Passive Transport Honors Biology
Objectives I can develop and use a model to illustrate the hierarchal organization of interacting systems that provide specific functions within multicellular organisms. I can plan and conduct an experiment to provide evidence that feedback mechanisms maintain homeostasis.
Recall: It’s all about homeostasis Activities of a cell depend on materials that enter and leave it To stay alive a cell must exchange materials (food, oxygen, waste) with its surroundings These materials must cross the cell membrane Small molecules (water, oxygen, carbon dioxide) move in and out freely since they can squeeze between the molecules of the membrane Large or charged molecules (proteins, sugars, ions) cannot The cell membrane is said to be selectively permeable and only allows certain molecules to pass through
Quick Chemistry Review Solvent: does dissolving Solute: gets dissolved Solution: combined mixture Concentration gradient: difference between the concentration of a solute in one place and its concentration in an adjacent area
Simple Diffusion Movement of molecules from an area of high concentration to one of low concentration Does not require cell to use energy Simplest type of passive transport Only small molecules can pass membrane by simple diffusion
Kinetic energy drives diffusion Temperature, size and type of molecules affect rates of diffusion Movement is always with concentration gradient Equilibrium: when concentration of molecules is the same throughout the space the molecules occupy
Diffusion in action How Diffusion Works
Facilitated Diffusion • Most molecules cannot cross the membrane by simple diffusion • Either too big or are ionic • Carrier proteins help molecules cross the membrane that otherwise could not • Example: glucose • Still high low concentration, so it is passive transport
Facilitated Diffusion in action How Facilitated Diffusion Works
Review: Passive transport does not require energy Molecules move from high low concentration Simple diffusion and facilitated diffusion are examples
Osmosis • Diffusion of water across a cell membrane • A form of passive transport • Water moves from high low concentration • 3 osmotic environments: • Hypertonic • Hypotonic • Isotonic
Hypertonic Environments • Less solvent (more solute) on the outside of the cell, greater on the inside of the cell • Water moves out of cell • “Hyper kids run out of school” • Animal and Plant cells: • Plasmolysis – cell shrinks as water moves out
Hypotonic Environments • More solvent (less solute) outside of the cell, less on the inside of the cell • Water moves into the cell • “HypO cell fills with water” • Animal cell: • Cytolysis – cell bursts • Plant cell: • Turgor pressure on cell wall • Best environment for plants
Isotonic Environments • Equal amounts of solvent (and solute) inside and outside of cell • Water moves in and out of cell at an even rate • “Like a full restaurant – one family must leave for another to enter” • Dynamic equilibrium – equally dispersed solvent on both sides of the membrane • Animal cell: • Best environment • Plant cell: • Flaccid (wilts)
Important to Remember… Molecules and solvent (water) are always moving Movement is from high concentration to low concentration All movement is in attempt to reach equilibrium Homeostasis!