Marieb s Human Anatomy and Physiology Marieb Hoehn Chapter 3 Cell Membranes Movement Across the Cell Membrane Lecture 7 1 The cell membrane Lecture Overview 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 1
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 Membrane Lipids Hydrophilic channel 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 2
Simple Diffusion 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) 8 Where Would You Rather Be? Spread out, would ya!? 9 3
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 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 Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 12 4
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, 12 th 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 Figure from: Hole s Human A&P, 12 solutes particles increases th edition, 2010 isotonic same osmotic pressure as a second solution hypertonic higher osmotic pressure hypotonic lower osmotic pressure 0.9% NaCl 5.0% Glucose Crenation The O in hypotonic 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 15 5
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 Figure From: Marieb & Hoehn, Human Anatomy & Physiology, 9 th ed., Pearson 16 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 17 Endocytosis Figures from: Hole s Human A&P, 12 th edition, 2010 18 6
Exocytosis reverse of endocytosis substances in a vesicle fuse with cell membrane contents released outside the cell release of neurotransmitters from nerve cells 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? CONCEN- TRATION GRADIENT GENERAL DESCRIPTION EXAMPLE IN HUMANS SIGNIFICANCE SIMPLE DIFFUSION NO spreading out of molecules to equilibrium O 2 into cells; CO 2 out of cells. Cellular Respiration FACILITATED DIFFUSION NO Using a special cm carrier protein to move something through the cell membrane (cm) Process by which glucose enters cells OSMOSIS NO water moving through the cm to dilute a solute maintenance of osmotic pressure of 0.9%. Same FILTRATION NO using pressure to push something through a cm (sprinkler hose) manner in which the kidney filters things from blood removal of metabolic wastes 23 7
Lecture Review TRANSPORT PROCESS IS ENERGY NEEDED? CONCEN- TRATION GRADIENT GENERAL DESCRIPTION EXAMPLE IN HUMANS SIGNIFICANCE ACTIVE TRANSPORT YES opposite of diffusion at the expense of energy K + -Na + -ATPase pump maintenance of the resting membrane potential ENDOCYSIS YES 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 EXOCYSIS YES expelling a substance from the cell into ECF Exporting proteins; dumping waste Same 24 8