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Transcript
Marieb’s Human Anatomy and Physiology Marieb w Hoehn Chapter 3 Cell Membranes Movement Across the Cell Membrane Lecture 7 1 Lecture Overview • The cell membrane • Osmotic pressure and tonicity • Movement of substances into and out of the cell 2 Cell Membrane • outer limit of cell; isolates cell • controls what moves in and out of cell selectively permeable • self-sealing •phospholipid bilayer • water-soluble “heads” form outer surfaces • water-insoluble “tails” form interior • permeable to lipid-soluble substances only • cholesterol stabilizes the membrane • proteins • receptors • pores, channels, carriers • enzymes • CAMS • self-markers 3 Cell Membranes Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 4 A Transmembrane Protein Figure from: Alberts et al., Essential Cell Biology, Garland Publishing, 1998 Hydrophilic channel Membrane Lipids 5 Movements Into and Out of the Cell Passive (Physical) Processes • require no cellular energy • simple diffusion • facilitated diffusion • osmosis Active (Physiological) Processes • require cellular energy • active transport • endocytosis • exocytosis • transcytosis 6 Simple Diffusion Figure from: Hole’s Human A&P, 12th edition, 2010 Solutes will evenly disperse in a solvent with time by diffusion. This is the lowest energy state. 7 Simple Diffusion • movement of solute from regions of higher concentration to regions of lower concentration (a physical process) Figure from: Hole’s Human A&P, 12th edition, 2010 8 Where Would You Rather Be? “Spread out, would ya!?” 9 Facilitated Diffusion • diffusion across a membrane with the help of a channel or carrier molecule • e.g, transport of glucose across cell membrane BUT…still from a region of higher concentration to a region of lower concentration Figure from: Hole’s Human A&P, 12th edition, 2010 10 Factors Influencing Diffusion Rates • Distance (shorter is faster) • Gradient size (bigger difference in concentration is faster) In the body, diffusion distances are typically limited to a maximum of about 125 µm • Molecule size (smaller is faster) • Temperature (warmer is faster) • Electrical forces (repulsion is better) 11 Diffusion and the Cell Membrane Carrier/channel proteins required for all but fatsoluble molecules and small uncharged molecules oxygen, carbon dioxide and other lipid-soluble substances diffuse freely through the membrane 12 Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 Osmosis • movement of water (solvent) through a selectively permeable membrane from regions of higher water concentration to regions of lower water concentration • *water always moves toward a higher concentration of solutes A special case of passive diffusion Figure from: Hole’s Human A&P, 12th edition, 2010 13 Osmotic Pressure/Tonicity Osmotic Pressure – ability of osmosis to generate enough pressure to move a volume of water *Osmotic pressure increases as the number of nonpermeable solutes particles increases 0.9% NaCl • isotonic – same 5.0% Glucose osmotic pressure as a second solution Figure from: Hole’s Human A&P, 12th edition, 2010 • hypertonic – higher osmotic pressure • hypOtonic – lower osmotic pressure Crenation The O in o hyp tonic 14 Filtration • smaller molecules are forced through porous membranes • separates large from small molecules • hydrostatic pressure; important in fluid movement • molecules leaving blood capillaries Think ‘sprinkler hose’ Figure from: Hole’s Human A&P, 12th edition, 2010 15 Active Transport • carrier molecules transport substances across a membrane from regions of lower solute concentration to regions of higher concentration, i.e., against a concentration gradient • sugars, amino acids, sodium ions, potassium ions, etc. Active transport is a physiological process since it requires cellular energy, e.g., ATP 16 Figure From: Marieb & Hoehn, Human Anatomy & Physiology, 9th ed., Pearson Endocytosis • cell engulfs a substance by forming a vesicle around the substance • three types • pinocytosis – substance is mostly water • phagocytosis – substance is a solid • receptor-mediated endocytosis – requires the substance to bind to a membrane-bound receptor Figure from: Hole’s Human A&P, 12th edition, 2010 17 Endocytosis Figures from: Hole’s Human A&P, 12th edition, 2010 18 Exocytosis • reverse of endocytosis • substances in a vesicle fuse with cell membrane • contents released outside the cell • release of neurotransmitters from nerve cells Figure from: Hole’s Human A&P, 12th edition, 2010 19 Transmembrane Potential Dependent upon active transport Cells maintain a transmembrane potential, with the inside of the cell membrane being slightly negative relative to the outside Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 22 Lecture Review TRANSPORT PROCESS IS ENERGY NEEDED? CONCENTRATION GRADIENT GENERAL DESCRIPTION EXAMPLE IN HUMANS SIGNIFICANCE SIMPLE DIFFUSION NO [HIGH] TO [LOW] spreading out of molecules to equilibrium O2 into cells; CO2 out of cells. Cellular Respiration FACILITATED DIFFUSION NO [HIGH] TO [LOW] Using a special cm carrier protein to move something through the cell membrane (cm) Process by which glucose enters cells OSMOSIS NO [HIGH] TO [LOW] water moving through the cm to dilute a solute maintenance of osmotic pressure of 0.9%. Same FILTRATION NO [HIGH] TO [LOW] using pressure to push something through a cm (sprinkler hose) manner in which the kidney filters things from blood removal of metabolic wastes 23 Lecture Review TRANSPORT PROCESS IS ENERGY NEEDED? CONCENTRATION GRADIENT GENERAL DESCRIPTION EXAMPLE IN HUMANS ACTIVE TRANSPORT YES [LOW] TO [HIGH] opposite of diffusion at the expense of energy K+-Na+-ATPase pump maintenance of the resting membrane potential ENDOCYTOSIS YES [LOW] TO [HIGH] bringing a substance into the cell that is too large to enter by any of the above ways; Phagocytosi: cell eating; Pinocytosis: cell drinking. Phagocytosed (foreign) particles fuse with lysosomes to be destroyed help fight infection EXOCYTOSIS YES [LOW] TO [HIGH] expelling a substance from the cell into ECF Exporting proteins; dumping waste Same SIGNIFICANCE 24