1. The Endoplasmic Reticulum (ER): This is the primary source. Proteins synthesized by ribosomes attached to the ER are translocated into the ER lumen (interior space) as they are being made. After folding and modification within the ER, these proteins are packaged into transport vesicles and sent to the Golgi apparatus.
2. The Cytosol: While less common, some proteins synthesized by free ribosomes in the cytosol can also enter the Golgi apparatus. These proteins are often destined for specific organelles like lysosomes or vacuoles, and they need to be sorted and packaged within the Golgi.
Here's a breakdown of the process:
1. Synthesis and Translocation: Ribosomes attached to the ER synthesize proteins that are targeted for the Golgi. These proteins are translocated into the ER lumen as they are being made, where they undergo folding and modifications.
2. ER to Golgi Transport: Proteins are packaged into transport vesicles that bud off from the ER membrane. These vesicles travel to the Golgi apparatus and fuse with its membranes.
3. Golgi Processing: Once inside the Golgi, proteins undergo further modifications, such as glycosylation (addition of sugar molecules), phosphorylation (addition of phosphate groups), and sorting. These modifications determine the protein's final destination and function.
4. Golgi to Destination: Proteins are packaged into new transport vesicles that bud off from the Golgi. These vesicles carry proteins to their final destination, which can be:
* The plasma membrane: for secretion outside the cell
* Lysosomes: for degradation
* Other organelles: like vacuoles or endosomes
In summary, the Golgi apparatus receives proteins primarily from the ER, where they are synthesized and undergo initial modifications. Some proteins from the cytosol can also enter the Golgi for further processing and sorting.
