Internalised Material Fate-Recycling

• The early endosome is the major sorting point in the endocytic pathway. From here the majority of  receptors are recycled back to the domain of the plasma membrane from which they came from origionally.

LDL Receptors

This is the pathway that the LDL receptor follows. After LDL has been transported to the early endosome, the acidic pH of the endosome causes LDL to dissociate from its receptor. The acidic pH is achieved through a pump, which pumps hydrogen (H+) ions into the endosome. The LDL receptor is recycled back to the plasma membrane in recycling vesicles which bud from early endosome. Recycling vesicles bud off the tubular part of the endosome. It is also possible for tubular parts of the early endosome to fuse and form recycling endosomes. These also deliver receptors back to the plasma membrane.

Transferrin Receptors

Transferrin is a soluble protein which carries iron around the body.The transferrin recptor is also recycled, but in contrast to the LDL receptors which degrade their ligand LDL, the transferrin receptor also recycles its transferrin. In the early endosome, transferrin releases its iron in response to the acidic pH encountered. Apotransferrin remains bound to its receptor and both are recycled back to the plasma membrane. As the neutral pH at the plasma membrane is encountered, transferrin dissociates from its receptor and will now participate in furthr internalisation cycles.

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