1. Endoplasmic Reticulum (ER): This is a network of interconnected membranes that serves as a factory for protein synthesis and folding, and lipid synthesis. Many substances are synthesized within the ER.
2. Golgi Apparatus: This is a stack of flattened sacs that act as a "packaging and shipping" center. It modifies, sorts, and packages proteins and lipids for export.
3. Transport Vesicles: These small, membrane-bound sacs bud off from the Golgi apparatus and carry the packaged substances to the cell membrane.
4. Cell Membrane: This is the outer boundary of the cell. It acts as a selective barrier, controlling what enters and exits the cell. The substances, now enclosed within vesicles, fuse with the cell membrane, releasing their contents outside the cell.
So, the journey of an exported substance typically follows this path:
ER → Golgi Apparatus → Transport Vesicle → Cell Membrane
Note: Some substances may be secreted directly from the ER without passing through the Golgi apparatus.
