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