What are endocytosis and exocytosis?
Endocytosis is the process by which a cell takes in the actively transporting molecules from the external surrounding into the vesicle by engulfment process. The substances may include essential nutrients required for the cell development or pathogens that may cause harm to our cells are engulfed and destroyed by our immune cells.
Endocytosis occurs when the cell membrane folds engulfing the molecules with extracellular fluid which breaks off vesicle facilitating to transport inside the cell.
Exocytosis functions are the opposite of endocytosis by pushing the molecules out from the cell into the extracellular environment. When a vesicle fuses with the plasma membrane, it allows its content to release out of the cell hence exocytosis occurs.
Endocytosis and exocytosis are used by all transport molecules which are unable to pass through the membrane passively.
Homeostasis enables an equal flow of molecules by endocytosis and exocytosis to ensure the balance of nutrients and wastes to maintain cell life and function.
Functions of endocytosis:
- The main function of endocytosis is to take in essential nutrients like proteins and lipids for cellular growth and repair.
- When pathogens infect our body, our immune cell engulfs and destroys them by different types of endocytosis process.
- When damaged or dead cells stop functioning, they must be disposed of safely which is done by the endocytosis process to prevent damage to other normal cells.
Functions of exocytosis:
- During ATP formation in the cell, wastes and toxins are produced that must be removed out of the cell to maintain homeostasis which is carried out by the exocytosis process.
- Cells facilitate signaling molecules like hormones and neurotransmitters to other cells via exocytosis.
- Exocytosis facilitates cell membrane growth and repair by fusing exocytotic vesicles with the cellular membrane.
Types of endocytosis:
- Receptor-mediated endocytosis
Phagocytosis is the process by which the cell targets large molecules that are not particularly specific. It is also known as the cell eating process and how the immune cells and white blood cells like macrophage and neutrophil engulfs and destroys pathogen and toxic compounds.
Process of phagocytosis:
- A pathogen or molecule binds to the cell surface receptor which stimulates the pseudopodia extended by phagocytes by membrane evagination.
- Phagocytes are macrophages and neutrophils that are part of white blood cells that encircles the molecule until their membrane fuse and forms a phagocytic vesicle known as phagosomes.
- A lysosome contains a digestive enzyme that digests the materials in the vesicle and fuses with phagosome forming a phagolysosome.
- Damaged and dead materials are expelled out by exocytosis.
In the pinocytosis process, the cell takes in small substances like water and nutrients from the extracellular fluid and is non-specific. It is also known as the cell drinking process which is common in plant and animal cells.
Process of pinocytosis:
- Molecules like water molecules or nutrients bind to the receptor on the cell surface.
- The portion of the membrane folds in and encircles the molecule forming a pinocytic vesicle and transports them into the cell lysosome.
- The lysosome fuse with the vesicle that degrades the cell layer and its content releases out into the cytoplasm.
Another specialized type of pinocytosis is receptor-mediated endocytosis where macromolecules require specific receptors to bind on the cell membrane surface.
One of the common examples of receptor-mediated pinocytosis is cholesterol uptake in the body.
Types of exocytosis:
- Regulated exocytosis
- Constitutive exocytosis
- Lysosome mediated exocytosis
The process of fusion of the membrane of cytoplasmic organelles with the plasma membrane which occurs in response to stimulation is regulated exocytosis. The molecules such as hormones or neurotransmitters or digestive enzymes created within the endoplasmic reticulum are packed in a secretory vesicle which is transported to the Golgi complex once excreted out from the ER. Then the molecules are packed again in a vesicle and move towards the plasma membrane and get released out. These released molecules are known as regulated exocytosis because the released materials are regulated by extracellular signals and cause membrane depolarization.
Constitutive exocytosis is the most common pathway performed by the body cells which do not require extracellular signaling. Some exocytotic vesicle remains in the plasma membrane even after exocytosis but other releases their content and returns to the interior of the cell. This mechanism is termed as the “kiss-and-run” pathway.
Lysosome mediated exocytosis:
Lysosome mediated exocytosis is a process of fusion of cell vesicles and cell lysosomes. An enzyme contained in the lysosome breaks down the molecules and carries them into the cell membrane where it fuses and clears out.
Process of exocytosis:
- A vesicle is formed in early endosomes or endoplasmic reticulum and Golgi complex and travels to the cell membrane.
- The vesicle links to the cell membrane which is termed as tethering.
- The vesicle membrane attaches to the cell membrane and begins to merge with each other.
- Priming occurs only in the regulated exocytosis process which is a specific modification in certain cell membrane molecules that requires extracellular signaling.
- The contents are then released out and fusion of the vesicle takes place which is either complete fusion or kiss-and-run fusion.