What is exocytosis in a cell? examples, process, and function

Endocytosis and exocytosis are examples of bulk transport mechanisms used to move materials into or out of the cell. The exocytosis process function in transporting molecules and substances that are too large to directly pass through the lipid bilayer of the cell membrane. The cell membranes of organisms are semi-permeable and thus allow certain small molecules and ions to passively diffuse through them.

Some other small molecules still find their way into or out of the cell via carrier proteins or channels. Nevertheless, some molecules can’t pass through the plasma membrane of a cell or move through a transport protein because they are too large. Due to this, cells make use of two active transport processes to move such macromolecules into or out of the cell.

For instance, there are times a cell will need to release a hormone or engulf a bacteria. In such instances, bulk transport mechanisms are required. Endocytosis and exocytosis are examples of the bulk transport mechanism used in such instances.

Generally, endocytosis and exocytosis are means of transport used by eukaryotes. Endocytosis and exocytosis are types of bulk transport that require the expenditure of energy (ATP). They are the bulk transport mechanism used in eukaryotic organisms and are active transport processes because they require energy. Some of the substances moved through the cell membrane via these processes include large molecules, waste products, and microorganisms. There are basically, three types of exocytosis which include constitutive exocytosis, regulated exocytosis, and lysosome mediated exocytosis.

Exocytosis functions as an important process of plant cells and animal cells and is the opposite function of endocytosis. In the process of endocytosis, substances that are outside the cell are brought into the cell, whereas in exocytosis, substances within the cell are brought out of the cell.

Cells use endocytosis and exocytosis because the majority of chemical substances that are essential to them are large polar molecules that can’t pass through the hydrophobic portion of the cell membrane by passive means.

Exocytosis definition in biology

Exocytosis can be defined in biology as the process by which cytoplasmic secretory vesicles or vacuoles fuse with the cellular membrane, in order to release contents to the extracellular space.

In order words, exocytosis is the process by which neurotransmitters are released from synaptic vesicles into the synaptic cleft. Cells use exocytosis to transport substances from the interior of the cell to the exterior of the cell. As an active transport mechanism, exocytosis requires energy.

During exocytosis, membrane-bound vesicles that contain cellular molecules are transported to the cell membrane. The vesicles then fuse with the cell membrane and expel their contents outside the cell. Hence, the exocytosis process involves the use of energy to transport molecules such as neurotransmitters and proteins out of the cell.

It is a form of bulk transport because a large number of molecules are released. This process takes place via secretory portals at the cell plasma membrane called porosomes. Porosomes are permanent cup-shaped lipoprotein structures located at the cell plasma membrane. Secretory vesicles transiently dock and fuse at these porosomes to release intra-vesicular contents from the cell.

Types of exocytosis

1. Constitutive exocytosis
2. Regulated exocytosis
3. Lysosome mediated exocytosis

There are three main pathways (types) of exocytosis.

Constitutive exocytosis

The constitutive exocytosis is performed by all cells and involves the regular secretion of molecules. Constitutive exocytosis transport membrane proteins and lipids to the cell’s surface and expels substances to the exterior of the cell. This exocytosis process doesn’t require any extracellular signals and many molecules that travel to the cell membrane use this pathway.

Some exocytotic vesicles are incorporated into the cell membrane after exocytosis, whereas others return to the interior of the cell after they have emptied their contents (kiss-and-run pathway). Others, on the other hand, remain docked to the membrane where they can be used several times (kiss-and-stay pathway).

Regulated exocytosis

The regulated exocytosis unlike the constitutive exocytosis depends on the presence of extracellular signals in order to expel the materials within vesicles. The majority of exocytotic vesicles have substances produced within the endoplasmic reticulum such as hormones or neurotransmitters. A vesicle (a layer of membrane) has these molecules packaged up within them.

These vesicles are excreted from the endoplasmic reticulum and transported to the Golgi apparatus for further modification. The molecules are packaged again in a vesicle that finds its way to the cell membrane.

Regulated exocytosis doesn’t happen in all cell types, it is common in secretory cells. Products or molecules such as neurotransmitters, hormones, and digestive enzymes are stored in secretory cells and then released only when triggered by extracellular signals. The secretory vesicles are not incorporated into the cell membrane, rather, they fuse only long enough to secrete their contents.

These vesicles reform and returns to the cytoplasm once the delivery has been made. Therefore, the release of these products/molecules from the cell is known as regulated exocytosis. This is termed so, because the expulsion of these products is regulated or controlled by extracellular signals that cause membrane depolarization.

Lysosome mediated exocytosis

The fusion of vesicles with lysosomes is a third pathway for exocytosis in cells. Lysosomes are organelles that contain acid hydrolase enzymes that break down microbes, waste materials, and cellular debris. They carry their digested material to the plasma membrane, then fuse with the membrane and release their contents into the extracellular matrix.

Furthermore, in the context of neurotransmission, cells release neurotransmitters through exocytosis from the synaptic vesicles into the synaptic cleft. Also, these neurotransmitters can be released through reverse transport via membrane transport proteins.

Process of exocytosis

1. A vesicle forms within the endoplasmic reticulum and Golgi apparatus or early endosomes.
2. The vesicles containing molecules travel within the cell to the plasma membrane (cell membrane).
3. The vesicle membrane fuses to the plasma membrane and the two bilayers merge.
4. This fusion causes the vesicle contents to be released into the extracellular space (outside the cell).
5. Then the vesicle either fuses with the plasma membrane or separates from it.

Trafficking

In this process, the vesicles are transported to the plasma membrane along the microtubules of the cytoskeleton. The movement of the vesicles is engineered by the motor proteins kinesins, dyneins, and myosins.

Tethering

As the vesicle reaches the plasma membrane, it becomes linked to the plasma membrane and pulled into contact with it.

Docking

In this process, the vesicle membrane attaches to the plasma membrane. During docking, the phospholipid bilayers of the vesicle membrane and plasma membrane begin to merge.

Priming

The process of priming takes place in regulated exocytosis and not in constitutive exocytosis. Priming involves specific modifications that must occur in certain plasma membrane molecules for exocytosis to happen. These modifications are needed for signaling processes that trigger exocytosis to occur.

Fusion

In exocytosis, there are basically two types of fusion that can occur which include complete fusion and kiss-and-run fusion.

During complete fusion, the vesicle membrane fully fuses with the plasma membrane. In this fusion, ATP provides the energy required to separate and fuse the lipid membranes. A fusion pore is created from the fusion of the membranes. As the vesicle becomes part of the plasma membrane, this fusion pore allows the contents of the vesicle to be expelled.

In kiss-and-run fusion, the vesicle temporarily fuses with the plasma membrane. This fusion is long enough to create a fusion pore and release its contents to the exterior of the cell. The vesicle then pulls away from the plasma membrane and reforms before it returns to the interior of the cell.

Exocytosis function in the cell

1. Removal of molecules, toxins, and waste products from the cell.
2. The process of exocytosis functions in chemical signal messaging and cell-to-cell communication.
3. It is used to rebuild the cell membrane.

Removal of toxins, molecules, or waste products from the cell’s interior

In order to maintain homeostasis, cells have to remove waste or toxins that must have been created. hence, cells use exocytosis to secrete waste substances and molecules such as proteins and hormones. In aerobic respiration, for example, cells produce carbon dioxide and water as waste products during ATP formation. The carbon dioxide and water are then removed from these cells through exocytosis.

A number of cells in the body using exocytosis to transport proteins is a real world example of exocytosis in human cells. For instance, merocrine glands classified as glands release their secretions by exocytosis. These glands are the most common and use exocytosis to release their secretory products (usually proteins).

Facilitates chemical signal messaging and cellular communication

Exocytosis is essential for chemical signal messaging and cell-to-cell communication. Cells produce signaling molecules such as neurotransmitters and hormones which are delivered to other cells when released from the cell via exocytosis. This is a major exocytosis example in the human body.

Facilitates cellular membrane growth, repair, signaling, and migration

During endocytosis, cells absorb materials from outside the cell by using lipids and proteins from the plasma membrane to create vesicles. However, as certain exocytotic vesicles fuse with the plasma membrane, they replenish the plasma membrane with these materials. Hence, cells use exocytosis to rebuild the cell membrane by fusing proteins and lipids removed via endocytosis back into the membrane.

Exocytosis examples

1. Exocytosis occurs in the pancreas
2. Exocytosis takes place in neurons

Exocytosis in the pancreas

An example of exocytosis occurs in the pancreas. Small clusters of cells in the pancreas called islets of Langerhans produce glucagon and insulin hormones that are stored in secretory granules. These hormones are then released by exocytosis when signals are received. This is an exocytosis example in the human body.

Insulin is released from islet beta cells when the glucose concentration in the blood is too high. This causes cells and tissues to take up glucose from the blood. When blood glucose levels fall too low, the pancreas releases glucagon via exocytosis which is secreted from islet alpha cells.

The glucagon makes the liver convert stored glycogen into glucose. The glucose is then released into the bloodstream which causes blood glucose levels to rise. Additionally, the pancreas uses exocytosis to secrete digestive enzymes such as proteases, amylases, and lipases.

Exocytosis example in neurons

One of the examples of exocytosis is the synaptic vesicle exocytosis that takes place in neurons of the nervous system. Nerve cells interact and communicate by chemical (neurotransmitters) or electrical signals that are passed from one neuron to the next. Through the transmission of neurotransmitters, some neurons communicate.

Neurotransmitters are chemical messages that the synaptic vesicles transport from nerve to nerve. These synaptic vesicles are membranous sacs formed via endocytosis of the cell membrane at presynaptic nerve terminals. Once these synaptic vesicles are formed, they are filled with neurotransmitters and sent towards an area of the cell membrane known as the active zone.

The synaptic vesicle awaits a signal which is an influx of calcium ions brought on by an action potential. This allows the vesicle to dock at the pre-synaptic membrane. Until the second influx of calcium ions takes place, the actual fusion of the vesicle with the pre-synaptic membrane doesn’t occur. It is after the second signal that the synaptic vesicle fuses with the pre-synaptic membrane to create a fusion pore.

As the two membranes merge as one, this fusion pore expands and the neurotransmitters are released into the synaptic cleft. The synaptic cleft is the gap between the pre-synaptic neuron and the postsynaptic neuron. These neurotransmitters are transmitted via exocytosis and the released neurotransmitters can bind to receptors on the post-synaptic neuron. The binding of the neurotransmitters may either excite or inhibit the postsynaptic neuron.

Exocytosis diagram

Endocytosis vs exocytosis

Endocytosis and exocytosis are means of transport used by eukaryotic cells to move substances into or out of the cell, respectively. They are bulk transport mechanisms used to transport large molecules, waste products, and microorganisms. Endocytosis and exocytosis are types of active transport mechanisms in cells.

Despite the difference between exocytosis vs endocytosis, there are several different types of endocytosis as in exocytosis. The different types in both processes are similar in that the main underlying process involves the cell membrane forming an invagination or pocket as well as surrounding the underlying substance that needs to be transported into the cell. Phagocytosis, pinocytosis, and receptor-mediated endocytosis are the three major types of endocytosis. Whereas, the types of exocytosis involve constitutive, regulated, and lysosome-mediated exocytosis.

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