LabManual_Osmosis - Foundation of Biological Sciences I Biological Membranes Osmosis 1 Biological Membranes Osmosis Hbel G 2008 Biological

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Foundation of Biological Sciences I Biological Membranes & Osmosis - 1 Biological Membranes & Osmosis Höbel, G. 2008. Biological Membranes & Osmosis. Foundations of Biological Sciences I – Lab Manual. Life on earth is believed to have originated in water, and living cells consist of up to 85% water. Most substances entering and leaving cells are dissolved in water, making it the most important solvent for lives processes. The combination of a solvent (like water) and a dissolved substance ( solute ) is called a solution . The cytoplasm of living cells contains numerous solutes, like sugars and salts, in solution. The boundary of all cells consists of plasma membranes. These are composed of a phospholipids bilayer that contains different kinds of embedded or surface proteins (Fig. 1). Material crosses this outer cell boundary by several processes. Large particles are engulfed in membrane, forming a vesicle that can pass into or out of the cell. Small molecules diffuse though the spaces between lipid molecules in the membrane. Others bind with membrane proteins and are transported into or out of the cell. Solutes in a solution are in constant motion because of their kinetic energy. As temperature increases, the speed of movement increases so that he solutes move more rapidly. Diffusion results from kinetic energy of molecules, and the movement of molecules is – on average – away from regions of high concentration towards regions of low concentration. The gradual difference in concentration over distance between regions of high and low concentrations is called the concentration gradient . Diffusion takes place down a concentration gradient. The steeper the concentration gradient, the faster the rate of diffusion. The rate of diffusion is affected by temperature and molecule size. It is directly proportional to temperature, but inversely proportional to the molecular weight of the substance (small molecules move faster). The spaces between membrane molecules can be passed by small solutes which can enter and leave the cell by diffusion. Diffusion occurs without the expenditure of cellular energy. Once inside the cell, the solutes move through the cytoplasm by diffusion, sometimes assisted by cytoplasmatic streaming. For many substances, however, the cell membrane represents a barrier, either because they of their size or their charge. Membranes that block or otherwise slow passage of certain substances are said to be “ selectively permeable ”. This property of membranes accounts for the phenomenon of osmosis. Water (the solvent), also moves across the membranes, and the movement of water across selectively permeable membranes is called osmosis . You can think of osmosis as a special form of diffusion, one occurring from a region of higher water concentration to a region of lower water concentration.
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This note was uploaded on 11/22/2009 for the course BIO SCI 150 taught by Professor Geraldbergstrom during the Fall '08 term at Wisconsin Milwaukee.

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LabManual_Osmosis - Foundation of Biological Sciences I Biological Membranes Osmosis 1 Biological Membranes Osmosis Hbel G 2008 Biological

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