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Transcript
Unit 3: Homeostasis and Cell Transport Sect 1 Passive Transport Sect 2 Active Transport Objectives • Explain how an equilibrium is established as a result of diffusion. • Distinguish between diffusion and osmosis. • Explain how substances cross the cell membrane through facilitated diffusion. • Explain how ion channels assist the diffusion of ions across the cell membrane. Passive Transport: Diffusion • Movement of molecules from high to low concentrations across a concentration gradient. • Simplest type of cell transport. • Due to molecule’s kinetic energy. • Results in equilibrium (even distribution of molecules). • Molecules can diffuse across a cell membrane by dissolving in the phospholipid bilayer or by passing through membrane pores. Passive Transport: Osmosis • Diffusion of water from high to low concentrations across a membrane. Direction of Osmosis • Net direction is determined by the relative solute concentrations on the two sides of the membrane. Direction of Diffusion-Hypotonic Solution • when the solute concentration outside the cell is lower than in the cytosol, the solution outside is hypotonic to the cytosol; water will diffuse into the cell. Direction of Osmosis- Isotonic Solution • When the solute concentrations outside and inside the cell are equal, the solution outside is isotonic, and there will be no net movement of water. Direction of Osmosis-Hypertonic Solution • When the solute concentration outside the cell is higher than that in the cytosol, the solution outside is hypertonic to the cytosol, and water will diffuse out of the cell. How Cells Deal with Osmosis • Contractile vacuoles pump excess water from unicellular, freshwater organisms. • Cell walls in plants resist turgor pressure in hypotonic conditions. • Plasmolysis (wilting) occurs in plant cells in hypertonic conditions. • Cytolysis (bursting) occurs in animal cells in hypertonic conditions. Facilitated Diffusion • Large molecule binds to a carrier protein on one side of the cell membrane. • Carrier protein changes shape and transports the molecule down its concentration gradient to the other side of the membrane. ex. glucose transport Diffusion Through Ion Channels • Ion channels: proteins, groups of proteins, that provide passageways across the cell membrane through which specific ions can diffuse. Active Transport •Active transport: moves molecules from an area of lower concentration to an area of higher concentration across the cell membrane. •Requires energy. •May be done by a carrier proteins. Active Transport: Sodium-Potassium Pump •Sodium-Potassium Pump –Moves three Na+ ions out of the cell for every two K+ ions it moves into the cytosol. –ATP supplies the energy to drive the pump. Active Transport: Endocytosis • Cells ingest materials by enfolding them into a pouch. • Pouch pinches off and becomes a membrane-bound vesicle. • Includes: pinocytosis: vesicle contains solutes or fluids phagocytosis: vesicle contains large particles or cells. Active Transport: Exocytosis • Vesicles within the cell fuse with the cell membrane. • Contents are released outside of cell.