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Unformatted text preview: Activity 2A: MOVEMENT OF MOLECULES ACROSS MEMBRANES The cell membrane is a selectivel y permeable membrane that permits certain molecules to pass through it , while other s require carriers or transporters to do so. It is a lipid bilayer , composed mostly of phospholipids and protein mol e cules , that regulate the entry and exit of molecules in and out of the cell . Small molecules that are lipid soluble, such as Hg ions , O 2 , and C0 2 , can pass through the membrane easily. However , other molecules , such as water, are not lipid soluble but can still penetrate the membrane because they pass through pores which are passageways formed by protein molecules (Mader 1990). Generally, small molecules pass through the membrane by diffusion. Diffusion is the movement of molecules from an area of greater concentration to an area of lesser concentration until equilibrium is reached. Osmosis involves the diffusion of water through a membrane. It happens whenever there is unequal concentration of water on either side of the selectively permeable membrane. If one side is highly concentrated and the solute (proteins or large molecules) involved cannot cross the membrane , then there will be a net movement of water through the membrane from the area of lower concentration to the area of higher concentration until equilibrium is reached . Once the water enters, a " back pressure," called osmotic pressure , builds up , thereby preventing further net gain of w ater ....
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This note was uploaded on 07/19/2011 for the course SC 101 taught by Professor David during the Fall '10 term at American Intl. University.
- Fall '10