The endomembrane system links several of a cell's organelles, the endoplasmic reticulum, the Golgi apparatus, lysosomes and the cell's own outer membrane, into a single connected production and delivery network. A newly made protein destined to leave the cell entirely or to embed itself in the cell membrane typically moves through that network in a specific, largely one-way sequence, rather than simply diffusing freely through the cell's interior until it happens to end up in the right place.
Each organelle in the network has its own specific role in the pipeline
The rough endoplasmic reticulum, studded with ribosomes, is where a targeted protein is actually synthesised and begins its initial folding, while the smooth endoplasmic reticulum, lacking ribosomes, specialises instead in lipid synthesis. The Golgi apparatus receives proteins arriving from the endoplasmic reticulum and further modifies, sorts and packages them, attaching the specific molecular tags that determine where each individual protein is ultimately headed. Small membrane-bound vesicles physically shuttle material between each of these compartments in turn, carrying cargo rather than letting it drift freely through the cell's cytoplasm.
A protein's route through this system is a directional pipeline, not random diffusion
A protein destined for secretion or for the cell membrane typically follows a specific sequence: synthesis and initial folding in the rough endoplasmic reticulum's lumen, vesicle transport to the Golgi apparatus for further modification and sorting, then a final vesicle carrying it to its actual destination, the cell membrane itself or release outside the cell. Specific molecular sorting signals attached to each protein direct it correctly at each step, which is exactly what keeps the whole system's traffic organised despite an enormous number of different proteins moving through the same shared compartments simultaneously.
What we're still unsure about
That the endomembrane system's organelles are functionally connected, and that secreted and membrane-bound proteins follow a broadly directional route through them, are well established, extensively documented cell biology. What's more genuinely an active area of ongoing research is exactly how the sorting machinery reliably keeps each individual protein's specific destination separate from every other protein's moving through the same crowded compartments at the same moment, and how errors in that sorting process, a protein ending up at the wrong destination, get caught and prevented from accumulating, remains a genuinely open question researchers are still actively working out rather than a fully mapped mechanism.
This sits inside The Endomembrane System, one of seven topics in Cell Biology, one of six domains in Biology, one of seventeen subjects the app can quiz you on.