Golgi apparatus: modification and sorting of proteins
The Golgi apparatus, a stack of flattened membrane-bound sacs, modifies proteins from the rough endoplasmic reticulum (the ribosome-studded network of membranes), then sorts them for delivery.
Remember
Think of the Golgi as a sorting office for proteins: they arrive from the rough endoplasmic reticulum in vesicles (small membrane-bound bubbles), are modified and tagged, then sent on to their destinations.
Memory aid
Cis is In and Trans is Transport out.
The cis face is the entry side of the Golgi, and the trans face is the exit side.
The Golgi apparatus is the cell's sorting office for proteins. Proteins made on the rough endoplasmic reticulum reach it inside vesicles, which fuse with the entry side of the stack. The Golgi is a stack of flattened sacs called cisternae, and the stack has a direction: material enters on one face and leaves from the other.
The entry side is the cis face, the forming side nearest the rough endoplasmic reticulum. As proteins pass through the stack they are modified, most often by the addition of short sugar chains, a change called glycosylation, and they are tagged so that they reach the right place.
At the exit side, the trans face, signals on each protein are read and the proteins are directed into separate vesicles with different destinations. The destinations may be lysosomes (the cell's digestive compartments), the plasma membrane, or secretion outside the cell.
Lysosomal enzymes (digestive enzymes that work in acid) show why tagging matters. In the Golgi they are given a tag called mannose 6-phosphate. Receptors for this tag capture the tagged enzymes, which are then transported to the lysosomes. If the tagging step fails, the enzymes are not delivered to lysosomes, so the lysosomes are short of the enzymes they need. This is the picture in I-cell disease (mucolipidosis II), in which dozens of lysosomal enzymes are deficient inside the cell.
The Golgi works on proteins after they have been made, and it does not digest anything itself.
Key facts
- Structure
- a stack of flattened sacs (cisternae) with a direction, so cargo enters on one face and leaves from the other.
- Entry side
- the cis face, nearest the rough endoplasmic reticulum, where vesicles arrive.
- Modification
- proteins are modified as they pass through, most often by adding short sugar chains (glycosylation), and are tagged for the right place.
- Exit side
- the trans face, where proteins are sorted and packaged into vesicles.
- Destinations
- lysosomes, the plasma membrane, or secretion outside the cell.
- Lysosomal tag
- mannose 6-phosphate is added to lysosomal enzymes in the Golgi, and its receptors carry the tagged enzymes to lysosomes.
Exam traps
- Making ATP by oxidative phosphorylation (the main ATP-making process, using oxygen) does not belong to the Golgi apparatus, because it takes place on the inner membrane of the mitochondria.
- Digesting worn-out material is the job of lysosomes, the cell's digestive compartments, not of the Golgi apparatus: the Golgi helps to assemble lysosomes by sending them their enzymes, but it does not carry out the digestion itself.
- Phospholipids are made mainly in the smooth endoplasmic reticulum, the membrane network without ribosomes, so the Golgi receives and finishes lipid products but is not where they are first made.
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