“The Golgi apparatus or Golgi complex is an organelle essential in the process of sorting, packaging, and delivering proteins to their final destination in or out of an eukaryotic cell. The organelle is made up by stack of flattened cisternae. The number and size of stacks and cisternae varies from cell to cell depending on the cell type and metabolic status. Each cisterna is membrane-bound and contains enzymes included in protein modification.

The cisterna closest to the endoplasmic reticulum (ER), so called cis, represents the first step of the journey that protein takes in order to reach its destination. Proteins, synthesized into the ER, then travel through the organelle, enveloped in a vesicle, to the cisterna facing the plasma membrane (trans cisterna). Passing through cisternae, proteins are subject to constant modifications. These modifications include glycosylation, sulfation and phosphorylation.

At the end, a vesicle carrying the proteins buds off from the trans cisterna and goes to its final destination including lysosomes, the plasma membrane, secretory vesicles, or out of the cell. The sorting of the vesicle is based on specific protein markers. For example, lysosomal enzymes are marked with mannose-6-phosphate.

In order to explain the movement of proteins through the Golgi complex scientists proposed two models: the vesicular transport model and cisternal maturation model. According to the former, vesicles budding off from cis-cisterna fuse with the next cisterna. The process repeats until the vesicles fuse with trans-cisterna. The latter model assumes that cis-cisterna is continually formed by the fusion of vesicles from the ER. Moving through the Golgi apparatus, the cisterna arrives to the trans side and breaks up. Meanwhile, new cis-cisterna is built.
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