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Cell Membrane Transport
One of the many functions of the cell membrane is to control the movement of ions and molecules into and out of the cell. But what exactly determines the ability of the molecule to pass through the membrane? It turns out that two factors influence the ease with which molecules move through and these factors are polarity and size. Since the cell membrane is predominantly non-polar, nonpolar molecules tend to move quite easily through the membrane while polar molecules find it very different. Generally, large and polar molecules cannot pass through the phospholipid bilayer and must be assisted by protein. There are three major modes of membrane transportation - passive diffusion, facilitated diffusion and active transport. Passive diffusion is the movement of molecules through the cell membrane without the use of energy or some sort of integral protein. Passive diffusion involves movement down the molecules electrochemical gradient. Cholesterol and water are two molecules that can move via passive diffusion. The movement of water via the cell membrane is also known as osmosis. In osmosis, the water moves from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration). In hypotonic solutions, the solute concentration inside the cell is higher than the outside and water will move into the cell and the cell will swell up. Under hypertonic conditions, the solute concentration outside the cell is higher than inside and water will move out of the cell, thereby shrinking the cell. Under isotonic conditions, there is no net movement of water because the solute concentrations are equal. The second form of membrane transport is passive diffusion (or passive transport), which involves the movement of large and/or polar molecules across a cell membrane down their concentration gradient via some type of integral (transport) protein. This does not require energy. The third mode of membrane transport is known as active transport. This type of transport involves the usage of integral proteins moving molecules agaisnt their concentration gradient and therefore it requires the use of adenosine triphosphate (ATP), an energy source.
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