ID #2462

I'm still a little puzzled between early, late endosomes and lysosomes. Are these different structures or does it go through stages where it morphs from an early all the way to a lysosome? Does an early endosome collect endocytic vessicles or is it formed from a merging of them? does an early endosome include the receptors or is the early endosome just the material that is going to be degraded after sending off the receptors to the recyling endosome?

Early endosomes are located near the plasma membrane. They have ATP-dependent proton pumps so the pH begins to drop. They are also irregularly shaped due to the constant flow of traffic to them (from the plasma membrane) and from them (to recycling endosomes). Late endosomes have a lower pH because protons are continuing to be pumped. In addition, acid hydrolases are delivered to late endosomes (and M6P receptors are recycled from them back to the trans face of the Golgi. Some degradation of material can occur in a late endosome, but not at the highest levels. Mature lysosomes have the lowest pH, so optimal degradation can occur. They also lack the M6P receptors since they have already been recycled. An early endosome can mature into a late endosome, or vesicles containing material that needs to be degraded can bud off of an early endosome and fuse with an existing late endosome. Thus, material that needs to be degraded can reach a late endosome in multiple ways. 
An early endosome likely forms by the fusing of endocytic vesicles, and then it maintains its structure through the constant traffic of vesicles from and to the plasma membrane. Receptors that recognize signal molecules or other extracellular molecules will be part of an early endosome briefly when the material that needs to be degraded is dropped off, and then the receptors are sent to a recycling endosome before being trafficked back to the plasma membrane. 

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