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[LAB3] Diff-Os.5A.10w

[LAB3] Diff-Os.5A.10w - Biology 05LA Winter Quarter 2010...

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Biology 05LA – Winter Quarter 2010 Lab 3 – page 1 LAB 3 : DIFFUSION, OSMOSIS AND THE PERMEABILITY OF CELL MEMBRANES The movement of materials between cells and their external environment as well as within the cell is essential to life. For example, the raw materials for cellular metabolism must be imported and the metabolic waste products resulting from this metabolism must be exported. Within the cell, substrates for the cell’s enzymes must move into the active site of the enzyme and the soluble products of these reactions then distributed throughout the cell. Given the fundamental importance of this traffic, an understanding of the mechanisms whereby these materials move is essential. Since both the internal and external cellular environment are aqueous, the materials to be moved are dissolved in water and thus form a solution (remember that in an aqueous solution, water is called the solvent and the materials dissolved in the water are called solutes ). Solute movement may be separated into two broad categories. One of these requires the direct output of energy by the cell and is referred to as active transport. Here, a solute is moved against a concentration gradient of that solute, that is, from an area of low solute concentration to an area of high solute concentration. Thus active transport allows cells to generate concentration gradients that support a diverse array of cellular processes. You will be hearing much more about active transport in lecture. The other category of solute movement does not involve a direct expenditure of energy by the cell and is called diffusion. Diffusion will be the primary focus of today’s lab. Your text defines diffusion as: “the spontaneous movement of a [solute] down its concentration gradient from an area where it is more concentrated to an area where it is less concentrated.” This movement continues until the solute is evenly distributed throughout the available space. The driving force for diffusion comes from the activity of the water (solvent) molecules in the solution. Water molecules are not static. In reality, they are in constant motion that is driven by the heat energy of the system. Evidence for this can be seen at high magnification in the light microscope when viewing suspensions of small particles rapidly vibrating in a random manner. This motion, referred to as Brownian motion, is the result of randomly directed collisions between the water molecules and the suspended particles. Similar interactions occur between solvent and solute molecules. The random nature of these collisions and the probability of more frequent collisions occurring in areas where solutes are more concentrated work together to move the solute molecules into the available space, that is, down their concentration gradient. Thus, anything that influences the number of collisions between solvent and solute can, in turn influence the rate of diffusion. Relevant possibilities here include: the
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