Plasma Membrane

Definition, Function, Structure, Vs Cell Wall


Definition:  What is the Plasma Membrane?


Essentially, the plasma membrane refers to the cell membrane that defines the boundaries of a cell (and cell organelles). As such, it forms a barrier (with controlled interaction) between two aqueous compartments; between the intracellular and intracellular environments.

Although this membrane serves to protect the cell and its components, it is also surface through which substances are exchanged and information shared with other cells.

Like the other cellular membranes, the plasma membrane is made up of lipids and proteins that contribute to the functions and characteristics of the membrane. While the plasma membrane is the only barrier between the internal environment of a cell and the extracellular environment in some organisms, some organisms have an additional barrier known as a cell wall.

Several differences exist between a plasma membrane and a cell wall (as well as a cell membrane) as will be discussed in detail in this article.



Main components of a plasma membrane includes:

  • A phospholipid bilayer
  • Proteins - E.g. glycoproteins and peripheral membrane proteins
  • Cholesterol
  • Carbohydrates - E.g. carbohydrates that form glycoproteins and glycolipids


Structure of a Plasma Membrane



Fluid Mosaic Model


Although this model has undergone several changes over time (since it was proposed in 1972), the fluid mosaic model is the model often used to describe the structure of plasma membrane.

According to this model, the plasma membrane is composed of freely moving (in a fluid-like manner) components (phospholipids, proteins, and cholesterol). Its consistency has been likened to that of salad oil at body temperature. Here, then, the plasma membrane depicted in books may be thought of as a snapshot of a structure that is always in motion.

* Due to the fluid nature of the plasma membrane, it would continue to flow around an object (e.g. very fine needle) if the object was inserted into a cell.


Phospholipid bilayer of plasma membrane


The phospholipid bilayer is one of the main components of the plasma membrane. It's composed of fatty acids (two), glycerol and a phosphate group and takes the following chemical structure:



As the basic fabric of the plasma membrane, the phospholipid (a bilayer lipid) has a number of characteristics/features that contribute to its functions.


Some of these characteristics include:


Amphipathic


As an amphipathic structure, the phospholipid bilayer has regions that are hydrophilic and hydrophobic in nature. Whereas the hydrophilic region of phospholipid is essentially "water-loving", the hydrophobic region is "water hating" and thus insoluble in water.



Hydrophilic region of phospholipids


The hydrophilic region of phospholipids consists of a phosphate group that is negatively charged as well as a small group that varies from one organism to another (may be polar or charged) attached to glycerol.

As the hydrophilic region of phospholipid, the head remains in contact with the aqueous environment both inside and outside the cell. This contact is enhanced by hydrogen bonds.

* As a polar molecule, water forms electrostatic interactions with the hydrophilic region of phospholipid (phospholipid heads).

As the part that forms the surface of the bilayer, the hydrophilic region of phospholipid helps attract water inside and outside the cell. This is an important feature of the phospholipid bilayer that contributes to the exchange of substances between a cell and its immediate environment.

While the plasma membrane acts as a barrier that separates the internal environment of the cell from the external environment, intake, and excretion of substances is essential for growth, communication as well as the proper functioning of cell and organelles. Here, then, the hydrophilic region of phospholipids play an important role in that it attracts water (the aqueous fluid) in which these substances are suspended.


Hydrophobic region of phospholipids


The hydrophobic region of phospholipids is composed of unsaturated (as well as some saturated) fatty acids that form long hydrocarbon chains. This region makes up the tail part of phospholipids while the hydrophilic region makes up the head.

While this part is "water-hating", which means that it poorly interacts with water, it interacts well with other nonpolar molecules. For this reason, unlike the "water-loving" region, the tail part of a phospholipid is sandwiched between the two hydrophilic heads (where they interact with each other) away from water/aqueous fluid inside and outside the cell.

With the hydrophobic region occupying the interior region of the phospholipid bilayer, this area is impermeable to various biological molecules and ions that are soluble in water. While this region of the phospholipid bilayer poorly interacts with water (as well as various water-soluble molecules and ions), it makes the plasma membrane a good barrier by regulating substances that cross the membrane.

Being a hydrophobic region of the phospholipid bilayer, water, and other water-soluble substances cannot easily move in and out of the cell. As such, it can be said to help maintain cell homeostasis.


The phospholipid layer is a viscous fluid.

As already mentioned, the plasma membrane is a dynamic structure that is in constant motion. This is made possible by one or to double bonds present on the fatty acids that not only make it difficult for the hydrocarbon chains to pack together, but also move freely.


This makes the membrane soft and flexible which has the following benefits:

· It contributes to the selective movement of substances across the membrane.


· In such organisms as the amoeba, this characteristic of plasma membrane makes it easier for them to engulf food material.


· It makes it possible for some cells to squeeze through other cells to reach given destinations.


· In this structure, proteins and the phospholipid layer itself are capable of lateral diffusion within the membrane which is important for all its functions. 

* The length and bulk of phospholipid tails determine the type of end product. Whereas a small tail of phospholipids results in the formation of micelles while tails that are bulkier in nature produce liposome.

 

Some of the other important components of a plasma membrane include:

Proteins are also major components of the plasma membrane and make up about 50 percent of the plasma membrane by weight.


They are divided into two main categories that include:

  • Integral proteins
  • Peripheral proteins


Integral proteins of the plasma membrane


Also known as intrinsic proteins, integral proteins are inserted/embedded in the phospholipid bilayer. These proteins vary in length with some stretching from one end of the membrane to the other (transmembrane proteins) while the shorter ones are not as extended (thus not fully integrated into the phospholipid bilayer).

Given that the central region of the plasma membrane is hydrophobic in nature, integral proteins possess a hydrophobic region (residues with hydrophobic side chains) that allows them to remain anchored in the membrane. However, parts of these proteins that are exposed to the inner environment of the cell (cytoplasm) and the outer surface of the membrane are hydrophilic (water-loving).

According to studies, transmembrane proteins (ones that stretch from one side of the phospholipid bilayer to the other) consists of between 20 and 25 amino acids arranged as membrane-spanning domains (α helices or multiple β strands) that range in length from 4 to a few hundred residues long.

Proteins remain anchored to the membrane. Therefore, this is different in comparison to integral proteins that are anchored to a single leaflet/layer of the membrane (they do not extend deep into the bilayer of the membrane) through covalently bound fatty acids.

* Non-polar interactions, as well as the external and internal force, ensure that integral proteins remain in place.

Depending on the cell, integral proteins have a number of functions. In some cells, integral proteins are largely involved in communication and thus serve to transfer signals between the internal of the cell and the extracellular space. As such, the proteins often act as hormone receptors that receive information allowing for the cell to respond appropriately.

Apart from their role in cell communication, some integral proteins also act as transporters and channels (e.g. the potassium channels) through which various substances are transported in and out of the cell.


Some of the other functions of integral proteins include:

  • Energy accumulation
  • Cell adhesion


Peripheral Proteins


As compared to integral proteins, peripheral proteins are located on the surfaces (outer or inside the cell) and thus do not extend from one side of the membrane to the other.

Here, peripheral proteins may be attached to the surface of phospholipids or integral proteins. They are loosely attached (and can therefore easily detach) given that they do not form stronger bonds as those formed between integral proteins and the membrane.

* Peripheral membrane proteins attach to the phosphate heads of phospholipids through a unique sequence of amino acids in their structure.

The loose attachment of peripheral proteins is crucial to their functions on the cell surface. For instance, for peripheral proteins involved in biochemical pathways, being loosely attached allows them to detach and attach to the membrane as they move substances in or out of the cell.


As they move from one location of the cell surface to another, peripheral proteins are also involved in:

· Support - They provide a point of attachment for cytoskeleton and various components of the extracellular matrix. In turn, this also provides support for the cell as a whole.

· Communication - Enzyme and protein activation - Message from the extracellular matrix is passed to integral proteins and ultimately to the peripheral proteins. Once this information reaches peripheral proteins, the appropriate cell response is initiated. This is enhanced by the fact that peripheral proteins can attach and detach from the membrane.

* Some peripheral membranes proteins act like enzymes:  Phospholipases located on the surface of the plasma membrane are involved in the hydrolysis of bonds located on the head group of phospholipids where they are involved in cell degradation.

· Molecule transfer/ transportation - By attaching to various molecules and electrons, peripheral proteins can also help in their transfer thus contributing to the electron transport chain process.

· Cell interaction - Attachment and detachment of peripheral proteins contributes to cell interaction. 


A section of the Plasma Membrane



Carbohydrates


Carbohydrates, which are also components of the plasma membrane are normally found on the exterior surface of the plasma membrane. Here, they are bound to proteins to form glycoproteins or lipids forming glycolipids (glycosphingolipids, glycoglycerolipids, and glycophosphatidylinositol).

While carbohydrates are not inserted in the phospholipid bilayer, some are inserted in the membrane as proteoglycans.

Carbohydrates attached to the plasma membrane are made up of 2 or more monosaccharides (as many as 60) and may appear straight or branched in shape.

* Plasma membrane carbohydrates are synthesized in the endoplasmic reticulum and modified in the Golgi complex.


* By weight, membrane carbohydrates make up between 2 and 10 percent of the plasma membrane depending on the type of organism and cell. 90 percent of these carbohydrates are covalently bound to proteins (to form glycoproteins) while the rest are bound to lipids.


On the plasma membrane, carbohydrates have two main functions.


These include:

Mediating interactions and sorting proteins - On the plasma membrane surface, carbohydrate protections play an important role in mediating the interaction of cells with their environment. In the process, they also serve to sort proteins into the appropriate compartments of the cell.

Cell recognition - On the plasma membrane, the structure and type of carbohydrates have made it possible to identify specific cells. Here, in addition to the proteins, carbohydrates act as markers that not only make it possible to identify the cells, but also making it possible for cells to recognize each other.

In the immune system, these markers make it possible for immune cells to recognize each other and thus differentiate between cells that belong to the body and foreign substances. For this reason, immune cells do not attack body cells.

With regards to cell identifications, carbohydrates (e.g. in glycolipids) play an important role in cell classification. Using glycolipids located on red cells, it became possible to group red cells into several types (A, B, AB, and O). For instance, if a blood type is classified as being type A, this means that it contains an enzyme which adds an N-acetylgalactosamine to the end of the chain.


Cholesterol


Cholesterol is present in the plasma membranes of some cells and may make up to 50 percent of the total lipid. On the plasma membrane, they have been shown to disrupt the arrangement (packing) of fatty acyl chains which in turn disrupt their general mobility.


Functions of the Plasma Membrane


As the barrier between the internal environment of a cell and the external medium, the plasma membrane plays a number of important roles that allow a cell to function properly.

By acting as the boundary that separates the cytoplasm (as well as the nucleus) from the extracellular environment, the plasma membrane helps regulate substances that enter or leave the cell (through characteristics of the phospholipid bilayer) which in turn provides ideal cellular conditions for the cell to function normally.


Some of the other functions of the plasma membrane may be classified as follows:

Transport - Transport is one of the main functions of the plasma membrane. While some substances are allowed into the cell, some are prevented from gaining entrance. Therefore, the plasma membrane is selectively permeable and thus does not allow all substances in and out of the cell.

While it protects the cell from some harmful substances that are denied entry, this action also helps maintain a balance between various material for cell functions.


There are two types main types of transport that occur through the plasma membrane including:

· Passive transport - this is the type of transportation that does not require the use of energy

· Active transport - through the use of energy given that substances have to be transported against a concentration gradient

Ingestion - Because of the nature of the plasma membrane, different types of cells are able to ingest a variety of substances into the cell. This is achieved through such processes as endocytosis, phagocytosis, and pinocytosis. Here, such cells as amoeba surround the substance or food particle (as well as other microorganisms) and engulf them as they are into the cell. These processes allow cells to feed or destroy other microorganisms/substances.

Cell division - As mentioned, the plasma membrane is a dynamic structure that is always in motion. This characteristic makes it easy for a cell to divide when need be to form two daughter cells from the original cell. Here, the plasma membrane pinches at the central part and separates to form two new cells.

Communication - Through structures on their surface (proteins and carbohydrates) cells are able to communicate with each other and interact through signaling. 



Plasma Membrane Vs Cell Wall


Like the plasma membrane, the cell wall also serves to protect the cell from various external forces that may otherwise affect the internal environment of the cell.

However, unlike the plasma membrane (which is present in all cells), a cell wall is only found in some organisms (e.g. plants, some bacteria, and fungi among a few others). In these organisms, the cell wall, which is thicker in diameter, is more rigid as compared to the plasma membrane which not only allows it to protect the cell, but also influences the overall shape of the cell.

The other difference between the cell wall and plasma membrane is with regards to their respective components. Whereas a cell wall consists of chitin, lignin, sugar, cellulose , and pectin among other molecules, the plasma membrane is mostly made up of the phospholipid bilayer. This difference makes a cell wall more elastic than the plasma membrane.

Differences between the two can also be identified in their functions. Whereas the cell wall simply serves to protect the cell (as well as the cell membrane which is located beneath it) the plasma membrane has a number of functions that include cell division, motility, signaling and reception, transportation as well as maintaining cell homeostasis. The plasma membrane, then, has many important functions that contribute to the proper functioning of a cell.

* Whereas a cell wall ranges between 20 to 80 nm in thickness, the plasma membrane may range between 7.5 and 10 nm.

Some of the other differences between the two include:

· Cell wall requires deposition while plasma membrane needs proper nutrition

· The thickness of cell wall changes over time

· The cell wall is inactive as compared to the plasma membrane which is metabolically active

· Cell wall does not have receptors on its surface while plasma membrane does

· Whereas plasma membrane is semi-permeable, the cell wall is completely permeable and only prevents very large molecules from entering the cell.


Plasma Membrane Vs. Cell Membrane



While the terms cell membrane and plasma membrane are used interchangeably in some books, there is a slight difference between the two.

The term cell membrane refers to the boundary of the cell as a whole, the term plasma membrane refers to the boundary of the cell or that of an organelle within the cell. Here, then, it would be right to say that membrane-bound organelles are surrounded by a plasma membrane.

The two are made up of similar components, but for the most part, the cell membrane, which is the plasma membrane that surrounds the cytoplasm, may contain a number of additional attachments such as cilia that allow for the locomotion of the cell.



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References


Rachna C. (2017). Difference between Plasma Membrane and Cell Wall. 

Raymond C. Stevens. (2007). the Structure and Function of the Plasma Membrane. , SCIENCE 318:1258, 2007; © 2007, Reprinted With Permission from Aaas.

Links

https://microbenotes.com/membrane-carbohydrate/

https://www.ncbi.nlm.nih.gov/books/NBK21570/


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