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Do Now 1. What maintains homeostasis for cells? THE CELL MEMBRANE 2. What is the term that means when the organelle from #1 only lets certain things in and out? • SELECTIVELY PERMEABLE Getting through the barrier of the cell membrane • Some substances pass through the membrane without any energy needed from the cell called passive transport • Some substances need energy from the cell to pass through the membrane called Active Transport Passive Transport Passive Transport: movement of molecules across a membrane WITHOUT any energy – Three types: 1. Diffusion 2. Facilitated Diffusion 3. Osmosis The Goal: maintain homeostasis using the cell membrane as a barrier. Diffusion Diffusion = movement of molecules of a substance from an area of HIGH concentration to an area of LOW concentration Ex: sugar cube in water food coloring home cooking air freshener Diffusion • There is a difference in concentration of the 2 substances (food coloring & water, water & sugar, air freshener & air) • The difference in concentration across space/area concentration gradient • When the concentration of a substance is the same all over Equilibrium! (molecules still moving… at random) Diffusion • Molecules tend to move DOWN the concentration gradient… – more (hi) concentration to less (low) concentration Diffusion Across Membranes • Some molecules can diffuse across a membrane (with no aid/help) if they are soluble in lipids – NOT all molecules can do this – Also depends on 1. size & type of molecule 2. structure of molecule Facilitated Diffusion • The movement of molecules with the gradient (to an area of lower concentration) using a carrier protein for passage • Carrier proteins are molecule specific, so only one type of molecule can move through them • Occurs when regular diffusion cannot occur fast enough as needed (large molecules) • Can go either into or out of cell depending on gradient OSMOSIS Osmosis Diffusion of water molecules through cell membrane Direction of osmosis is determined by the SOLUTE concentration DIRECTION OF OSMOSIS Depends on the concentrations of solutes & water . THE WATER MOVES WHERE THERE IS MORE SOLUTE! Hypertonic Solution • HIGH concentration of SOLUTE OUTSIDE the cell, • LOW concentration of SOLUTE INSIDE the cell • RESULT: WATER MOVES OUT OF THE CELL. HYPOTONIC • LOW concentration of SOLUTE OUTSIDE the cell • HIGH concentration of SOLUTE INSIDE the cell, • RESULT: – WATER MOVES INTO THE CELL. ISOTONIC • inside and outside concentrations are equal. • equal concentrations of solute & water outside and inside the cell RESULT: water diffuses in and out at constant rate, no net movement Osmosis in Cells OSMOSIS How does osmosis affect cells? • Many cells live in isotonic solutions so equilibrium is easily attained • But some do not: – Some unicellular freshwater organisms – Plant cells Unicellular freshwater organisms Organisms live in hypotonic environments (lots of salt inside their cells) so they constantly take in water. • If too much water enters cell dies, called cytolysis • They have an organelle called a contractile vacuole = that collects & pumps out the excess water. This requires energy to be expended Contractile Vacuole of Amoebae Osmosis in Plants • Plant cells also live in hypotonic solutions, so water enters the cell.. They like it • Why?... Osmosis continues until vacuole is full & the cell membrane is forced against the cell wall, called turgor pressure • When a plant cell loses turgor pressure, the membrane collapses away from the cell wall, causing the plant to wilt, called plasmolysis Plasmolysis