These bulk transport mechanisms are used to move materials into or out of the cell. The processes function in transporting molecules and substances that are too large to directly pass through the lipid bilayer of the cell membrane.
Table of Contents
What is endocytosis and exocytosis?
Endocytosis and exocytosis are types of bulk transport that require the expenditure of energy (ATP) to move substances in or out of the cell respectively. Hence, they are active transport processes. Some of the substances moved through the cell membrane via these processes include large molecules, waste products, and microorganisms.
The cell membranes of organisms are semi-permeable and thus allow certain small molecules and ions to passively diffuse through them. 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 the two active transport processes (endocytosis and exocytosis) 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 the mechanisms used in such instances. Therefore, endocytosis and exocytosis are means of transport used by eukaryotes.
Endocytosis vs exocytosis definition
Endocytosis involves the active transport of molecules into the cell by engulfing it with its membrane whereas, exocytosis pushes molecules out of the cell.
In living systems, the need for homeostasis causes an equal flow of molecules in and out of the cell. Hence, the number of molecules coming into the cell via endocytosis equals the number of molecules exiting the cell through exocytosis. In other words, the two main categories of vesicle transport are endocytosis and exocytosis.
What is endocytosis?
Endocytosis is a cellular process whereby cells ingest materials into the cell. The material or substance that is to be brought into the cell is surrounded by an area of the cell membrane which then buds off inside the cell to create a vesicle containing the ingested substance. This means endocytosis moves materials into a cell via membranous vesicles.
There are four types of endocytosis known which include:
- Receptor-mediated endocytosis
The process of endocytosis
Endocytosis transport materials from the external environment of the cell into the cell. During the process of endocytosis, the plasma membrane (cell membrane) folds itself around the material to be engulfed. This results in the formation of a vesicular structure that eventually pinches off from the membrane inside the cell. Therefore, the endocytosis process in cells involves steps that include the following:
- Vesicle formation
This is the first of the endocytosis steps which involves the cellular membrane folding and creating a cavity wherein the material to be engulfed is entrapped.
The next step is the cellular membrane folding back to form a vesicle with uniform vesicular membrane thickness. This vesicle formed contains the material to be engulfed.
After the formation of the vesicle, the vesicle detaches from the cellular membrane. The vesicle then proceeds for further processing by the cell.
What is exocytosis?
Exocytosis is a process by which cells move materials that are within the cell, outside, and into the extracellular fluid. The process of exocytosis occurs as a result of a vesicle fusing with the plasma membrane which allows its content to be released outside the cell.
Exocytosis can be defined in biology as the process by which cytoplasmic secretory vesicles fuse with the cellular membrane, in order to release contents to the extracellular space. Through exocytosis, 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.
There are basically, three types of exocytosis which include:
- Constitutive exocytosis
- Regulated exocytosis
- Lysosome-mediated exocytosis
The process of exocytosis
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. Here are the steps that occur during exocytosis:
- A vesicle forms within the endoplasmic reticulum and Golgi apparatus or early endosomes.
- The vesicles containing molecules travel within the cell to the plasma membrane (cell membrane).
- The vesicle membrane fuses to the plasma membrane and the two bilayers merge.
- This fusion causes the vesicle contents to be released into the extracellular space (outside the cell).
- Then the vesicle either fuses with the plasma membrane or separates from it.
Listed above are the five basic processes that occur during exocytosis which include vesicle trafficking, vesicle tethering, vesicle docking, vesicle priming, and vesicle fusing. In constitutive exocytosis, four of these processes occur while the whole five steps occur in regulated exocytosis.
Endocytosis vs exocytosis diagram
The image above is an endocytosis vs exocytosis simple diagram showing the difference in their process.
Endocytosis vs exocytosis examples
The table below shows some examples to compare and contrast endocytosis and exocytosis.
Phagocytosis in immune cells
In order to neutralize and eliminate pathogenic material from the body, immune cells such as dendritic cells, macrophages, and neutrophils exhibit phagocytosis e.g the white blood cell engulfing a bacterium.
Exocytosis 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 a typical example of exocytosis in the human body.
Pinocytosis in cells
The uptake of nutrients, hormones, and enzymes by cells occurs via pinocytosis. For instance, body cells take up hormones and enzymes via pinocytosis. Human egg cells and the microvilli cells of the intestines also take up nutrients from the surrounding environment via pinocytosis.
Exocytosis 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.
What is the difference between endocytosis and exocytosis?
The difference between endocytosis vs exocytosis involves the movement of substances into or out of the cell respectively. In endocytosis, substances and materials are transported from outside of the cell into the interior of the cell. Whereas, in exocytosis, substances or materials from inside the cell are moved and transported to the exterior of the cell. However, these two processes are both forms of active transport because they require energy.
What’s the difference between endocytosis and exocytosis? Let’s use a table to compare and contrast endocytosis and exocytosis.
Factors for comparison
This process involves the movement of a particle or substance from outside of the cell and transferring it inside the cell using a vesicle.
This process involves the movement of a substance or particle from inside of the cell and exiting it outside the cell using a vesicle.
(3) Receptor-mediated endocytosis
(1) Constitutive exocytosis
(2) Regulated exocytosis
(3) Lysosome-mediated exocytosis
How it works
An endocytic vesicle is formed surrounding the foreign substance
The vesicle containing the waste or substance fuses with the plasma membrane
Vesicle types formed
Internal vesicles such as phagosomes are formed
Secretory vesicles are formed
(1) Absorption of nutrients for cellular function
(2) Elimination of pathogens
(3) Disposal of old or damaged cells
(1) Removal of toxins or waste products
(2) Repair of the cell membrane
(3) Facilitates the communication between cells
White blood cells engulfing a virus and eliminating it is an example of endocytosis
The release of a neurotransmitter for cellular communication is an example of exocytosis
The tabular presentation above summarizes the difference between endocytosis and exocytosis.
How are endocytosis and exocytosis similar?
Endocytosis and exocytosis are means of transport used by eukaryotic cells. Regardless of the endocytosis vs exocytosis differences in function, they are similar in the sense that the main underlying process of both processes involves the cell membrane forming an invagination or pocket as well as surrounding the underlying substance that needs to be transported into the cell. Moreso, both processes require energy.
Endocytosis and exocytosis similarities
- Both mechanisms are forms of active transport.
- They both use energy to transport particles in or out of the cell.
- These processes both have different types that are similar because they both transport materials across the cell membrane by forming vesicle pores.
Phagocytosis vs endocytosis vs exocytosis
Endocytosis and exocytosis are examples of bulk transport mechanisms in cells whereas phagocytosis is a type of endocytosis. The comparison of phagocytosis vs endocytosis vs exocytosis is that phagocytosis is a type of endocytosis while endocytosis involves the movement of solid materials or fluids into the cell via the formation of cell membrane vesicle. Exocytosis, on the other hand, involves the movement of materials out of the cell to the extracellular environment.