a closer look at cell membranes chapter 5 part 2
TRANSCRIPT
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A Closer Look at Cell Membranes
Chapter 5 Part 2
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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
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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
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Endocytosis and Exocytosis
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Fig. 5-12, p. 86
Endocytosis ExocytosisA Molecules get concentrated inside coated pits at the plasma membrane.
coated pit
B The pits sink inward and become endocytic vesicles.
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.
C Vesicle contents are sorted.
F Some vesicles and their contents are delivered to lysosomes.
lysosome Golgi
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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
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Receptor-Mediated Endocytosis
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Fig. 5-13, p. 86
plasma membrane aggregated lipoproteins
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Animation: Phagocytosis
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Phagocytosis
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Fig. 5-14a, p. 87
A Pseudopods surround a pathogen ( brown ).
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Fig. 5-14b, p. 87
B Endocytic vesicle forms.
C Lysosome fuses with vesicle; enzymes digest pathogen.
D Cell uses the digested material or expels it.
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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
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Exocytic Vesicle
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Fig. 5-12, p. 86
F Some vesicles and their contents are delivered to lysosomes.
lysosome
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.
Golgi
Endocytosis
A Molecules get concentrated inside coated pits at the plasma membrane.
coated pit
Stepped Art
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Animation: Membrane cycling
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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
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5.6 Which Way Will Water Move?
Water diffuses across cell membranes by osmosis
Osmosis is driven by tonicity, and is countered by turgor
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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
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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
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Osmosis
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Fig. 5-16, p. 88
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.
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Animation: Tonicity and water movement
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Experiment: Tonicity
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Fig. 5-17a, p. 89
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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?
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2% sucrose
A What happens to a semipermeable membrane bag when it is immersed in an isotonic, a hypertonic, or a hypotonic solution?
Fig. 5-17, p. 89
2% sucrose 10% sucrose water
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
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Fig. 5-17 (b-d), p. 89
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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.
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Animation: Osmosis experiment
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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
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Hydrostatic Pressure in Plants
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Fig. 5-18a, p. 89
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5.6 Key Concepts: Osmosis
Water tends to diffuse across selectively permeable membranes, to regions where its concentration is lower
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Animation: Active transport
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Animation: Endocytosis and exocytosis
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Animation: Fluid mosaic model
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Animation: Passive transport II
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Animation: Solute concentration and osmosis
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Video: One bad transporter and cystic fibrosis
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Video: Diffusion of dye in water
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Video: Contractile vacuole