Secretory Proteins

Proteins for use in membranes or for secretion from the cell are transported through the cell within the endoplasmic reticulum. These proteins are produced on ribosomes on the outside surface of the endoplasmic reticulum and then pass into the lumen of the endoplasmic reticulum via small membrane channels. They eventually pass through the Golgi apparatus and reach the cell surface via membrane-bound vesicles. Phospholipid molecules are manufactured in the smooth endoplasmic reticulum (among other places) and are often added to the membranes of the endoplasmic reticulum at that time. These phospholipids reach the cell surface by contributing to the membranes forming the transport vesicles carrying proteins and other substances from the endoplasmic reticulum, to and between the layers of the Golgi apparatus, and then to the plasma membrane of the cell. Proteins destined for use in the cytosol are synthesized on free ribosomes.

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