The cell’s post office

Yes, the cell has a post office. It’s called the Golgi complex or Golgi apparatus after Camilo Golgi, the scientist who identified it.

So what does the Golgi complex do? Well, just like any post office, it receives packages, sorts them, checks their labels and delivers them to the proper destination. It also sends back packages that are defective. However, it is more than the ordinary post office. Why? Because aside from doing all those functions mentioned above, it can also modify the packages, insert more identifying markers, compact and repackage them, and even add some fancy gift tags and ribbons.

The packages I am talking about here are of course the products coming from the ER. If you remember from a previous post (Nov 23, rough ER), I mentioned that the Golgi together with the ER are involved in “membrane trafficking” wherein vesicles move between them and the other parts of the cell.

Yes, the Golgi also sends back to the ER packages that contain defective proteins. Aside from that, it can also insert identifying markers like mannose-6-phosphate, the marker for proteins destined to be part of the lysosome.

The Golgi is also involved in the synthesis of proteoglycans. As the name implies, these are molecules that contain protein and sugars. These molecules are very important components of the cell’s immediate surrounding or the extracellular matrix. Furthermore, the Golgi also adds sulfates and phosphates to various cell products. Sulfation and phosphorylation are important for signaling and sorting. All these add-ons are the fancy gift tags and ribbons that I mentioned.

So as you can see, the Golgi complex is the cell’s super post office.

Oh, and by the way, just like a post office, the Golgi also has a receiving window (its cis face) and a releasing window (its trans face).

rough ER

The presence of ribosomes attached to the cytoplasmic side of this membranous network gives it a rough appearance, thus the name. Its main function is protein synthesis.

Initially, the rough ER does not have any ribosomes yet. Synthesis actually starts in ribosomes that are freely floating in the cytoplasm. However, once the initial sequence of amino acids called the signal peptide, are formed, the ribosomes then attach to the ER and continue with the synthesis. Since different kinds of proteins are being synthesized regularly, a continuous stream of ribosomes attach to the ER thus making them practically integral parts of the ER.

Protein synthesis is a very systematic and closely monitored process. Proteins which are destined for use within the cell are processed separately from proteins that are “exported” out of the cell. Markers are added along the way as the synthesis of proteins progresses. The markers ensure that proteins reach their proper destinations.

The rough ER works closely with the Golgi apparatus. Between the two of them, there is a regular flow of proteins in vesicles. There is even a whole process of vesicular movement between the ER, the Golgi, and the cell membrane that is called “membrane trafficking”. Yes, there is traffic flow in cells, but unlike the chaotic traffic in our society, this cellular traffic is very well coordinated and there are no traffic snarls along the way. Maybe our traffic coordinators could learn a thing or two by observing how cells do their trafficking.