Proteins destined for export from the cell are synthesized with a signal sequence and begin their synthesis on free ribosomes in the cytosol.

Here's why:

* Signal Sequence: This short sequence of amino acids acts like a "zip code" directing the protein to the endoplasmic reticulum (ER), the cellular organelle responsible for protein folding and export.

* Free Ribosomes: Protein synthesis starts on free ribosomes, which float in the cytoplasm.

* ER Targeting: Once the signal sequence emerges from the ribosome, it binds to a signal recognition particle (SRP). This complex then binds to a receptor on the ER membrane, directing the ribosome to dock with the ER.

* Translocation: The ribosome moves to a protein translocator channel in the ER membrane, and the growing polypeptide chain enters the ER lumen (interior).

* Signal Sequence Removal: Once the polypeptide is fully translocated, the signal sequence is usually cleaved off by a signal peptidase enzyme.

In summary:

1. Protein synthesis begins on free ribosomes.

2. The signal sequence emerges and binds to the SRP.

3. The complex binds to the ER membrane.

4. The ribosome docks and the polypeptide enters the ER lumen.

5. The signal sequence is removed.

Let me know if you'd like more details on any specific aspect of this process!