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Explore the structure of cell membranes, the different methods of transport, and the importance of membrane dynamics in maintaining cellular functions. Learn about passive and active transport, diffusion, osmosis, and vesicular transport.
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Membrane Dynamics Cell membrane structures and functions Membranes form fluid body compartments Membranes as barriers and gatekeepers How products move across membranes i.e., methods of transport Distribution of water and solutes in cells & the body Chemical and electrical imbalances Membrane permeability and changes
The Cell Membrane • Fluid Mosaic Model • Phospholipids • Integral Proteins • Peripheral Proteins • Glycocalyx • Glycoproteins • MHC • Glycolipids • Cholesterol
Thickness ~ 8nm Cell Membrane Structure: Fluid Mosaic Model PLs Cholesterol Proteins: peripheral (associated) or integral
Passive Transport = Diffusion (Def?) – 3 types: • Simple diffusion • Facilitated diffusion (= mediated transport) • Osmosis Active Transport Always protein-mediated – 3 types: • Co-transport • Vesicular transport • Receptor mediated transport
Diffusion Process (Passive) • Uses energy of concentration gradient • Net movement until state of equilibrium is reached (no more conc. gradient) • Direct correlation to temperature • Indirect correlation to molecule size • Lipophilic molecules can diffuse through the phospholipid bilayer
Diffusion through Membranes • Oxygen, carbon dioxide, fatty acids, and steroid hormones are examples of nonpolar molecules that diffuse rapidly through the lipid portions of membranes. • Remember that lipophilic (lipid-loving) substances move through easily. • Polar molecules and hydrophilic (water-loving) do not diffuse readily through the membranes.
Facilitated Diffusion • Some molecules are too polar or too large to pass through the lipid bilayer. • Carrier proteins change shape after the molecules bind then envelopes the molecule and releases it • The binding site is moved from one side of the membrane to the other by a change in the confirmation of the carrier protein.
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html
Osmosis • The net diffusion of water across a membrane • Through channel proteins called aquaporins
Tonicity • Physiological term describing how cell volume changes if cell placed in the solution • Always comparative. Has no units. • Isotonic sol’n = No change in cell • Hypertonic sol’n = cell shrinks • Hypotonic = cell expands • Depends not just on osmolarity but on nature of solutes and permeability of membrane
Tonic solutions • Isotonic, hypotonic, and hypertonic solutions: • Isotonic solutions have the same concentration of nonpenetrating solutes as normal extracellular fluid. • Hypotonic solutions have a lower concentration of nonpenetrating solutes as normal extracellular fluid. • Hypertonic solutions have a higher concentration of nonpenetrating solutes as normal extracellular fluid.
Active Transport • Movement from low conc. to high conc. • ATP needed • Creates state of disequilibrium • 1o (direct) active transport • ATPases or “pumps” • Uniport and Antiport • 2o (indirect) active transport • Symport and antiport
Primary Active-Transporters • The Na+/K+-ATPase primary active transporter is found in every cell and helps establish and maintain the membrane potential of the cell. • In addition to the Na+/K+-ATPase transporter, the major primary active-transport proteins found in most cells are: (1) Ca2+-ATPase (2) H+-ATPase (3) H+/K+-ATPase
Cotransport • Symport • Molecules are carried in same direction • Examples: Glucose and Na+ • Antiport • Molecules are carried in opposite direction • Examples: Na+/K+ pump
Vesicular Transport Movement of macromolecules across cell membrane: • Phagocytosis (specialized cells only) Macrophage or Phagocytes 2. Pinocytosis “Cell drinking” 3. Receptor mediated endocytosis Down Regulation 4. Exocytosis
http://www.youtube.com/watch?v=KiLJl3NwmpU • http://www.youtube.com/watch?v=4gLtk8Yc1Zc&feature=related
Endocytosis • Movement of molecules into the cell via vesicles. • There are three general types of endocytosis that may occur in a cell: 1. Phagocytosis 2. Pinocytosis 3. Receptor-mediated endocytosis
Phagocytosis • Requires energy • Cell engulfs particle into vesicle via pseudopodia formation • E.g.: some WBCs engulfs bacteria • Vesicles formed are much larger than those formed by endocytosis • Phagosome fuses with lysosomes ?
Receptor Mediated Endocytosis • No. 1 uptake method in most cells • Receptors and substance is internalized into a coated pit-clathrin • Down Regulation
Exocytosis Intracellular vesicle fuses with membrane Requires energy (ATP) and Ca2+ Examples: large lipophobic molecule secretion; receptor insertion; waste removal
Clinical Case Study A 22-year-old woman was competing in her first marathon. She was in good health, but was completely inexperienced in long-distance runs. In the hour before the race, she drank two 20-ounce bottles of water in anticipation of the water loss she expected to experience due to perspiration during the race. The race took place on an unseasonably cool day in April. As she ran, she took a drink at each water station. At the 20-mile mark she was feeling extremely fatigued and her leg muscles began cramping. Thinking she was losing too much fluid, she drank additional water. At 23 miles she began to feel confused and disoriented and developed a headache. She finished yet another bottle of water even though she was not thirsty. Twenty minutes later, she collapsed, lost consciousness, and was taken to a local hospital. Her blood Na+ levels had decreased to 115 mM and she was diagnosed with exercise-associated hyponatremia. What was the effect of excessive water consumption on the osmolarity of this woman’s extracellular fluids? How would this affect ion gradients and cell volumes in areas such as her brain and skeletal muscles?
