Diffusion and Osmosis

Selectively Permeable Some substances can pass through the membrane while other cannot Why is this important? Cells must let in needed materials (oxygen, food) while getting rid of waste products. The cell membrane also acts as a selectively permeable membrane,controlling the materials that enter and leave the cell. Molecules like water, oxygen, and carbon dioxide are able to easily pass in and out of the cell through the cell membrane while other molecules like specific proteins are strictly kept in or out of the cell. Image from: www.wildrosegeo.com/ catalogue_sieves.php

Diffusion Molecules move from an area of higher concentration to an area of lower concentration What causes diffusion? Random motion of molecules Water Dissolved molecules 2 3 1 4 Substances move into and out of a cell by one of three methods: diffusion, osmosis, or active transport. Diffusion is the main process by which small molecules move across the cell membrane. Diffusion is defined as the process by which molecules move from a region of higher concentration to a region of lower concentration. Concentration is the amount of substance in a given area.

Osmosis Diffusion of water molecules through a selectively-permeable membrane Dissolved molecules (i.e. glucose, starch) are called solutes The liquid they are dissolved in is called the solvent Osmosis refers to the specific diffusion of water through a selectively permeable membrane. Remember that molecules tend to move from an area of higher concentration to an area of lower concentration. In osmosis, water molecules move by diffusion from an area where they are highly concentrated to an area where they are less concentrated.

Effects of Osmosis on Cells [water] = [solute] Low [water] outside cell High [water] outside cell

Transportation of Molecules Passive Transport Movement of dissolved molecules through a membrane WITHOUT using cellular energy Active Transport Movement of molecules through a membrane USING cellular energy Passive transport includes the processes of diffusion and osmosis. In both forms of passive transport, molecules tend to move from a more crowded to a less crowded area in order to achieve a balance without using energy. Active transport is the movement of molecules from a less crowded to a more crowded area with the use of energy. Molecules are "carried" into or out of the cell using some of the cell's energy.

Egg-speriment Demonstrate the effects of osmosis using an egg to model a cell Dissolve eggshells in vinegar (weak acid) Submerge eggs in different aqueous environments Measure volume of liquid remaining and egg circumference to determine movement of water Vinegar Sugar syrup Distilled water Colored water Cola Diet Cola Salt water Alcohol