File:The Process of Autophagy.png

English: In a healthy cell, autophagy is a process that is continually occurring, but conditions within the body, such as low levels of nutrients, growth factors, or oxygen, send signals into the cell that speed up the assembly of autophagosomes. This assembly begins with a collection of protein and lipid fragments in the cellular cytoplasm forming a double-layered membrane sheet. This sheet then curves via the addition of a new double-layered membrane sheet, leaving an open space, taking an almost spherical shape with a gaping mouth much like Pac-man. This Pac-man shaped globule is known as a phagophore. When Pac-man eats snack pellets or when this phagophore engulfs damaged pieces of proteins or even damaged organelles such as mitochondria along with some cytoplasm, it closes its opening, encapsulating its contents and shedding membrane proteins that are recycled back into the cytoplasm. This enclosed form with its damaged cellular contents is then called an autophagosome. This autophagosome then transports its contents to a lysosome. The outer membrane of the autophagosome fuses with the lysosome forming an autolysosome, after which the lysosomal enzymes chomp through the autophagosome’s inner membrane where the collected cargo sits exposed, ready to be digested. Then the lysosomal enzymes break down the entire cargo, including proteins and macromolecules, delivering back into the cytoplasm basic biochemical building blocks. These basic building blocks, including many amino acids, serve as nutrients for the cell, which will fuel cellular functions. As mentioned earlier, autophagosomes will engulf malfunctioning organelles such as mitochondria, which may trigger untimely apoptosis or cell death if left in the cell’s cytoplasm. However, if the organelle damage is too severe for autophagy to clean up, then the cell must be removed from the organism to maintain overall health. The cell then may employ cell death type II or autophagy mediated cell death, which can occur in two ways, continued digestion until the cell dies or autophagy stimulated apoptosis. Many researchers regard autophagy as a fail-safe, which will act if apoptosis does not function properly and when necessary. Research has also led experts to hypothesize that the processes of apoptosis and autophagy communicate and may even receive information from the same signaling proteins. Researchers have found that a protein that signals the initiation of autophagy, named Beclin 1, actually binds to Bcl-2, a protein that prevents apoptosis from beginning. Decisions about whether the cell lives or dies can then be a result of the binding or unbinding of these two proteins. Therefore, if apoptosis fails, then autophagy takes over, causing an organized cell death instead of a harmful kind of cell death known as necrosis. Another proof of the connection between autophagy and apoptosis and one way that autophagy may take over the decision to cause cell death was found with the discovery of a protein fragment named Atg5. Atg5 is a protein that is utilized in the formation of autophagosomes, which can insert itself into mitochondria and there, it can convert its usual autophagic response to one that is apoptotic.