Exocytosis is a process in which a cell sends material inside secretory vesicles to be expelled from the cell via the cell membrane. This is also referred to as reverse pinocytosis; pinocytosis is the procedure by which little particles are brought into the cell in small vesicles. Likely contents of these secretory vesicles are soluble proteins to be sent out of the cell and lipids and membrane proteins that are designated to become parts of the cell membrane. Quite a number of cellular processes use exocytosis, such as when antibodies, enzymes and peptide hormones are secreted from the cell, when neurotransmitters are released from presynaptic neurons and when plasma membrane-bound receptors are recycled.
Exocytosis Takes Part in five Steps
Certain vesicle-trafficking steps require the translocation of a vesicle over a significant distance. For example, vesicles that carry proteins from the Golgi apparatus to the cell surface are likely to use motor proteins and a cytoskeletal track to get close to their target before tethering would be appropriate. Both the actin- and the microtubule-base are implicated in these processes, along with several motor proteins. Once the vesicles reach their targets, they come into contact with tethering factors that can restrain them.
It is useful to distinguish between the initial, loose tethering of vesicles with their objective from the more stable, packing interactions. Tethering involves links over distances of more than about half the diameter of a vesicle from a given membrane surface. Tethering interactions are likely to be involved in concentrating synaptic vesicles at the synapse.
The vesicles are also involved in regular cell’s transcription processes.
The term docking refers to the holding of two membranes within a bilayer’s distance of one another. Stable docking probably represents several distinct, molecular states: the molecular interactions underlying the close and tight association of a vesicle with its target may include the molecular rearrangements needed to trigger bilayer fusion. A common feature of many proteins that function in vesicle tethering and docking is their propensity to form highly extended, coiled-coil structures. Tethering and docking of a transport vesicle at the target membrane precedes the formation of a tight core SNARE complex.
In neuronal exocytosis, the term priming has been used to include all of the molecular rearrangements and ATP-dependent protein and lipid modifications that take place after initial docking of a synaptic vesicle but before exocytosis, such that the influx of calcium ions is all that is needed to trigger nearly instantaneous neurotransmitter release. In other cell types, whose secretion is constitutive (i.e. continuous, calcium ion independent, non-triggered) there is no priming.
The vesicle fusion is driven by SNARE proteins process of merging the vesicle membrane with the target one resulting in release of large biomolecules in the extracellular space.
The merging of the donor and the acceptor membranes accomplishes three tasks:
1. The surface of the plasma membrane increases. This is important for the regulation of cell size, e.g., during cell growth.
2. The substances within the vesicle are released into the exterior. These might be waste products or toxins, or signaling molecules like hormones or neurotransmitters during synaptic transmission.
3. Proteins embedded in the vesicle membrane are now part of the plasma membrane. The side of the protein that was facing the inside of the vesicle now faces the outside of the cell. This mechanism is important for the regulation of transmembrane receptors and transporters.
In addition to the expelling of waste material, exocytosis also increases the surface of the cell membranewith the addition of the wall of the vesicle, resulting in cell growth. If the expelled matter was hormones or neurotransmitters, their target receptors are signaled to do whatever they are required to do.
Is eExocytosis Passive or Active Transport?
Active transport in cells refers to movement against the concentration gradient. Energy is expended in accomplishing this and serves as an example of a process for which a cell needs energy. The energy is provided by ATP. A cell that possesses more glucose than its surroundings, yet takes in more glucose, is exhibiting active transport. Imagine trying to stuff more sweaters into an already full suitcase and then trying to close it. Similarly, if a cell is expelling its wastes when the extracellular space already contains plenty of waste molecules, this is also an example of active transport.
Passive transport is when particles move from an area of high concentration to an area of low concentration, and energy is not needed for this. A common scientific experiment which illustrates this is to submerge a bag of salty water inside a container of regular water. If a hole is poked in the bag and the water is tested after some time, it is found that the water in the bag has become less salty while the water in the container has become more salty. This is due to salt molecules leaving the bag with a higher concentration of salt and moving to the container with a lower concentration of salt. Or, in our other analogy, open up that stuffed suitcase and the sweaters just pop out by themselves without anyone having to expend any energy dragging them out.
Therefore, it can be said that exocytosis is a process of active transport.
What is Exocytosis Animation?
Reading the above description can sometimes be tedious or confusing. Enter animation. An animation is a series of pictures displayed quickly to give an illusion of movement. Cartoons watched by children are a typical example of animations. An exocytosis animation is a series of images depicting the cell, with a vesicle filled with little dots, representing molecules, nearing the cell membrane. The walls of the vesicle fuse with the cell membrane, and the vesicle opens up, and the little dots are sent free of the cell. These animations are usually clear and entertaining, with the cell membrane shown as a thick colored circle. Some animations have the wall of the vesicle maintain its own color, so when the vesicle fuses with the cell membrane, the viewer can still discern what was the original membrane and what is the addition from the vesicle.
Although the advantage of clarity from the animation is balanced by the disadvantage that many details of the process are left out, once the viewer has become familiar with the process of exocytosis through watching the animation, he or she can then read text explaining exactly how the process works. This additional information has a “destination folder” in the viewer’s brain of already understood knowledge of the process, which has been painlessly acquired by seeing the cartoon. These animations can be viewed over and over, and some have the option of the viewer controlling the rate at which exocytosistakes place.
A few of the animations also include the nucleus of the cell and the Golgi apparatus, showing how the vesicle travels around in the cell before it approaches the cell membrane. Other animations show endocytosis in addition to exocytosis, so the student can view how molecules or bacteria are engulfed by the cell as well as how they are expelled. Separate animations for the different kinds of endocytosis, namely, phagocytosis, pinocytosis and receptor-mediated endocytosis, can be viewed, providing their audience with a clear, enjoyable education of how these cell processes work.
How are Endocytosis and Exocytosis Similar
What is Endocytosis
Endocytosis is the way that cells take in molecules from the extracellular space by surrounding them with their cell membrane. The cell membranesurrounds the particle to be ingested and folds inward. A small piece of the cell membrane is pinched off, forming a bubble or vesicle, also known as an endosome. All the cells of the body need endocytosis in order to get food, for mitosis, for the acceptance of neurotransmitted impulses and for the acceptance of medicines into the cell. When the particle is engulfed by the cell and is surrounded by an endosome, it is able to be transported through the cell.
Phagocytosis is the term for cell eating. It involves large endosomes, is only carried out by specialized cells and occurs sporadically. Pinocytosis, comparable to cell drinking, uses small endosomes, is used by all cells and happens continuously. The endosomes eventually deliver their contents to lysosomes, whose digestive enzymes break down their meal. Another type of endocytosis is receptor-mediated endocytosis, where the cell is more selective regarding its “diet.” Only specific molecules, that attract and bind to desired ligands that those molecules possess, are invited to be ingested. When the preferred molecules arrive at the receptors, they are engulfed by the cell membrane and ingestion occurs.
Some bacteria use this process to their advantage, disguising themselves as the desired molecules by clothing themselves in the ligand in which the receptor is interested. Examples are the Epstein Barr virus, Listeria monocytogenes and tuberculosis. By this subterfuge, the bacteria gain entrance to the cell wherein they can wreak the destructive illness.
What is Exocytosis
Exocytosis is a process in which a cell sends what’s inside secretory vesicles to be expelled from the cell membrane. It is the opposite of endocytosiswherein little particles are brought into the cell in small vesicles. Vesicles in general are little sacs enclosed by a membrane; they have the ability to move around the cell, transporting their contents from one area to another. Likely contents of these secretory vesicles are soluble proteins to be sent out of the cell and lipids and membrane proteins that are designated to become parts of the cell membrane. Quite a number of cellular processes use exocytosis: when antibodies, enzymes and peptide hormones are secreted from the cell, when neurotransmitters are released from presynaptic neurons and when plasma membrane bound receptors are recycled.
Both endocytosis and exocytosis involve active transport, that is, energy must be expended to move particles against the concentration gradient. Regardless whether there is already more glucose in the cell than in the surrounding extracellular space, the cell will still take in more articles of glucose by endocytosis. If there is more waste outside the cell than inside, the cell will still expel its waste using exocytosis. Both endocytosis and exocytosis involve the formation of vesicles: endocytosis forms them in order to take particles into the cell via the cell membrane; exocytosis forms them in order to expel things from the cell via the cell membrane. Each of these processes is the opposite of the other. Endocytosis involves a reduction in cell membrane area, as part of the membrane is pinched off to form a vesicle, and exocytosis results in an increase in cell membrane, as the vesicle wall joins that of the cell membrane and is incorporated into it. While endocytosis creates new vesicles, exocytosis gets rid of them by merging them with the cell membrane. Thus, the two processes also serve to balance each other.