_{I. Endocytosis: Pathway: endocytic vesicle......early endosome......>late endosome.....>lysosome (review this by yourself)}

• _{Types of endocytosis:}
• _{1) phagocytosis (cellular eating): triggerd process}
• _{2) pinocytosis (cellular drinking): constitutive}
• _{A)phagocytosis by macrophages}
• _{Mechanism: e.g. Antibody mediated: Fc regions and Fc receptors}
• _{B) Pinocytosis: two types}
• _{1) constitutive and non-selective}
• _{2) constitutive but selective: receptor-mediated endocytosis}
• _{II) Significance of receptor-mediated endocytosis}
• _{a) selectivity of the molecules ingested}
• _{b) a mechanism of concentrating the endocytosed molecules}
• _{III) Mechanism of receptor-mediated endocytosis}

• _{Example (best studied): receptor-mediated endocytosis of LDL (cholesterol)}
• _{Structure of LDL particle: phospholipid monolayer, cholesterol esters, apo B protein}
• _{A)The Receptor:}

• _{-single-pass TMP}
• _{-N-terminal binds to LDL (extracellular domain)}
• _{-C-terminal binds to coated pit (cytosolic domain)}
• _{B) Genetic disorder: Familial Hypercholesterolemia (FH)}
• _{Problem: too much cholesterol in the blood....>atherosclerosis....>heart attack}

_{Reason: no LDL receptor or defective LDL receptor: LDL is not internalized}

• IV. Exocytosis: two pathways (see Figure 13-54; 13-55)
• 1) constitutive secretory pathway
• 2)regualted secretory pathway: needs signal
• A)Mechanism of sorting for constitutive and regulated pathways
• Question: How are the two different types of secretory proteins, all of which are soluble in the lumen of Golgi complex, sorted to the correct type of secretory vesicle?
• Hypothesis: protein sorting to regulated secretory vesicle is controlled by selective protein aggregation.
• B) Regulated pathway:
• Signal: is usually an extracellular chemical messenger (hormone)mediated increase in [Ca⁺⁺]_i
• _{Examples of regulated secretion: neurotransmitter release (Fig. 13-64), histamine release by mast cells (figure 13-60), cortical granule exocytosis in egg at fertilization, hormone release, etc.}
• _{Membrane recycling: exocytosis delivers new membranes and membrane proteins to the PM; endocytosis retrieves it (recycling)}