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