1. A Closer Look at Cell Membranes
Chapter 5 Part 2

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

3. Exocytosis and Endocytosis
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

4. 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

5. Receptor-Mediated Endocytosis

6. A Pseudopods surround a pathogen ( brown ).
Figure 5.14
Phagocytosis. (a) Micrograph of a phagocytic cell with its pseudopods (the extending lobes of cytoplasm) surrounding a pathogen. (b–d) Diagram showing what happens inside a phagocytic cell after pseudopods (the extending lobes of cytoplasm) surround a pathogen. The plasma membrane above the bulging lobes fuses and forms an endocytic vesicle. Inside the cytoplasm, the vesicle fuses with a lysosome, which digests its contents.
Fig. 5-14a, p. 87

7. B Endocytic vesicle forms.
Figure 5.14
Phagocytosis. (a) Micrograph of a phagocytic cell with its pseudopods (the extending lobes of cytoplasm) surrounding a pathogen. (b–d) Diagram showing what happens inside a phagocytic cell after pseudopods (the extending lobes of cytoplasm) surround a pathogen. The plasma membrane above the bulging lobes fuses and forms an endocytic vesicle. Inside the cytoplasm, the vesicle fuses with a lysosome, which digests its contents.
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

8. 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

9. concentrated inside coated pits at the plasma membrane.
Endocytosis
A Molecules get
concentrated inside coated pits at the plasma membrane.
coated pit
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.
Golgi
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
Figure 5.12
Endocytosis and exocytosis.
Stepped Art
Fig. 5-12, p. 86

10. 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

11. 5.6 Which Way Will Water Move?
Water diffuses across cell membranes by osmosis
Osmosis is driven by tonicity, and is countered by turgor

12. Osmosis Osmosis Tonicity
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

13. 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

14. Osmosis

15. Figure 5.17
(a) A tonicity experiment. (b–d) The micrographs show human red blood cells that were immersed in fluids of different tonicity.
Fig. 5-17a, p. 89

16. Figure 5.17
(a) A tonicity experiment. (b–d) The micrographs show human red blood cells that were immersed in fluids of different tonicity.
Fig (b-d), p. 89

17. 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

18. Hydrostatic Pressure in Plants

19. 5.6 Key Concepts: Osmosis
Water tends to diffuse across selectively permeable membranes, to regions where its concentration is lower