In this video, we're going to introduce the third organelle of the endomembrane system, which is the Golgi apparatus. It's important to note that molecules synthesized or built in the endoplasmic reticulum (ER), such as proteins built in the rough ER and lipids built in the smooth ER, are transported by vesicles (little membrane bubbles) to the Golgi apparatus. The Golgi apparatus is essentially a stack of flat membranous sacs, and being membranous, it is part of the endomembrane system. These flat membranous sacs are specifically referred to as cisternae. The term cisternae refers to the flat membranous sacs that make up the Golgi apparatus.
The Golgi apparatus functions as a processing center. It receives vesicles, modifies the contents of those vesicles, then repackages and distributes the modified contents to different parts of the cell or outside the cell. The Golgi apparatus has a receiving end, called the cis end, and a shipping end, called the trans end. Some vesicles are shipped from the Golgi apparatus and fuse with the cell membrane, allowing for the secretion of their contents. Secretion, in this context, is the release of substances into the environment of the cell. This explains why the Golgi apparatus is one of the final organelles involved in protein secretion.
If we take a look at our image below, we can get a better understanding of the Golgi apparatus. On the left-hand side, we are showing you an image from our previous lesson videos. You can see the nucleus of the cell here, surrounded by the endoplasmic reticulum, the rough ER and the smooth ER. In this image, we are introducing the Golgi apparatus. On the right-hand side, we are showing you a bird's-eye view, a different vantage point of the same organelles. You'll notice the nucleus of the cell and the rough ER, studded with ribosomes, which make proteins that end up in the lumen of the rough ER and then in vesicles that transport proteins to the Golgi apparatus.
The Golgi apparatus has a receiving end, where it receives packages called the cis end or the cis face of the Golgi, and these packages are modified throughout the Golgi apparatus until they reach the trans end or the trans face of the Golgi. This is the shipping end where the modified contents are repackaged into vesicles. These vesicles can either be distributed to different locations within the cell or travel to the cell membrane where they fuse with the cell membrane and release their contents. Each of these flat membranous sacs is named cisterna. This is the pathway for protein secretion: Proteins are made by ribosomes in the rough ER, go to vesicles, then to the cis face of the Golgi, be modified in the Golgi, go to the trans face of the Golgi, be repackaged into vesicles, and then make their way to the cell membrane where those proteins are secreted.
This concludes our brief introduction to the Golgi apparatus, and we'll be able to get some practice applying these concepts as we move forward. I'll see you all in our next video.