A Closer Look at Cell Membranes
Chapter 5 Part 2
5.5 Membrane Trafficking
 By processes of endocytosis and exocytosis,
vesicles help cells take in and expel particles
that are too big for transport proteins, as well as
substances in bulk
 Membrane trafficking
• Formation and movement of vesicles formed from
membranes, involving motor proteins and ATP
Exocytosis and Endocytosis
 Exocytosis
• The fusion of a vesicle with the cell membrane,
releasing its contents to the surroundings
 Endocytosis
• The formation of a vesicle from cell membrane,
enclosing materials near the cell surface and
bringing them into the cell
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
Receptor-Mediated Endocytosis
A Pseudopods surround
a pathogen ( brown ).
Fig. 5-14a, p. 87
B Endocytic
vesicle forms.
C Lysosome fuses
with vesicle;
enzymes digest
pathogen.
D Cell uses the
digested material
or expels it.
Fig. 5-14b, p. 87
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
Endocytosis
Exocytosis
A Molecules get
concentrated
inside coated pits
at the plasma
membrane.
D Many of
the sorted
molecules
cycle to the
plasma
membrane .
coated pit
B The pits
sink inward
and become
endocytic
vesicles.
E Some vesicles
are routed to the
nuclear envelope
or ER membrane.
Others fuse with
Golgi bodies.
C Vesicle
contents
are sorted.
F Some vesicles
and their
contents are
delivered to
lysosomes.
lysosome
Golgi
Stepped Art
Fig. 5-12, p. 86
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
5.6 Which Way Will Water Move?
 Water diffuses across cell membranes by
osmosis
 Osmosis is driven by tonicity, and is countered
by turgor
Osmosis
 Osmosis
• The movement of water down its concentration
gradient – through a selectively permeable
membrane from a region of lower solute
concentration (more water) to a region of higher
solute concentration (less water)
 Tonicity
• The relative concentrations of solutes in two fluids
separated by a selectively permeable membrane
Tonicity
 For two fluids separated by a semipermeable
membrane, the one with lower solute
concentration is hypotonic, and the one with
higher solute concentration is hypertonic
• Water diffuses from hypotonic to hypertonic
 Isotonic fluids have the same solute
concentration
Osmosis
Fig. 5-17a, p. 89
Fig. 5-17 (b-d), p. 89
Effects of Fluid Pressure
 Hydrostatic pressure (turgor)
• The pressure exerted by a volume of fluid against
a surrounding structure (membrane, tube, or cell
wall) which resists volume change
 Osmotic pressure
• The amount of hydrostatic pressure that can stop
water from diffusing into cytoplasmic fluid or other
hypertonic solutions
Hydrostatic Pressure in Plants
5.6 Key Concepts:
Osmosis
 Water tends to diffuse across selectively
permeable membranes, to regions where its
concentration is lower