell motility: the plasma membrane is best understood with the fluid mosaic model. This is where the layer is within a fluid, giving mobility to the cell along with other features that carry out particular functions of the membrane.
  
  
• Support: the membrane helps reinforce tissues, giving them support, shape and helping to bind cells. This is achieved thanks to the fact that the plasma membrane is joined to the cytoskeleton by certain intracellular proteins. Discover more about the function and structure of the cytoskeleton.
  
  
• Cellular signaling: eukaryotic cells have a system for signal transduction in which signals are activated from substances external to the cell. This happens when a certain receptor is activated, allowing for a particular function to be carried out.

Learn about the difference between prokaryotic and eukaryotic cells in our related guide.

As we can see above, cell membranes have different functions depending on the type of cell. Just as the function varies, the structure of the membrane differs according to type. Despite this fact, all plasma membranes have the following :

• Lipids: the cell membrane itself is a lipid bilayer, meaning the ends have a hydrophilic head that attracts water and a hydrophobic tail that repels water. This simple principle that lipids repel water is what allows cells to be delimited by the cell membrane.
  
  
• Proteins: these can be intrinsic or extrinsic. They are intrinsic when inserted in the lipid bilayer. Extrinsic proteins are located on either surface of the plasma membrane, but they do not enter the bilipid layer of the membrane. These proteins are shown in the diagram above.
  
  
• Carbohydrates: they attach to lipids and proteins via covalent bonding, forming glycolipids and glycoproteins, respectively.

In addition to these basic structures, plasma membranes can have variations or additional structures depending on the type of protein. Some examples are:

• High level of cholesterol: animal cells have a lot of cholesterol to give stability and firmness to the cells. This helps since they do not have a cell wall like plant cells. Learn about other differences between plant and animal cells in our related article.
  
  
• Anchoring of motile structures: some prokaryotic cells have structures that help them move, such as flagella. These are released from the plasma membrane.
  
  
• Glycocalyx: it is found in some plasma membranes of bacterial cells, as well as in mucosal cells in animals. This glycocalyx helps secrete a layer of mucilage which serves as protection from the extracellular environment.
  
  
• Exoenzymes: are found particularly in prokaryotic cells. They are the result of catabolism, forming enzymes that can be used outside the cell. Learn about this process with our article on the difference between anabolism and catabolism.

Now you know more about the plasma membrane, including its structure and function, you may want to learn more about other vital parts of the cell. You can do this with our article which asks what is the cytoskeleton and its function?

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