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BIO 121 - September 18th

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BIO 121
Jason Wiles

BIO 121, Class Notes September 18 Selectively Permeable Membranes • Allow passages of some substances but not others • Controls volume and internal composition of cell’s ions and molecules Osmosis • Atype of diffusion • H2O molecules pass through a selectively permeable membrane • From higher effective H2O concentration • To lower effective H2O concentration Osmotic Pressure • Determined by concentration of solutes • Cells regulate internal osmotic pressure • To prevent shrinking or bursting Tonicity - Only mean something if you’re comparing to something else • Isotonic solutions • Equal solute concentration • Hypertonic solution • In solute concentration > the cell’s • Cell loses water to surroundings • Hypotonic solution • In solute concentration < the cell’s • Water enters, cells swell Plant Cells • Plasmolysis - if you don’t water the plant, the plant will start falling over • In hypertonic solution, water moves out • Plasma membrane splits from cell wall • Turgor pressure • In hypotonic solution, water moves in • Cells swell, build pressure against cell walls Key Concept • Diffusion is passive - does not require the cell to expend metabolic energy • Active transport requires direct expenditure of energy Simple vs. Facilitated • Simple diffusion • Molecules or ions move directly through the membrane • Down their concentration gradient • Facilitated diffusion • Specific transport proteins move solutes across a membrane • Down their concentration gradient • Facilitated diffusion from highest concentration to lower concentration does not cost the cell energy Active Transport • Cell uses metabolic
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