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

Endocytosis and
Exocytosis

Endocytosis
A Molecules get concentrated inside coated pits at the plasma membrane.
Exocytosis coated pit
B The pits sink inward and become endocytic vesicles.
C Vesicle contents are sorted.
F Some vesicles and their contents are delivered to lysosomes.
lysosome
D Many of the sorted molecules cycle to the plasma membrane .
Golgi
E Some vesicles are routed to the nuclear envelope or ER membrane.
Others fuse with
Golgi bodies.
Fig. 5-12, p. 86

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

plasma membrane aggregated lipoproteins
Fig. 5-13, p. 86

Animation: Phagocytosis

Phagocytosis

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

Exocytic Vesicle

Endocytosis
A Molecules get concentrated inside coated pits at the plasma membrane.
coated pit
B The pits sink inward and become endocytic vesicles.
C Vesicle contents are sorted.
Exocytosis
D Many of the sorted molecules cycle to the plasma membrane .
E Some vesicles are routed to the nuclear envelope or ER membrane.
Others fuse with
Golgi bodies.
F Some vesicles and their contents are delivered to lysosomes.
lysosome
Golgi
Stepped Art
Fig. 5-12, p. 86

Animation: Membrane cycling

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 to a region of higher solute concentration
 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

hypotonic solution hypertonic solution solutions become isotonic selectively permeable membrane
A Initially, the volume of fluid is the same in the two compartments, but the solute concentration differs.
B The fluid volume in the two compartments changes as water follows its gradient and diffuses across the membrane.
Fig. 5-16, p. 88

Animation: Tonicity and water movement

Experiment:
Tonicity

Fig. 5-17a, p. 89

2% sucrose
2% sucrose 10% sucrose water
A What happens to a semipermeable membrane bag when it is immersed in an isotonic, a hypertonic, or a hypotonic solution?
Fig. 5-17a, p. 89

2% sucrose
2% sucrose 10% sucrose water
A What happens to a semipermeable membrane bag when it is immersed in an isotonic, a hypertonic, or a hypotonic solution?
B Red blood cells in an isotonic solution do not change in volume.
C Red blood cells in a hypertonic solution shrivel because water diffuses out of them.
D Red blood cells in a hypotonic solution swell because water diffuses into them.
Stepped Art
Fig. 5-17, p. 89

Fig. 5-17 (b-d), p. 89

B Red blood cells in an isotonic solution do not change in volume.
C Red blood cells in a hypertonic solution shrivel because water diffuses out of them.
D Red blood cells in a hypotonic solution swell because water diffuses into them.
Fig. 5-17 (b-d), p. 89

Animation: Osmosis experiment

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

Fig. 5-18a, p. 89

5.6 Key Concepts:
Osmosis
 Water tends to diffuse across selectively permeable membranes, to regions where its concentration is lower

Animation: Active transport

Animation: Endocytosis and exocytosis

Animation: Fluid mosaic model

Animation: Passive transport II

Animation: Solute concentration and osmosis

Video: One bad transporter and cystic fibrosis

Video: Diffusion of dye in water

Video: Contractile vacuole