Final answer:
The most likely pathway for an enzyme secreted by pancreatic cells is from the endoplasmic reticulum to the Golgi apparatus, and then in vesicles that fuse with the plasma membrane to release the protein outside the cell via exocytosis.
Step-by-step explanation:
The most likely pathway for an enzyme secreted by pancreatic cells that has been tracked using a radioactive label is: ER → Golgi → vesicles that fuse with plasma membrane.
Explanation in 200 words: The pathway of a secretory protein such as an enzyme from the pancreas typically starts with its synthesis on the rough endoplasmic reticulum (RER). After initial synthesis, the protein is encapsulated into vesicles that bud off from the RER and move towards the Golgi apparatus. At the Golgi apparatus, these proteins are further modified, tagged, and sorted. The subsequent packaging into vesicles occurs, which then travel to the cell's surface. These vesicles fuse with the plasma membrane to release the protein outside of the cell, a process known as exocytosis. This is the classic secretory pathway for proteins that need to be exported from the cell.
Contrastingly, proteins that are destined for lysosomes follow a slightly different route. These proteins are also synthesized in the RER and then sent to the Golgi apparatus. However, post-modification, they are packaged into vesicles that become lysosomes. These lysosomes contain hydrolytic enzymes and do not typically secrete their contents outside the cell but rather, are involved in intracellular digestion.