Which cellular structure do all proteins enter the secretory pathway through?

The secretory pathway for proteins begins in the endoplasmic reticulum (ER). When a protein is destined for secretion or for use in the cell membrane or organelles, it must first be synthesized in the rough ER, which is studded with ribosomes that translate mRNA into polypeptides. As proteins are synthesized, they are translocated into the lumen of the ER where they undergo folding and post-translational modifications, such as glycosylation. This process is critical for proper protein function.

Following their synthesis and processing in the ER, these proteins are packaged into vesicles and transported to the Golgi apparatus for further modification, sorting, and eventual delivery to their final destinations, which may include secretion outside the cell. Therefore, the endoplasmic reticulum is the key cellular structure through which all proteins enter the secretory pathway, making it the correct answer to the question.