Exocytosis – what is it? Well, in short, it is regarding the transport of various waste products, recycled materials and other substances from the body and into the cells for use.
First, these substances are enclosed in a vesicle, which is a container that includes a membrane and liquid. This vesicle then fuses itself to the membrane where it then opens out and delivers the contents to the requisite tissue.
But what is the difference between endocytosis and exocytosis? How do these different processes affect the body of the plant? What various enzymes and proteins are involved in this process?
Well, if you want to know the answer to these questions and a lot more, then we would suggest that you continue reading.
What Is Exocytosis?
As we mentioned above, exocytosis is when the contents of a cell vacuole are transported when it is attached to the membrane and release its contents, usually proteins or enzymes.
The material is usually released through the extracellular environment, which is where it is utilized in various different areas.
There are various substances within the body that can be easily diffused in the body, such as oxygen and carbon dioxide.
However, other elements are too large, such as molecules, so they need a specialized method of transportation to get from one place to another.
These types of cells require being transported in large bulks using vesicles, as it is hard to convert the molecules in the extracellular membrane.
Why Is Exocytosis Important?
There are many reasons why it is important for exocytosis to occur. Here are a few different functions that exocytosis is essential for:
It allows cells to communicate – Cells need to communicate with each other by releasing certain molecules which will send signals from one molecule to another.
For example, in the brain, neurotransmitters are released, either dopamine or gamma-Aminobutylic acid, which are then absorbed and received by other atoms.
To allow the body to recycle certain substances – is where the differences between endocytosis and exocytosis become obvious.
Endocytosis is internalized into the cell, whereas during exocytosis, these receptors are recycled and absorbed back into the cell membrane.
Taking waste from the cell – taking waste material from the cell is very important for maintaining the cell’s optimum efficiency.
Things such as lactic acid and nitrogen compounds are not required. These are enclosed in vesicles and transported to extracellular membranes.
Transmission of cell secretions – in addition to signals and waste material, exocytosis can be used to transport enzymes, proteins and antibodies.
But exocytosis takes different forms, depending on what the process is used for in the first place. Here are a few of the different types of exocytosis and the function they perform.
Exocytosis And The Different Forms It Can Take
In basic terms, the process of exocytosis is basically when the cellular material is enclosed in a vesicle and transported into the extracellular environment. However, this can be expressed in many different forms:
When proteins are enclosed by the Golgi apparatus, they need to be transported to the surface of the cell where they can be converted into energy or used to build more cells. So this is where the constitutive exocytosis comes in.
This is basically a continuous pathway through which the synthesized proteins are transported into the cellular matrix.
The Golgi apparatus is where the proteins are synthesized and packaged using vesicles. They are usually transported through the endoplasmic reticulum, which is a series of discs through which materials are synthesized.
Once the vesicle is filled with this material, it detaches itself from the Golgi complex and moves towards the cell membrane, where it is then transferred via osmosis through the cell wall.
Microtubules are what is responsible for the movement of vesicles. These are propelled through the microtubules in a single direction by motor proteins.
Once these vesicles are transported to the cell membrane, they go through several steps in order to get release the contents into the extracellular matrix.
This is done in much the same method as a cargo ship might dock at a port, tethering and locking before shipping the goods to the shore.
Once the vesicle is fully tethered to the cell membrane, then it opens up to eject the contents through the cell membrane. This process is regulated through enzymes and large SNARE proteins.
How Do Proteins Play A Part In Vesicle Formation?
There are a few proteins that help with creating vesicles. There are proteins called coat proteins that shape transmembrane proteins into spheres. Some of these proteins contain COPII, COP1 and clathrin.
Clathrin is one of the main substances that is involved in the formation of vesicles. When it coats the membranes, it does not do so directly, merely doing so through adaptor proteins.
Certain enzyme proteins such as disulfide-isomerase are found in the endoplasmic reticulum and are often returned there once the vesicles have moved and deposited the protein.
This is why constitutive exocytosis is so important because it is responsible for the constant rotation of these cellular materials, as well as transporting them through the cell membrane and removing them from the cell in the first place.
Through this level of continual transport, you can be sure that cell material is constantly recycled and re-used on the surface of the cells.
Serum, milk proteins and proteoglycans are also transported through exocytosis. These are also important for the overall function of cells.
What Is Regulated Exocytosis?
This type of exocytosis is very different from constitutive exocytosis in that it is not a continuous process. It will only be triggered by certain signals, which occur when the cell is stimulated in a particular way.
This type of transportation only really happens in certain specialized cells such as the secretion of hormones, enzymes and neurotransmitters. This will only kick start when these substances are subjected to certain stressors.
Mast cells are also characterized by regulated exocytosis. These are a type of white blood cell that is found near the connective tissue in the body. They are near blood vessels and lymph vessels.
Other cells that use regulated exocytosis are neurons, endocrine and exocrine cells.
The material that is secreted by vesicles is contained in the cell cytoplasm until they are needed. This will usually happen when the cells are triggered in the correct way.
What Are The Islets Of Langerhans?
These are cells that occur in the pancreas and are heavily responsible for the secretion of insulin, as well as glucagon.
Insulin is a peptide hormone that regulates the dissolution of carbs, proteins and fats. Glucagon is also a peptide hormone that increases the level of glucose and fatty acids in the bloodstream.
These pancreatic cells are also known to be a site where regulated exocytosis occurs. As with many other proteins of its type, this is where precursors to insulin are produced using ribosomes.
Once this insulin progenitor has been produced, then it undergoes a further transformation in the endoplasmic reticulum, which then leads to the creation of insulin.
Once this insulin is produced, then it is enclosed in a vesicle using the Golgi complex which is then released into the cytoplasm.
However, as we have mentioned above, the process that the insulin undergoes is only triggered in certain circumstances, which is why it is characterized as regulatory exocytosis.
In the case of insulin being transported to the cells, this is triggered by a spike in blood glucose levels. Once cells have traced a high level of glucose in the blood, then 5 steps occur in order to release the insulin.
- Trafficking – this is when the vesicle moves through the microtubules through the membrane where it will deliver the contents.
- Tethering – this is when the vesicle locks onto the membrane, where the contents are then transferred into the extracellular matrix.
- Docking – this is when the membranes of the cell and the vesicle fuse together.
- Priming – this is when the cell membrane modifies itself in response to certain stimuli or triggers.
- Fusing – this is when the membrane of the cell and the vesicle merge together so that the material enclosed inside the vesicle can be released. This fusion can either be temporary or permanent, depending on the function. If it is temporary, then the vesicle will return to the cell.
The type of fusion where the cell membrane only touches the vesicle briefly or only partially is called a kiss-and-run fusion.
During regulated exocytosis, some of the contents of the vesicles are released into the cytosol.
What Is Lysosome Mediated Exocytosis?
This is the type of transportation where the vesicles are fused with lysosomes, which often have similar features and characteristics of vacuoles.
Lysosomes are organelles – small organ-like systems that serve many functions – which contain a lot of enzymes that can break down all types of bio-material including proteins, carbs and lipids. They are usually enclosed in a membrane.
The process of lysosome-mediated exocytosis allows the material from cells to be degraded, which will allow the cells to maintain homeostasis.
When you are clearing out cells, there are numerous other processes that take place. These processes include bone resorption, antigen presentation and melanocyte function.
But there is a link between lysosome-mediated exocytosis and membrane repair which is not often discussed.
How Does Lysosome-Mediated Exocytosis Help With Membrane Repair?
The membrane of a cell is very important when it comes to moving fluid and material in and out of the cell itself. This membrane also contains many different types of organelle, which helps will the regulation of cells.
If the cell membrane is damaged due to chemical or physical breaches. There are 5 different types of membrane damage: mechanical, chemical, microbial, immune, and intracellular stressors.
When a cell membrane has been damaged, lysosomes will move towards the cell and begin to move and fuse with it. It will then begin to repair the damaged cell. This is when a surge of calcium ions will enter the ruptured membrane.
When the calcium ions congregate in this area, then it activates a calcium sensor which undergoes changes that interact with the SNARE proteins of the cell membrane.
Once this interaction occurs, then the formation of a fusion pore creates a space for the lysosome to be released, repairing the rupture or damaged membrane through sealing.
What Are Endocytosis And Exocytosis?
In basic terms, these two processes are the opposite. Endocytosis involves the movement of the extracellular material into the cell whereas exocytosis involves the transportation of material from the cell to the extracellular matrix.
Both of these processes are extremely important for the body to function as it does. Here are a few things that endocytosis and exocytosis are crucial for:
- Ingestion of nutrients – when it comes to regulating metabolism and the creation of ATP energy, then both these systems are key features. This allows cells to uptake nutrients such as sugars and lipids.
- Phagocytosis – this is when phagocytes (also known as immune cells) are able to tackle invading pathogens and microbes.
- Recycling old and damaged cells – this is when the same immune cells are able to get rid of damaged or dead cells.
Focusing on endocytosis for a second, here are the three main types of this cellular transportation.
Three Different Types Of Endocytosis
- Phagocytosis – this is when the cells eat each other. This is basically when cells are able to engulf larger invading microbes, which is a large part of disease prevention. It also refers to the consumption of dead and damaged cells as well as absorbing nutrients.
- Pinocytosis – this is known as ‘cell-drinking’, where cells take on some particles from outside themselves in the extracellular matrix. This is when the cell effectively ‘drinks’ some of the fluid from the surrounding environment.
- Receptor-mediated endocytosis – this is when different types of molecules and particles including viruses are internalized as soon as they come into contact with the receptors in the cell.
Now that we’ve looked at some of the ways in which these two processes of transportation differ, let’s have a look at what ways they are the same.
How Are Endocytosis And Exocytosis The Same?
There are a few ways in which these processes are the same as well as how they interlink to form a system.
- Active transport – this means that energy is required for both of these processes to occur, especially during the transportation of the material through the cell membrane. This energy takes the form of Adenosine triphosphate (ATP) which will help the formation of vesicles.
- Both involve vesicles – Both these processes involve the creation of vesicles in order to move the material from one cell to another. These vesicles contain a lipid bilayer that encloses the material. In endocytosis, vesicles are formed in the cell membrane while in exocytosis, they are formed in the membrane of the Golgi apparatus.
How Are Endocytosis And Exocytosis Different?
There are a few major ways in which these two processes are different:
Both of these systems involve transporting material from one cell to another, however, they have different functions.
As we have discussed earlier, exocytosis involves the transportation of proteins from the cell to the extracellular matrix, whereas endocytosis move material the opposite way.
By bringing materials to the cell, endocytosis is very important to maintaining the function of the cells. It also is responsible for giving much-needed energy and protein to the cells.
Exocytosis is also very important for getting rid of waste material which, if left in the cell, will start to rupture and swell. This also helps send out signal molecules, which helps the cells to communicate.
Formation Of Vesicles
The vesicles that are produced during the process of exocytosis are made in the Golgi complex and the endoplasmic reticulum, whereas those vesicles that are formed during endocytosis are created in the cell membrane itself.
Phagocytosis, pinocytosis, clathrin-mediated endocytosis and cavaelae-mediated endocytosis are only associated with endocytosis.
Calcium ions triggered by non-constitutive exocytosis (known as regulated exocytosis) and non-calcium ions triggered by constitutive exocytosis are only associated with exocytosis.
What Happens During Phagocytosis?
We discussed phagocytosis in brief above, but here is a more in-depth explanation of how this form of endocytosis works:
- Binding – this is when a particle binds to the surface of the cell via the receptors. This then stimulates the release of a plasma membrane that is filled with cytoplasm, also called pseudopodia.
- This plasma then surrounds the object so that the membranes fuse, which then forms what is called a phagocytic vesicle.
- Detachment – this vesicle then removes itself from the cell membrane and enters the cell completely.
- Fusion – once this is complete, the vesicle fuses with lysosomes which then re-use or obliterate the material inside the vesicle.
What Happens During Pinocytosis?
Next, we’ll have a more detailed look at the other version of endocytosis called Pinocytosis:
- Binding – once again, the molecules will bind to the surface of the membrane of the cell.
- Folding – the plasma of the membrane then folds in to form a pinocytic vesicle that houses all the fluid and the molecules themselves.
- Detachment – this pinocytic vesicle then removes itself from the cell membrane.
- Fusion – this vesicle then fuses with endosomes which then sort out the contents and deliver them to the appropriate cells.
Why Is It Necessary For Cells To Bulk Transport?
Cells only let smaller particles and ions pass through them, as they are semi-permeable. This is one of the main methods that only certain molecules can get into these cells.
However, smaller molecules can also get through this membrane through carrier proteins and channels.
However, they are much larger molecules that the cells need that are simply too big to permeate the cell membrane. This is why bulk transportation is important, to ensure the delivery of certain proteins and amino acids.
There are also times when a cell will need to entirely swallow up another bacteria, especially if it needs to release a hormone. It is during these situations that bulk transport is required.
Both endocytosis and exocytosis require energy in order to work, which is why they are called active transportation processes.
What Are Early and Late Endosomes?
Early endosomes are usually located on the edge of the cells and they are responsible for receiving the vesicles that are coming from the surface of the cell.
These endosomes are mildly acidic and are mainly concerned with sorting organelles. They also recycle many of the receptors by transporting them to the cell surface via tubules.
Late endosomes received endocytosed materials that are en route to the lysosomes, either going from the trans-Golgi network in the biosynthetic pathway or from the phagosomes in the phagocytic pathway.
These late endosomes are much more acidic and are speculated to mediate the sorting events that occur prior to the delivery of proteins and other material to the lysosomes.
We hope that our explanation of the differences and similarities of endocytosis and exocytosis has helped you better understand how these processes work.
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