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 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 ). 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

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

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

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

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 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

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

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

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. 5-17 (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