Pinocytosis vs Endocytosis – What’s The Difference?

If you are interested in cells, then chances are you have heard the term pinocytosis at some point and want to know what it means. 

The cell process of pinocytosis was first discovered in the 1920s and is associated with the human body and how it consumes nutrients.

Because of this, the term is often confused with endocytosis, which is another process that uses different materials and receptors. But what are the other core differences between these two phrases?

In the article below, we have created a guide all about pinocytosis and how it differs from others such as endocytosis. We have also included valuable information about each process and examples of how they impact your cells. 

What Is Pinocytosis? 

The term pinocytosis was first coined in the 1920s by Warren Lewis and was defined as a form of endocytosis that used the extracellular matrix as a source of fluids and dissolved solutes for various human cells. 

This process would be activated when various molecules came into contact with the cell membrane, which would include the development of small pockets or folds on the surface of the membrane and around the extracellular fluid.

The same process would also impact the eventual internalization of these various materials. 

Because of this, pinocytosis could be classed as a continuous process. One that serves a number of important actions: 

• The absorption of nutrients 

• The transport of important substances into the cell

• The development of immune functions (such as macrophages and dendritic cells)

• The transmission of molecular signals 

The name pinocytosis is derived from two Greek words: pino which means drink and cyto which means cell. 

What Are Some Examples Of Pinocytosis? 

When examining pinocytosis, the process can be recorded in various cells, which means there are several kinds of pinocytosis to consider.

These include the following: 

The Gastrointestinal Tract & Nutrient Absorption

One of the most common examples of pinocytosis involves the absorption of nutrients into the small intestine, which are broken down by the microvilli of the main cells. These notable substances include extracellular fluid and dissolved solutes. 

Based on research and microscopic examination, the cells are able to absorb these nutrients through vesicles that feed on internalized materials, which means the process can run in a continuous loop.

The Female Reproductive System & Nutrient Intake

Pinocytosis also has an important role in the female reproductive system, as the process is used to source unfertilized cells with nutrients.

Once the female reproductive system begins to mature, it will become surrounded with various cells – the most common begin the mucosa cells of the fallopian tube. 

During this development, the ovum will absorb these materials from the extracellular fluid, which is considered a form of pinocytosis. 

The Renal Tubular Cells & Pinocytosis 

Renal tubular cells are located in the ducts of the kidney, where pinocytosis often contributes to the formation of urine.

During this process, extracellular fluid and dissolved nutrients are broken down by the cell ducts and separated into important materials, with the remaining debris being expelled from the body. 

This process ensures that vital substances are retained in the body, while the residual waste is removed and deposited. 

The Impact Of Pinocytosis On Immune Cells

In most cases, cells found in the immune system will use a process called phagocytosis to absorb or combat damaged cells. 

However, recent studies have also confirmed that these cells can also use pinocytosis to test their environment for toxins and harmful materials. 

While this process is not universal with all immune cells, a certain few are able to absorb the extracellular fluid and dissolved solutes to determine the presence of foreign substances, which allows them to respond as needed. 

To learn more about pinocytosis, we have provided the diagram below, which outlines the process of endocytosis in its various forms: 

How Does Pinocytosis Work? 

In basic terms, the process of pinocytosis is comprised of several important mechanisms, which we have outlined below: 

Mechanism One: The Initiation 

This process sees a molecule in the extracellular fluid bind itself to the cell membrane, which activates pinocytosis. 

These molecules can take on various forms, with the most common being ions, sugars, and proteins.

When they come into contact with the cell membrane, the cells induce the process that results in their internalization, which means the cells are transformed into what is known as inducers. 

Mechanism Two: The Folded Membrane 

Once the molecules have come into contact with the cell membrane, the membrane will become stimulated and fold inwards around the extracellular fluid and dissolved molecules, which results in the formation of small folds or pockets. 

Mechanism Three: The Consumption Of Dissolved Molecules 

With the extracellular fluid and dissolved molecules now contained in the small folds, the cell membrane is able to invaginate and connect at the open end, which encloses the residual fluid and contains it inside the cell. 

Mechanism Four: The Pocket Detachment 

Following the formation of the fold, the vesicle will disconnect itself from the cell membrane and transport its contents to the cytosol.

Since the vesicle contains the fluid and dissolved molecules, these substances are unable to enter the cell and impact common cellular functions. 

Note: Once the fluid and dissolved solutes have been internalized, the following events will vary depending on the contents inside the vesicle. 

What Kinds Of Pinocytosis Are There? 

When it comes to pinocytosis, there are two common varieties, which are determined by the size of the material absorbed into the cell. 

These include the following: 

Micropinocytosis 

From the name, it is easy to deduce that this form of pinocytosis involves the absorption and internalization of small molecules and solutes into the main cell.

Because of this, the vesicles on these cells will often measure around 0.1um in diameter. While there are various forms of micropinocytosis, the most common example is caveolin-mediated pinocytosis, which consists of plasma membranes called little caves. 

Macropinocytosis 

In contrast to micropinocytosis, macropinocytosis is a form of pinocytosis that involves the absorption and internalization of large molecules that are often present in extracellular fluids. 

Because of this, the vesicle on these cells (which carry fluid and solutes) will often measure between 0.5 – 5um in diameter.

In most cases, macropinocytosis is used by macrophages to detect the presence of antigens within other materials, which could indicate the presence of harmful microorganisms. 

Are There Other Kinds Of Pinocytosis? 

While pinocytosis is often divided into two varieties based on the size of the material inside the cell, there are also several other kinds that are defined by the mode of action the cell uses to absorb the vital materials. 

The four varieties of pinocytosis based on the mode of action include: 

• Macropinocytosis 
• Clathrin-mediated pinocytosis 
• Clathrin-independent pinocytosis 
• Caveolae-mediated pinocytosis

To learn more about these different processes, we have provided a detailed explanation for each one in the section below. 

Macropinocytosis 

Macropinocytosis is considered to be nonspecific, which means the process can consist of various materials from antigens to nutrients.

Because of this, large macropinosomes can form when inducers come into contact with the receptors on the cell’s surface, which can activate the absorption process. 

Once the process has been induced, the polymerization of actin filaments will increase and result in the folding of the membrane.

This allows the membrane to contain the extracellular fluid and dissolved molecules, which are then encapsulated into macropinosomes (vesicles) and later internalized. 

During this process, the vesicle is also able to fuse itself with lysosomes, which allow it to absorb the wanted materials and discard the leftover debris. 

The process of macropinocytosis has since been observed in numerous cells, with the most common being dendritic and immune cells. 

Clathrin-Mediated Pinocytosis 

In contrast to macropinocytosis, clathrin-mediated pinocytosis is a form of pinocytosis that allows cells to consume various molecules from proteins to metabolites.

Because of this, the cell membrane is also known to take on a conformational change, which is activated by the introduction of molecules to the receptors. 

Once these molecules have been processed, a protein called clathrin will adapt the chosen materials and polymerize at the center of the membrane to produce coated pits.

This is due to the protein’s triskelion structure, which contributes to the pits and their development.

Once activated, the adaptor molecules will combine themselves with the triskelion and membrane receptor proteins, which results in the formation of a clathrin shield around the exterior of the molecules. 

Following their formation, the clathrin vesicles will consist of three distinctive layers, which consist of clathrin, adaptor protein, and lipid membrane.

Once developed, the clathrin-coated pits will be used to contain the extracellular fluid and dissolved molecules, while the open end is closed off through the polymerization of dynamin. 

When the open end has been sealed, the vesicle is able to detach itself from the central membrane and relocate the materials into the cell.

Of course, this process can also vary based on the materials contained inside the membrane, as certain materials will need to be dissolved, while others can be resourced. 

Caveolae-Mediated Pinocytosis 

Caveolae-mediated pinocytosis is a common form of micropinocytosis, which means the process sees the transport of small materials into the cell.

Because of this, common cells associated with this process include endothelial cells and fat cells, which are both found around blood vessels. 

Since this process includes notable proteins (caveolins and cavins) and lipids, the caveolae pits that form on the cell membrane are often identified by these molecules.

After the ligands have interacted with the caveolae complex, the proteins present will begin to internalize and the overall process will be initiated. 

In some cases, the process can be activated by certain molecules and solutes, these include notable proteins such as albumin and alkaline phosphatase, as well as viruses such as HIV and folic acid.

Once the holes begin to form, the extracellular fluid around the molecules will become trapped inside the small caves. 

Like the entries above, the vesicle will detach itself from the cell membrane once the cave has been formed, which will allow it to transport the materials into the cell.

In recent studies, the vesicle’s detachment from the membrane has been connected to dynamin, which means it requires a lot of power to reach its destination.

For this reason, the process is now classed as a form of active transport. 

Once the vesicle has been internalized, certain materials will be recycled and transported back to the cell membrane, while others will remain inside the caveosomes in stasis until they are needed for future actions. 

While this process is known to contribute to the transportation of vital proteins, recent studies have also shown that certain pathogens can use this method to transport themselves to the center of the cell. 

What About Clathrin & Caveolae-Independent Pinocytosis? 

As you can see from the name, this kind of pinocytosis works independently of the receptors and other coat proteins used in the development of the vesicles.

Because of this, the overall process allows a diverse spectrum of materials to be transported around the cell, which includes various pathogens and toxins. 

In most cases, this process relies on actin and its associated proteins to develop vesicles and other forms of protection. 

Pinocytosis Vs Phagocytosis 

Both pinocytosis and phagocytosis are varieties of endocytosis, which means they transport materials from the extracellular matrix through the invagination of the cell membrane.

Because of this, both processes also use a lot of power to form vesicles, which are used to contain fluid and dissolved substances. 

Since both varieties require sources of energy, they are considered forms of active transport, even though they differ in several ways. 

These include the following: 

Cell Eating & Cell Drinking 

One of the most notable differences between pinocytosis and phagocytosis is the materials they transport into the cell.

In most cases, the latter process involves the ingestion and internalization of solid material, whether that be invading pathogens or foreign particles.

In contrast to this, pinocytosis is associated with the absorption of fluids from the extracellular matrix, as well as some dissolved solutes. 

For this reason, the process of pinocytosis is sometimes referred to as cell drinking, since it consists of fluid consumption.

On the other hand, phagocytosis is known as cell eating, as the process consists of solid materials being transported into the heart of the cell through the process of invagination. 

Types Of Cells 

While cells are present in both processes, phagocytosis is often associated with the immune system and immune cells such as granulocytes.

This is in contrast to pinocytosis, which can be observed in numerous cells across the body, such as endothelial cells, epithelial cells, and immune cells. 

Because of its association with immune cells, phagocytosis is known to combat corrupted cells, as well as invading organisms and unwanted particles, which it absorbs and degrades.

On the other hand, pinocytosis can be found across various cells and is able to transport extracellular fluid and dissolved solutes into the cell’s core.

For this reason, pinocytosis is also able to absorb fluids and proteins needed by the cell, which also sets it apart from other forms of endocytosis. 

In basic terms, both processes use different cells to achieve different functions, which is the main difference between them. 

The Continuous Process 

As we mentioned before, there are various forms of pinocytosis, which can be found in many cells throughout the body. In some cases, these processes use factors such as receptors and other proteins.

However, pinocytosis has since been defined as a continuous process, which allows the cell to absorb extracellular fluid and various molecules for the benefit of the main membrane. 

This is often attributed to the fact that pinocytosis is also nonspecific, which means it can be observed in various cells. Because of this, the process is able to transport various molecules present in the fluid with the help of ligands. 

In contrast to this, phagocytosis is only induced when certain materials come into contact with the cell and activate the process. For example, monocytes will only respond to certain activators, which means it is not continuous. 

Size Of Vesicles 

In most cases, smaller vesicles are attributed to the process of pinocytosis, especially when compared to those formed during other forms of endocytosis. Because of this, phagocytes are able to consume larger particles of cell debris.

They can even be used to combat invading organisms and other foreign bodies.

Pinocytosis Vs Endocytosis – What’s The Difference? 

Despite its individual characteristics, pinocytosis is actually a form of endocytosis, which puts it in the same faction as phagocytosis and receptor-mediated endocytosis.

In the section below, we have outlined some of the key differences between pinocytosis and its various cousins. 

Since we have already noted the differences between pinocytosis and phagocytosis, we will use the section below to discuss the differences between pinocytosis and receptor-mediated endocytosis. 

These differences include the following: 

Types Of Materials 

As we have already discussed, the process of pinocytosis consists of the absorption of extracellular fluid and dissolvable molecules in the cell. In contrast, receptor-mediated endocytosis involves the absorption of specific molecules into the cell.

Because of this, the materials associated with the receptors are granted access to the membrane, while other debris is ejected from the cell. 

Receptors 

Unlike pinocytosis, receptor-mediated endocytosis is dependent on the addition of receptors to activate the process and absorb materials into the cell.

Because of this, specific molecules have to come into contact with the surface receptors in order to be transported through the cell membrane. 

However, in pinocytosis, receptors are not always needed during the transport of materials, which sets it apart from its dependent counterparts.

Instead, small pits will form on the cell membrane, which will then absorb the fluids and solutes present and deposit them into the cell.

It is also important to remember that clathrin and caveolae-independent pinocytosis do not depend on receptors or other notable proteins. 

The Specifics 

Since receptor-dependent endocytosis relies on receptors to absorb molecules, the process is also selective and is only compatible with certain substances. Because of this, it can only transport certain materials, usually, those identified by the receptor. 

In contrast to this, receptors do not have an important role in pinocytosis, since the process is nonspecific and present in countless cells.

For this reason, the process can be used to transport various materials, as it is not limited in its composition. 

Final Thoughts 

While there are some who might class pinocytosis and endocytosis as different entities, it is important to remember that the former is actually a variety of the latter.

However, this does not mean that the two processes are similar, as one is limited due to its composition and the other can be observed in various cells throughout the human body. 

In basic terms, pinocytosis is a process that transports extracellular fluid and molecules to the cell, which are contained within small holes that form on the surface of the membrane.

While the process does share some similarities with its various cousins, it also comes with various differences, which include its nonspecific nature, transport of fluid and dissolved molecules, and independent design. 

Because of this, the process is often mistaken for phagocytosis, which is used to transport solid material to the cell.

For this reason, phagocytosis is sometimes referred to as cell eating, while pinocytosis is called cell drinking due to its association with fluid, molecules, and dissolvable solutes.

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