Final answer:
Proteins entering the cis Golgi network can either move backward to the ER or move forward through the Golgi, where they are modified, sorted, and eventually packaged into vesicles. Some vesicles deliver their contents internally, while secretory vesicles fuse with the plasma membrane to release their contents outside the cell.
Step-by-step explanation:
When proteins enter the cis Golgi network, they undergo a sorting process to determine their final destination within the cell. One path proteins can take is to move backward through the Golgi stack, which means they can be returned to the endoplasmic reticulum (ER) if needed. Alternatively, proteins may move onward through the Golgi stack, undergoing further processing such as glycosylation and phosphorylation. These modifications often include the addition of carbohydrate or phosphate groups, which act as sorting signals. For instance, mannose-6-phosphate is a signal that directs proteins to lysosomes. Ultimately, the proteins reach the trans Golgi network, where they are packaged into vesicles. These vesicles can either deposit their content into other parts of the cell or they can become secretory vesicles that fuse with the plasma membrane, releasing their contents outside the cell.