Sunday, 9 November 2008

cell biology - Does endo- and (or) exocytosis require energy? Do they belong to active / passive transport?

Just to extend the answer from @Amory slightly, I think that the terms active and passive transport are best kept for describing transmembrane movement of molecules.



In the case of exocytosis the only transmembrane event is when a secreted protein is first inserted (usually cotranslationally) across the endoplasmic reticulum membrane. I'm not aware of any evidence that this uses more energy than that already expended during polpeptide elongation by the ribosome. At that point the secreted protein is topologically extracytoplasmic, and everything else is achieved by rounds of vesicle formation and fusion of vesicles with target membranes.



The same is true in reverse for endocytosis. Any molecule that is internalised in an endocytic vesicle is still extracytoplasmic unless some process specifically moves it across the membrane of the vesicle, or a downstream organelle such as the endosome. At that point whether the transport process was active or passive would depend upon the properties of the carrier system.

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