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• Solution - a homogeneous mixture of 2 substances • Solute - the dissolved substance • Solvent - the dissolving agent Diffusion • Random movement of molecules from an area of their higher concentration to an area of their lower concentration. • Lighter molecules diffuse faster than heavier molecules Diffusion occurs in both liquids and gases. Examples: sugar in water; perfume in air. Osmosis • Diffusion of water molecules through a semipermeable membrane from an area of higher water concentration to an area of lower water concentration • (An area of high water concentration has a low solute concentration, and visa versa) Figure 3.7 Generation of osmotic pressure by osmosis. Copyright © 2009 Pearson Education, Inc. The plasma membrane is a semi-permeable membrane. The cytoplasm is completely enclosed. Solute concentrations of outside solutions relative to the inside of a cell: • Isotonic: the same solute concentration • Hypertonic: higher solute concentration • Hypotonic: Lower solute concentration Osmosis and Cells Osmosis and cells • In hypertonic environment, the cell shrinks. In a plant cell, this is called plasmolysis: the cell shrinks inside the cell wall. • In hypotonic environment, the cell swells. An animal cell will burst, which is called lysis. Differences between dialysis tubing and the cell membrane (both are selectively permeable): 1. Dialysis tubing has pores. Whether or not a molecule can cross depends only on its size. 2. The cell membrane is made of a lipid bilayer. Whether or not a molecules can cross depends on its chemical properties. 3. Proteins are embedded in the membrane, and regulate the passage of most molecules across the membrane. Membrane Lipids: basic structure head Copyright © 2009 Pearson Education, Inc. tails The plasma membrane. (DON’T NEED TO KNOW THE DETAILS) Copyright © 2009 Pearson Education, Inc.