Honors Biology

 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

 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

 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

 How Diffusion Works How Diffusion Works

 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

 How Facilitated Diffusion Works How Facilitated Diffusion Works

 Passive transport does not require energy  Molecules move from high  low concentration  Simple diffusion and facilitated diffusion are examples

 Diffusion of water across a cell membrane  A form of passive transport  Water moves from high  low concentration  3 osmotic environments: 1. Hypertonic 2. Hypotonic 3. Isotonic

 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

 More solvent (less solute) outside of the cell, less on the inside of the cell  Water moves into the cell  “ Hyp O cell fills with water”  Animal cell: Cytolysis – cell bursts  Plant cell: Turgor pressure on cell wall Best environment for plants

 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)

 Molecules and solvent (water) are always moving  Movement is from high concentration to low concentration  All movement is in attempt to reach equilibrium  Homeostasis!