endomembrane system

Meaning: The endomembrane system is the set of eukaryotic compartments that exchange membrane and contents directly or by vesicles: nuclear envelope, rough and smooth ER, Golgi apparatus, lysosomes, and transport vesicles. Many textbooks also count the plasma membrane, because vesicles fuse with it. Proteins that will be secreted, placed in the plasma membrane, or sent to lysosomes are made on ribosomes bound to the rough ER. A signal sequence directs them into the ER, where they are folded and glycosylated. Vesicles carry them to the Golgi (cis to trans) for modification and sorting, and further vesicles deliver them to lysosomes or to the cell surface by exocytosis. The lumen of each compartment is topologically equivalent to the outside of the cell.

On the MCAT: Passages trace a protein's route and ask what happens if one step is blocked. For example, a protein that cannot leave the ER accumulates there, and a missing signal sequence leaves the protein on free ribosomes in the cytosol. Questions also contrast secreted, membrane, and lysosomal proteins with cytosolic ones.

Common confusion: Mitochondria and chloroplasts are not part of the system. They arose by endosymbiosis, have their own DNA and ribosomes, and do not bud from the ER or Golgi. Most of their proteins are made on free ribosomes and imported.