A eukaryotic cell resembles a big city with several districts. Each of them is specialized in a particular role, such as energy production, manufacturing products, exportation, importation, communication with other cities, recycling, and so on. To work properly, a rich and complex communication system is needed between districts. This system is composed of carriers that go through a variety of pathways.
Cellular districts are the intracellular compartments, and many of them are membrane-bound compartments, i.e., organelles. Each organelle is specialized in one or several functions. For instance, the endoplasmic reticulum is in charge of synthesizing proteins for secretion and lipids for making membranes, the Golgi apparatus produces carbohydrates to be attached to proteins and lipids or released to the extracellular milieu, lysosomes are the main digestion centers, mitochondria and chloroplasts synthesize ATP, lipid droplets are energy storage centers, and so on.
Communication between many membrane bound organelles is mediated by vesicles, which carry molecules both in the interior and as part of the membrane of the vesicle. Vesicular trafficking includes all the communication mediated by vesicles (Figure 1).
There are two main roads on this trafficking road map. One, known as the secretory pathway, starts in the endoplasmic reticulum, that sends vesicles to the Golgi apparatus, which in turn sends vesicles targeted to the plasma membrane (exocytosis). This pathway releases molecules into the extracellular space and also carries molecules to the plasma membrane. The other pathway is an importing pathway that begins at the plasma membrane, where vesicles and other large compartments are formed after membrane invagination (endocytosis). These vesicles fuse with the endosomes, and endosomes fuse with lysosomes. Lysosomes degrade the endocyted molecules, both those fetched from the extracellular space and those forming the membrane of the vesicles. It is a degradation pathway. There are many other communication pathways mediated by vesicles, so it looks like that all organelles are connected to one another by vesicles. Furthermore, there seems to be a rule: communication by vesicles between two organelles is bidirectional, i.e., the organelle A sends vesicles to the organelle B, and at the same time the organelle A receives vesicles from the organelle B. For instance, the endoplasmic reticulum sends vesicles to the Golgi apparatus, which in turn sends vesicles back to the endoplasmic reticulum. The same happens between the plasma membrane and endosomes and between the Golgi apparatus and endosomes.
Vesicular trafficking is a way of transporting material to the degradation centers and for the secretion of molecules. However, molecules transported by vesicles perform other functions too. For instance, vesicles carry degradative enzymes to the lysosomes, the glucosidases to the Golgi apparatus, and also receptors to the plasma membrane. Thus, the vesicular trafficking is needed for the organelles in order to carry out their specific functions. The identity of the organelles is set by molecules, proteins and lipids, mainly found in their membranes, which arrive at the organelles as part of the incoming vesicle membranes. Organelle identity is important because a vesicle coming from the endoplasmic reticulum has to be fused with the Golgi apparatus but not with an endosome, and this recognition is done through membrane proteins.
There are organelles, such as mitochondria, chloroplasts and peroxisomes, that are not commonly included as part of vesicular trafficking. Although they can send vesicles, they do not seem to play a major role in vesicular trafficking. However, these organelles communicate with other organelles by other means. For example, by membrane contact sites. It is frequently observed that the mitochondrial external membrane is in close apposition to the endoplasmic reticulum membrane. Some authors propose that there is a high transfer of molecules, mostly lipids, through these areas of membrane contacts.