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Transcript
Cell Membrane – Regulates what enters and leaves the cell and also provides protection and support Nearly all cell membranes are composed of a double-layered sheet called a lipid bilayer. In addition to lipids, most cell membranes contain protein molecules that are embedded in the lipid bilayer. Carbohydrate molecules are attached to many of these proteins. Some of the proteins form channels and pumps that help move material across the cell membrane. Solution – mixture of two or more substances Solute – substances dissolved in the solution Concentration – mass of solute in a given volume of solution, or mass/volume If you dissolved 12 grams of salt in 3 liters of water, the concentration of the solution would be 12 g/3L or 4 g/L (grams per liter). Which solution has the higher concentration? ◦ 4 g/L or 2 g/L? The tendency of molecules of a substance (solute) to move from an area where they are more concentrated to an area where they are less concentrated Does not require energy When the concentration of the solute is the same throughout a system, the system has reached equilibrium. Solute Most membranes are selectively permeable, meaning that some substances can pass across them and others cannot. Water passes quite easily across most membranes, even though many solute molecules cannot. Osmosis is the result. It is the diffusion of water through a selectively permeable membrane. Does not require energy 1. There are more sugar molecules on the right side of the membrane than on the left side. That means that the concentration of water is lower on the right side than it is on the left. 2. The membrane is permeable to water, but not to sugar. This 3. As a result, there is a net means that water can movement of water from the cross the membrane in area of high concentration to both ways, but sugar the area of low concentration. cannot. Hypotonic – Solution has a lower solute concentration than cell -- Water moves into cell Isotonic– Concentration of solutes is same inside and outside cell – water moves into and out of cell Hypertonic – Solution has a higher solute concentration than cell -Water moves out of cell A few molecules, such as glucose, seem to pass through a cell membrane much more quickly than they should. How does this happen? ◦ Protein channels act as carriers, making it easy for certain molecules to pass. ◦ Red blood cells have membrane proteins with carrier channels that allow glucose to pass through them. Only glucose can pass through here. This is known as facilitated diffusion because the protein channels are said to facilitate, or help, the diffusion of glucose across the membrane. Does not require energy Cells must sometimes move materials in the opposite direction – against a concentration gradient – from low to high This is accomplished through active transport. Requires energy. Usually carried out by transport proteins or “pumps.” that are found in the cell membrane. Quiz Time! Yahoo! Yipee! You get to take a quiz! Yea! Yeah, man…this is exciting! www.cstephenmurray.com/onlinequizes/biology/cells/osmosisvsdiffusion.htm Many cells use proteins to move calcium, potassium, and sodium ions across cell membranes. Changes in protein shape, play an important role in the pumping process. A considerable portion of the energy used by cells in their daily activities is devoted to providing the energy to keep this form of active transport working. Endocytosis – process of taking material into the cell by means of infoldings, or pockets, of the cell membrane. The pocket that results breaks loose from the outer portion of the cell membrane and forms a vacuole within the cytoplasm. Large molecules, clumps of food, and even whole cells can be taken up this way. Two examples of endocytosis are phagocytosis and pinocytosis. Phagocytosis – cell eating ◦ Extensions of the cytoplasm surround a particle and package it within a food vacuole. The cell then engulfs it. ◦ Amoebas use this method of taking in food. ◦ Requires considerable amounts of energy Pinocytosis – ◦ Process of taking up liquid from surrounding environment ◦ Tiny pockets form along cell membrane, fill with liquid, and pinch off to form vacuoles within the cell. The membrane of the vacuole surrounding the material fuses with the cell membrane, forcing the contents out of the cell The removal of water by means of a contractile vacuole is one example of this kind of active transport