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

Endocytosis and Exocytosis

Fig. 5-12, p. 86 EndocytosisExocytosis A 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

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

Fig. 5-13, p. 86 plasma membraneaggregated lipoproteins

Animation: Phagocytosis

Phagocytosis

Fig. 5-14a, p. 87 A Pseudopods surround a pathogen ( brown ).

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.

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

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

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

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.

Animation: Tonicity and water movement

Experiment: Tonicity

Fig. 5-17a, p. 89

2% sucrose 10% sucrosewater A What happens to a semipermeable membrane bag when it is immersed in an isotonic, a hypertonic, or a hypotonic solution?

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

Fig (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.

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