The Plasma Membrane – A Look At Its Definition And Function

The components of a cell are often quite mysterious to the average person with little to no medical training.

This is not because of a lack of interest or confusion, it is simply because your body and your body’s cells are filled to the brim with millions of different components.

One of the most important is the plasma membrane or, as some know it, the cell membrane.

Considering how small cells are compared to pretty much everything else in the universe, it is only natural to assume that they would crumple against the forces of the outside world.

Yet, the cell membrane holds it firmly together and protects against these outside forces.

As such, we decided to dive into our knowledge of the plasma membrane and give a run down on its definition, its function, its structure, and the differences between it and the cell wall.

What Is The Plasma Membrane?

The plasma membrane is a biological membrane found in almost all types of cells.

It separates the inside of the cell from the outside environment, which is known as extracellular space. Normally, this outside environment is other parts of the plant or animal that the cell inhabits – for example, other cells or plasma material.

Plasma membranes are present in almost all cells and without them the cells would simply cease to be. These membranes rely on some components with the main ones being:

• Proteins.
• Carbohydrates.
• Cholesterol.
• The lipid bilayer.

The plasma membrane is formed by the lipid bilayer. Lipids are molecules found in both the cell membrane and cell walls of many organisms.

They are composed of a hydrophobic part and a hydrophilic part, which are held together by a covalent bond.

This allows the molecules to be soluble in both water and oil. In the cell, lipids act as an important barrier to protect the inside from external influences.

The Plasma Membrane’s Structure

Our study of the cells now uses a certain model known as the Fluid Mosaic model to define the structure of the cell.

This model has undergone several revisions and changes throughout its usage, but has remained as the go to model for cell study since its introduction in 1972, replacing the Davson-Danielli model.

This model posits that biological membranes can be considered two-dimensional liquids rather than three-dimensional. In this way, molecules – specifically lipid and protein – would diffuse more easily through the membrane itself.

While the foundation of the membrane is made of lipids which can form two-dimensional liquids, it is important to note that large numbers of proteins exist in the membrane that are far more structured.

If we take this model as fact, then it would be important to note that the plasma membrane is not one solid, static, structure, but a free-moving, fluid-like one whose components are free moving as well.

This is beneficial to the body, as the plasma membrane can move around invasive objects or simply avoid them, but not really beneficial to our study of them.

A good example is that a membrane would be hard to inject with a syringe or needle, as they would just move around the syringe.

The Lipid Bilayer Of The membrane

We briefly mentioned the lipid bilayer earlier, but they are the core of the plasma membrane itself.

The plasma membrane is made of lipids, a biomolecule, that are used to create two layers which in turn form a thin polar membrane. This barrier is made of lipids and certain proteins attach to this membrane to regulate its activities.

The plasma membrane has a lipid bilayer or two-layer structure. Although lipids are tiny, when formed together they create a dense and highly fluid membrane towards the outer surface of the cell.

This helped by the hydrophilic and hydrophobic regions of the lipid themselves as well.

Due to the importance and necessity of the Lipid Bilayer to the membrane itself, we need to look over its characteristics and structure in detail:

The Amphipathic Structure

The amphipathic structure of the plasma membrane’s lipid molecules is important. Any cell or object in nature that has an amphipathic structure is one that has a polar region and an apolar region.

Polarity is the state of being for an object or living being that has two contradictory or opposite tendencies.

For example, magnets have polar and apolar regions, when one side of a magnet is attracted to an object, the other side will not be. Polarity exists in a lot of forms in the universe, from the cells in plants to the poles of the world itself.

The amphipathic structure of lipids is a mixture of hydrophobic and hydrophilic parts, rather than magnetic parts as mentioned previously. Hydrophobic is a complete aversion to water, whereas hydrophilic is an attraction to water.

In cells, this would mean that the hydrophilic regions of a cell would be soluble in water, whereas the hydrophobic regions would be insoluble in water.  

Hydrophilic And Hydrophobic Regions Of The Lipids

These two regions form a huge part of the plasma membrane and how it works. As such, we will look at them each individually to categorize them properly.:

Hydrophilic Regions Of The Lipid Bilayer

The hydrophilic region of the lipids that form the plasma membrane of the cell are the actual barrier itself. They form the barrier that separates the outside environment beyond the cell with the internal components and environment of the cell itself.

Since there are two layers of lipids, one layer will be the inner barrier of the membrane and this is in contact with the internal environment and one layer will be the outer environment and this is in contact with the external environment.

Each hydrophilic region is the external part of the lipid, otherwise known as the head.

These heads will attract water to them and take them into the cell. In this way, the membrane allows the passage of not just water, but other substances outside and inside the cell as transit through.

The membrane uses these biomolecules to funnel necessary substances into the cell, like sugars or proteins, and to excrete waste material or substances that are of no use.

In a way, it is useful to think of them as a bridge into a city on an island, but the city is the cell in this case. Without the bridge allowing transport to the city, the city would slowly crumble, much like the cell would.

Hydrophobic Regions Of The Lipid Bilayer

The hydrophobic region of the lipids that form the plasma membrane of the cell are the internal parts of the barrier.

They form a secondary barrier that is entirely within the membrane of the cell and interact only with what has passed through the hydrophilic sections of the membrane.

While the hydrophilic heads interact with the internal environment of the cell and the external environment outside the cell, the hydrophobic heads interact with each within the center of the membrane, where there is no naturally occurring water or fluid, which enables them to do their job correctly.

Their very hydrophobic nature is why they are here, as if the hydrophilic regions are the bridge, then the hydrophobic regions are the border guards or police officers, making sure that anything being allowed in is actually something the cell needs or wants.

A lot of biological molecules and ions are water soluble, so they interact with water well, but that doesn’t mean we want them anywhere near the cell.

In fact, most of the substances that do try to come through the barrier are cells you do not want near your body.

The hydrophobic cells will not allow these molecules through, creating an internal barrier that regulates and manages entrance to the cell, by making it difficult for any water based substance that is unwelcome to maneuver around them.

This is possible because the membrane is fluid and free moving, as mentioned before.

The cell can move and adapt quickly to circumstances, it can move and expand to catch substances it needs and move and contract to expel or avoid substances it doesn’t.

This is the role of lipids in the Lipid Bilayer, but there is another significant molecule that makes up a large proportion of the cell as well. That is proteins.

Proteins In The Plasma Membrane

Generally, there are two types of protein that can be found in the membrane, these are: the intrinsic proteins and the peripheral proteins. The main differences in the two is where they are found and what they are used for:

Intrinsic Proteins

Intrinsic or Integral proteins are found inside the Lipid Bilayer, as either inserted or embedded components of the Bilayer itself. These proteins regulate the lipid-protein interactions within the lipid bilayer.

While these proteins are hydrophobic in nature and remain anchored within the membrane where the hydrophobic regions of the lipids reside, parts of these proteins are exposed to either the inner and outer environment where water resides as well.

These proteins are incredibly important to the membrane’s function and perform a number of tasks that make the membrane work. The primary task they perform is communication.

This is done via the hormone receptors the proteins have that receive information from the body.

The information is then delivered from the internal or external part of the membrane to the other side and the cell can respond appropriately to this new information.

This means that important information is not potentially blocked from moving through the cell wall.

They can also act as a transportation system separate from the hydrophilic and hydrophobic lipids, allowing substances that may have been otherwise blocked from entering or exiting the cell to do so.

While it is not the main function of these proteins, they can also help with Cell Adhesion – keeping this fluid mass altogether – and Energy Accumulation – helping to maintain energy levels through absorption.

Peripheral Proteins

These proteins are not embedded in the membrane’s inner layer, instead these proteins reside on the outer layer of both sides of the membrane – the one in contact with the internal environment and the one in contact with the external environment.

However, they do not go through the membrane and remain on whatever side they are on until they move.

These proteins are not embedded or entrenched in the membrane, but instead loosely attached and so can easily detach when it is necessary.

This is crucial to their function, as their role is defined by their ability to move in and out of the cell itself.

The first is communication. While the intrinsic proteins take the information from the outer membrane, they do not leave the membrane and must pass it to the peripheral proteins.

These proteins then relay the information to the appropriate region which activates an appropriate cell response.

The peripheral proteins can also act like support components to the outer and inner membrane. Their attachment, while often brief, gives a little extra structure to the fluid and free-moving cell membrane, keeping it in place for certain tasks or activities.

This ability to attach and detach is particularly useful for transfers and the transportation of certain molecules, as well as the cells attachment and detachment to other cells that may need to be interacted with.  

Smaller Components Of The Membrane

While Lipids and Proteins make up the vast majority of the plasma membrane’s structure, they are not the only components that are a part of the membrane itself. There are two other smaller components of the cell and these are: carbohydrates and cholesterol.

Carbohydrates

The carbohydrates you would find that are used for the plasma membrane are usually found on the surface of the external part of the plasma membrane.

These carbohydrates bind to either proteins or lipids to form glycoproteins or glycolipids and while they do not insert themselves into the membrane, they do attach to the outside.

There are two functions that carbohydrates will perform on or for the plasma membrane itself:

The first is one of categorizing and mediation. The carbohydrates on the outside of a cell will act as a bit of a filing machine, where they are regulating how interaction between cells and the outer environment goes.

And also they will sort the various proteins that come their way and push them to the appropriate areas of the cell they are needed in.

Carbohydrates also are invaluable in determining what cells their cell is interacting with and recognising it. The carbohydrates on the outside of the membrane will act as identifiers or markers.

When a cell that is not attached to the carbohydrate comes into contact with the carbohydrates attached cell, they can recognise what kind of cell it is, thanks to the carbohydrates form and identifying features.

This is incredibly important to immune system responses, as it means the immune system will recognise native cells and attack foreign ones based on these markers.

Cholesterol

Cholesterol is found in the plasma membrane inside the lipids themselves and regulates the movement of certain substances across the membrane. This is important for maintaining the structure of the membrane and the movement of water and ions.

Otherwise, they are shown to interact in the same way lipids do.

The Plasma Membrane’s function

The plasma membrane is an essential part of every cell. Given this essentiality to the cell, it is important to know what it actually does to the cell that allows for the cell to function properly.

In essence, it acts as a semi-permeable shield that helps regulate or dictate what can and can’t come into the cell itself.

The structure of the Lipid Bilayer within the membrane means that this shield can provide the optimum conditions for the cell within its borders and creates a perfect way for the transfusion of substances to within the cell itself, thus keeping it running.

There are several functions that this ideal environment needs to keep running, the most important ones of which are seen below:

Transportation

As stated in the previous section, the membrane allows for the movement of substances through its surface. The plasma membrane contains a wide range of proteins that are responsible for the transportation of molecules across the membrane.

The membrane is selectively permeable and will only allow the appropriate molecules and substances into the cell, while also keeping out less desirable substances.

This helps maintain the perfect balance for the cell and for various cell functions between different interacting groups.

There are two main types of transportation in and out of the cell. The first is passive transportation, which is a form of transportation that does not require the use of energy.

The second is active transportation, which is a form of transportation that does require the use of energy.

Cell Division

When a cell divides into two daughter cells, the plasma membrane of the parent cell becomes a membrane for the two new cells.

As the two daughter cells are now genetically the same as each other, the plasma membrane of the two new cells is genetically the same as the original parent cell plasma membrane.

If not for the plasma membrane becoming the membrane for both cells, then one of the cells would cease to be before it even started existing.

Communication

The plasma membrane is also responsible for the transmission of signals between the inside and outside of the cell.

These signaling proteins are known as cell receptors, as they are the receptors for the signals being transmitted. These receptors are able to detect a number of different molecules, both inside and outside the cell.

Some of these molecules include hormones, neurotransmitters, and regulatory molecules that control gene expression. The cell membrane is also responsible for transmitting electrical signals between the inside of the cell and the outside.

In order for this to happen, the inside of the cell must absorb certain molecules that are found outside the cell. These molecules are then pumped into the cell, allowing for an electrical signal to be transmitted between the inside and outside of the cell.

Ingestion

Lastly, the plasma membrane of a cell is responsible for the uptake of substances into the cell. This process is known as ingestion, and it is often used by the body to satisfy its nutritional needs.

Ingested substances are absorbed through special proteins that line the cell membrane.

These proteins are specific for each type of substance and, once inside the cell, they are broken down by enzymes. The cell membrane is also responsible for excreting substances back out of the cell.

These substances are often wastes that are produced by the cell and must be transported outside the cell.

Alternatively, a cell’s flexible membrane means that they can surround another substance or cell completely and digest it, thus gaining the nutrients and substances that they need to survive.

The Differences Between A Plasma Membrane And A Cell Wall

Plasma membranes and cell walls are often spoken of as a thing that is one and the same, however there are a variety of differences between the two.

While they both serve to protect the internal structure of a cell, they are structured differently and use different components as well.

The biggest difference between a plasma membrane and a cell wall is where they are present. A plasma membrane is present in every single cell in one capacity or another, whereas a cell wall is only present in some cells in some organisms.

These organisms are often plant or fungi based with a few cell walls appearing in the cells of certain bacteria.

In these creatures, when a cell wall does appear, it appears as a very rigid and thick structure that affects the movement and shape of a cell.

While a plasma membrane will be fluid and flexible allowing it to move as it wishes, a cell wall’s rigidity will force the cell into a certain shape and affect its ability to move.

In order to achieve this rigidity, many of the components of the cell wall are different from that of the plasma membrane, which is another big difference between the two.

A cell wall is made up of sugar, cellulose, pectin, chitin, and lignin, which form a thick layer on the surface of the cell, whereas a plasma membrane is made up of mostly lipids and proteins, which are free-flowing and moving parts of the outer cell.

Obviously, these key differences will affect how each of these outer portions of the cell function. Surprisingly though, the cell wall has very few functions at all. Its only function, in fact, is to protect the cell from outside threat.

It does not filter or regulate what goes through it, it simply stops certain things from penetrating it and that is all it does.

The reason it can do this is that even cells with cell walls have plasma membranes, meaning all the functions that regulate, maintain, and aid the cell are done by the plasma membrane beneath the cell wall.

It is hard to determine why cell walls developed in such a manner and why they are necessary, however we can speculate.

One of the most prominent theories is that cell walls developed as an answer to develop multicellular life for static or non-independently moving creatures.

Their rigid but changing structures means that plants and fungi can maintain their shape, while continuing to grow upward without compromising their integrity.

A plant’s cell wall may help the overall plant stay upright instead of collapsing and help it still grow.

Since other organisms developed internal and external skeletons that provided support, they did not need to develop a structure within their cells to keep them supported, whereas other organisms did.

Yet, this is still a theory and may yet be completely disproven.

The Differences Between A Plasma Membrane And A Cell Membrane

Plasma membranes and cell membranes are used as terms quite interchangeably in many text books. However, unlike the plasma membrane and the cell wall, the differences between these two is subtle and comes down really to semantics.

A cell membrane refers to the boundary of the cell in totality, whether that be a boundary between two cells after one has been consumed or the boundary on the outside of a cell.

A plasma membrane is the boundary of the cell that serves as the boundary between the inner components of a cell and outside of the cell itself.

If I was to say: ‘the cell membrane is long’ we would be talking about the entire cell in width across its surface, but I was to say: ‘the plasma membrane is long’ we would be talking about the length of the plasma membrane around the cell itself.

As you can see, because these terms are used interchangeably, it can be quite confusing.

Conclusion

The plasma membrane is incredibly important to the function and protection of a cell. Understanding its functions is one of the ways in which we can understand cells themselves and as such anything related to cells, which is almost every living thing on the planet.

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