Final answer:
The GTPase Sar1 is involved in exocytosis, specifically in the formation of COPII-coated vesicles transporting proteins from the ER to the Golgi, while GTPase ARF is involved in creating vesicles for endocytosis.
Step-by-step explanation:
The process that involves the GTPase Sar1 is associated with the transport of proteins from the endoplasmic reticulum (ER) to the Golgi apparatus, known as COPII-mediated vesicular transport. This is a key step in the secretory pathway of eukaryotic cells. On the other hand, the GTPase ARF (ADP-ribosylation factor) is involved in the formation of coated vesicles like clathrin-coated vesicles, which are important for trafficking proteins in both exocytic and endocytic pathways. In light of this information, the correct answer to the student's question is:
b. Sar1 in exocytosis, ARF in endocytosis
Sar1 is involved in the exocytosis process where proteins are packaged in vesicles at the ER and then transported to the Golgi apparatus for further processing and sorting. ARF plays a role in the formation of vesicles that mediate endocytosis, which is the process of internalizing substances into the cell.