Endo/exocytosis

Regulated Secretory pathway

Triggered by hormones or neurotransmitters

Constitutive secretory pathway

Pathway proceeds without need for stimulus

Endosome

Vesicles that sort and facilitate transport of internalized materials

rough ER function

Newly-synthesized proteins from rough ER are released as vesicles → Golgi body for further modification

Golgi Body Function

Protein modification, directs newly-synthesized proteins to appropriate subcellular destinations

Cis face of Golgi body

Faces rough ER

Trans face of golgi body

faces plasma membrane

Pathway A

ribosomes (with ER signal sequence) → rER → Golgi body → vesicles → exocytosis

Example of pathway A

Insulin release from pancreatic beta cells

Pathway B

Free Ribosomes in cytoplasm (without ER signal sequence) → uptake by mitochondria/nucleus/peroxisome

Phagocytosis

Ingestion of solids in phagocytes

Pinocytosis

Ingestion of liquids

Receptor-mediated endocytosis

Endocytosis in response to a macromolecule binding to receptors of the plasma membrane

Proteins responsible in receptor-mediated endocytosis

Adaptor proteins and clarithins

Role of clarithins and adaptor proteins in endocytosis

Form a coated pit around the macromolecule to form a coated vesicle

Clathrin structure

3-legged structure (Triskeleton)

Importance of clathrin-coated vesicle

Only ligand-receptor complexes bound to adaptors/clathrin triskeletons are internalized

Internalization of LDL in liver

LDL binds to receptors, internalized by clathrin-coated pit → coated vesicle

Fate of coated vesicle in endocytosis

Depolymerizes to form early endosome, clathrin de-coat from vesicle and recycled

Fate of early endosome in endocytosis

Forms late endosome → low pH causes receptors to dissociate and return to cell surface → fuses with lysosome to degrade contents