The DNA in a cell's nucleus encodes proteins that are eventually targeted to every membrane and compartment in the cell, as well as proteins that are targeted for secretion from the cell.
For example, consider these two proteins:
1. Phosphofructokinase (PFK) is an enzyme that functions in the cytoplasm during
glycolysis.
2. Insulin, a protein that regulates blood sugar levels, is secreted from specialized
pancreatic cells.

Assume that you can track the cellular locations of these two proteins from the time that translation is complete until the proteins reach their final destinations.

For each protein, identify its targeting pathway: the sequence of cellular locations in which the protein is found from when translation is complete until it reaches its final (functional) destination. (Note that if an organelle is listed in a pathway, the location implied is inside the organelle, not in the membrane that surrounds the organelle.)

Options:
Cytoplasm only, ER --> cytoplasm, ER --> Golgi --> outside cell, cytoplasm --> ER --> outside cell, Golgi --> ER --> outside cell, cytoplasm --> Golgi --> outside cell, nucleus --> cytoplasm, ER --> Golgi --> cytoplasm

The PFK protein has to function in the same cell in which it is produced. Such kinds of proteins are produced by the ribosomes which are dispersed in the cytoplasm. As they do not have to undergo any packaging or modification hence, they are produced by the ribosomes dispersed in the cytoplasm and are directly released in the cytoplasm for functioning.

The Protein, insulin, is made by the pancreatic cells and it has to be moved outside the cell for its functioning. This protein is made by the ribosomes of the Rough Endoplasmic Reticulum. From there, it is moved to the Golgi- complex where it is modified and packaged and then it is transported outside the cell.