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Transcript
Biology 12 – Membrane Notes Fluid-Mosaic Model • Cell membranes are sometime described as a fluid mosaic ! Why Mosaic?... the proteins and carbohydrate chains in/on the phospholipid bilayer form a pattern (mosaic) within the bilayer ! Why Fluid?... the proteins and carbohydrate chains in/on the phospholipid bilayer can drift around in the bilayer (fluid) Selectively Permeable • Cell membranes are selectively permeable (not just semi-permeable) ! What’s the difference? o Semi-permeable = a membrane (e.g. dialysis tubing) that materials can pass through simply based on size and concentration gradient o Selectively permeable = a living membrane that can use energy to select molecules (even if they are too big or the concentration gradient is going in the opposite direction) Six Ways In/Out of Cells • Molecules enter/leave cells based on size, shape of molecule, charge of the molecule, concentration gradient, and cell “need” " some molecules can pass right through the bilayer (①Diffusion and ②Osmosis) " some molecules travel through protein channels/carriers (③Facilitated Transport and ④Active Transport) " some molecules enter/leave using vesicles or vacuoles (⑤Endocytosis and ⑥Exocytosis) ① Diffusion • natural physical process where molecules move from an area of high concentration to an area of low concentration o passive process (doesn’t need extra energy) o occurs within cells (cytoplasm), outside cells (extracellular fluid) and through the bilayer ! *molecules must be small and neutral (eg. O2, CO2, alcohol) o rate of diffusion affected by: # temperature # size of the molecule # concentration gradient # fluid viscosity # fluid currents ② Osmosis = the diffusion of water across a semi-permeable (or selectively permeable) membrane o water moves from an area of high [water] " low [water] OR water will move to dilute a solute • Important Definitions o Osmotic Pressure = the force that results from the osmotic flow of water # can “pop” an animal cell or keep a plant cell turgid o Tonicity = the amount of solute in a solution (in mol/L or %) # Isotonic = equal solute concentration # Hypertonic = greater solute concentration • Animal cells (e.g. Red blood cells) placed in a hypertonic solution will lose water and shrink/shrivel (“crenation” for RBCs) • # Plant cells in a hypertonic solution will wilt (“plasmolysis”) Hypotonic = lower solute concentration • Animal cells placed in a hypotonic solution will gain water and burst (“lysis” or “hemolysis” in RBCs) • Plant cells in a hypotonic solution will get tight (“deplasmolysis” or “turgor”)