A Closer Look at Cell Membranes  Aim: How do large particles enter and exit cells?  Do Now: Name some molecules/materials that enter and exit the cell. How would you describe the cell membrane that allows passage of these materials?

Exocytosis and Endocytosis  Exocytosis (out of the cell) The fusion of a vesicle with the cell membrane, releasing its contents to the surroundings  Endocytosis (into the cell) The formation of a vesicle from cell membrane, enclosing materials near the cell surface and bringing them into the cell

Endocytosis and Exocytosis Examples

Three Pathways of Endocytosis  Bulk-phase endocytosis Extracellular fluid is captured in a vesicle and brought into the cell; the reverse of exocytosis  Receptor-mediated endocytosis Specific molecules bind to surface receptors, which are then enclosed in an endocytic vesicle  Phagocytosis Pseudopods engulf target particle and merge as a vesicle, which fuses with a lysosome in the cell

Phagocytosis (“engulfment”)

Membrane Cycling  Exocytosis and endocytosis continually replace and withdraw patches of the plasma membrane  New membrane proteins and lipids are made in the ER, modified in Golgi bodies, and form vesicles that fuse with plasma membrane

Exocytic Vesicle

5.5 Key Concepts: Membrane Trafficking  Large packets of substances and engulfed cells move across the plasma membrane by processes of endocytosis and exocytosis  Membrane lipids and proteins move to and from the plasma membrane during these processes