Vesicular Transport (Overview)
3 pathways of vesicular transport
- Secretory pathway: delivers cargo to the plasma membrane.
- Endocytic pathway: uptake cargo from the plasma membrane.
- Retrieval pathway: recycles cellular molecules.
Key facts about vesicular transport:
- Compartment lumens mix via the transport intermediate.
- The membrane of each vesicle maintains its orientation.
- If the cell is growing, the secretory pathway is more active than the endocytic pathway.
Steps in secretory pathway
- Transport vesicles bud from the ER and carry content away from it to cis side of Golgi.
- Vesicular budding and fusion mediates the transport of cargo through the Golgi stacks, from cis to trans side.
- Cargo exits the Golgi via a transport vesicle on trans side.
- Transport vesicles fuse with plasma membrane or with endosomes (and then lysosomes).
Steps in endocytotic pathway
- Early endosome forms from plasma membrane and extracellular materials.
- Early endosome targets cargo to late endosomes.
- Late endosomes then deliver cargo to lysosomes, which degrade cargo.
The retrieval pathway takes several forms
- Endosomes can return cargo to the cell surface via recycling endosomes.
- Cargo in early and late endosomes can also return to the Golgi for reuse.
- Vesicles can deliver proteins from the trans face to the cis face of the Golgi.
- Vesicles can return proteins from the golgi to the ER as well.
3 steps of vesicular formation
- Cargo selection. Incorporation of cargo into a vesicle is carefully regulated to ensure that only the correct cargo gets transported.
- Vesicular budding. deformation of the hydrophobic membrane bilayer and breaking off of the membrane into a vesicle
- Vesicular targeting and fusion. Highly regulated just like cargo selection.
Cellular compartments are topologically equivalent when:
• Molecules can get from one to another without having to cross a membrane.
• Nuclear envelope, ER, Golgi, transport vesicles, endosomes, lysosomes, and extracellular space = topologically equivalent