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Cellular Membranes

Cellular Membranes. Chapter 7. Membrane Structure. Composed of lipids and proteins Phospholipid bilayer each protein type has a specific function in animal cells cholesterol acts as a buffer for membranes to resist changes in temperature

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Cellular Membranes

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  1. Cellular Membranes Chapter 7

  2. Membrane Structure • Composed of lipids and proteins • Phospholipidbilayer • each protein type has a specific function • in animal cells cholesterol acts as a buffer for membranes to resist changes in temperature • membrane carbohydrates (extracellular surface) aid in recognition of other cells • glycolipids (lipid + carb) • glycoproteins (protein + carb)

  3. Cellular Membranes • Selectively Permeable • allows some substances to cross more easily than others • regulates transport across cell boundaries • non-polar molecules cross easily (CO2, O2, hydrocarbons) • hydrophobic • polar molecules can’t cross easily (hydrophilic) • ions, sugars, water--pass slowly • Fluid Mosaic Model • membrane is a fluid structure that has a variety of proteins scattered throughout • Membranes float in or on phospholipidbilayer

  4. Membrane Proteins • Proteins determine the membrane’s function • different cell types contain different membrane proteins • Transport • Enzymes • Signal Transduction • Cell-Cell Recognition • Intercellular joining • Attachments to ECM (extra-cellular matrix) • Integral Proteins: into the membrane • Peripheral Proteins: not embedded in the membrane; loosely bound to the surface

  5. Transport Proteins • proteins that allow the passage of specific ions and hydrophilic substances • carrier proteins--hold onto substances and change shape which allows them to pass through membrane • channel proteins--hydrophilic channel that molecules/ions can use to tunnel through the membrane • water passes through proteins called aquaporins

  6. Passive Transport • Primary role is importing resources and exporting wastes from cell • Diffusion: movement of molecules from an area of high to low concentration • any substance will diffuse down a concentration gradient (high to low) • eventually movement of molecules will reach equilibrium (moving at the same rate) • no energy is required to move molecules across membrane • Non-polar molecules diffuse easiest

  7. Passive Transport • Osmosis: diffusion of water across a selectively permeable membrane • osmoregulation--ability for cells/organisms to control solute concentrations and water balance • most important in cells without walls • Freshwater vs. saltwater fish • tonicity: ability of a solution to gain or lose water (depends on concentration of solutes that can’t pass through membrane) • isotonic--no net movement of water • hypertonic--solution with high concentration of solutes unable to cross membrane • Ex. increased salinity • hypotonic--solution with low concentration of solutes unable to cross membrane • Ex. Distilled water

  8. Passive Transport • Facilitated Diffusion: transport proteins help certain polar molecules and ions pass through the cell membrane • molecules move down a concentration gradient, so no energy required! (high to low concentration) • provides efficient passage of a solute through the membrane • channel and carrier proteins • ion channels (gated channels)--open and close in response to a stimulus • glucose channels

  9. Active Transport • membrane proteins move solutes against a concentration gradient (all carrier proteins) • requires energy, provided by ATP • allow cells to maintain different internal concentrations of smaller molecules than their environment (create concentration gradients) • sodium/potassium pumps: high internal K+ concentration and low Na+ concentration

  10. Bulk Transport • Large molecules (polysaccharides and proteins) move in vesicles • Exocytosis: vesicle membrane and cell membrane fuse, releasing material OUT of the cell • Endocytosis: vesicles form from cell membrane to take IN materials • Phagocytosis “eating” • Pinocytosis “drinking”

  11. http://www.bozemanscience.com/water-potential/ Water Potential • Used to predict the direction in which water will diffuse through living plant tissues • solute potential (solute concentration) • pressure potential (pressure +/- on a solution). • In an open system, pressure potential will be the same as atmospheric pressure and can be ignored (so water potential = solute potential) • Water will move from an area of higher water potential to an area of lower water potential • Water potential of solutions at equilibrium will be zero • Higher M concentration = lower water potential (hypertonic) • Increase solute = decrease in water • Water will diffuse in the direction of the highest M at atmospheric pressure • Pure water (WP= 0) • Turgor pressure (cell wall exerts pressure back to prevent excessive uptake of water and bursting of cell)

  12. Water Potential Practice

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