端木·宇 2008-6-19 22:25
The Cell Membrane
The cells of all organisms, prokaryotic and eukaryotic alike, aresurrounded by a thin sheet called the cell membrane. This barrier keepscellular materials in and foreign objects out. The membrane is key tothe life of the cell. By regulating what gets into and out of the cell,the membrane maintains the proper chemical composition, which iscrucial to the life processes the cell carries out.
[b] Structure of the Cell Membrane[/b]
The cell membrane is made up of two sheets of special fat molecules called [b]phospholipids[/b], placed on top of each other.
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This arrangement is known as a [b]phospholipid bilayer[/b].Phospholipid molecules naturally arrange in bilayers because they havea unique structure. The long chains of carbon and hydrogen that formthe tail of this molecule do not dissolve in water; they are said to behydrophobic or “water fearing.” The hydrophilic phosphorous heads areattracted to water. Forming a bilayer satisfies the water preferencesof both the “heads” and “tails” of phospholipids: the hydrophilic headsface the watery regions inside and outside the cell, and thehydrophobic tails face each other in a water-free junction. The bilayerforms spontaneously because this situation is so favorable.
[i] The Fluid Mosaic Model[/i]
Phospholipids form the fundamental structureof the cell membrane, but they are not the only substance found there.According to the fluid-mosaic model of the cell membrane, specialproteins called membrane proteins float in the phospholipid bilayerlike icebergs in a sea.
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The sea of phospholipid molecules andgatekeeper membrane proteins is in constant motion. The membrane’sfluidity keeps the cell from fracturing when placed under strain.
[b] Transport Through the Cell Membrane[/b]
The most important property of the cellmembrane is its selective permeability: some substances can passthrough it freely, but others cannot. Small and nonpolar (hydrophobic)molecules can freely pass through the membrane, but charged ions andlarge molecules such as proteins and sugars are barred passage. Theselective permeability of the cell membrane allows a cell to maintainits internal composition at necessary levels.
Molecules that can pass freely through themembrane follow concentration gradients, moving from the higherconcentration area to the region of lower concentration. Theseprocesses take no energy and are called [b]passive transport[/b]. Themolecules that cannot pass freely across the phospholipid bilayer canbe carried across the membrane in various processes that require energyand are therefore called [b]active transport[/b].
[u]Passive Transport[/u]
There are three main types of passivetransport: diffusion, facilitated diffusion, and osmosis. In fact,osmosis is simply the term given to the diffusion of water.
[i] Diffusion[/i]
In the absence of other forces, substancesdissolved in water move naturally from areas where they are abundant toareas where they are scarce—a process known as diffusion. If there is ahigher concentration of carbon dioxide gas dissolved in the waterinside the cell than in the water outside the cell, carbon dioxide willnaturally flow out from the cell until its distribution is balanced,without any energy required from the cell.
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Nonpolar and small polar molecules can passthrough the cell membrane, so they diffuse across it in response toconcentration gradients. Carbon dioxide and oxygen are two moleculesthat undergo this simple diffusion through the membrane.
The simple diffusion of water is known as [b]osmosis[/b].Because water is a small polar molecule, it undergoes simple diffusion.SAT II Biology problems on osmosis can be tricky: water moves fromareas where it is in high concentration to areas where it is in lowconcentration. Remember, however, that water is found in lowconcentrations in places where there are many dissolved substances,called solutes. Therefore, water moves from places where there are fewdissolved substances (known as [b]hypotonic[/b] solutions) to places where there are many dissolved substances ([b]hypertonic[/b] solutions). An [b]isotonic[/b]solution is one in which the concentration is the same as that foundinside a cell, meaning osmotic pressure in both sides is equal.
Immersing cells in unusually hypotonic orhypertonic solutions can be disastrous: water can rush into cells inhypotonic conditions, causing them to fill up so fast that they burst.To combat this possibility, many cells that need to survive infreshwater environments possess contractile vacuoles to pump out excesswater.
[i] Facilitated Diffusion[/i]
Certain compounds important to thefunctioning of the cell, such as ions, cannot enter the cell throughsimple diffusion because they cannot pass through the cell membrane. Aswith water, these substances “want” to enter the cell if theconcentration gradient demands it. For that reason, cells havedeveloped a way for such compounds to bypass the cell membrane and flowinto the cell on the basis of concentration. The cell has proteinchannels through the phospholipid membrane. The channels can open andclose based on protein membranes. When closed, nothing can get through.When open, the protein channels allow compounds to pass through alongthe concentration gradient, which is diffusion.
[i] Active Transport[/i]
Quite often, cells have to transport a substance across the cell membrane [i]against[/i]the normal concentration gradient. In these cases, cells use anotherclass of membrane proteins. Instead of relying on diffusion, theseproteins actively pump compounds in the direction the cell wants themto go, a process that requires energy. Cells can turn active transporton and off as needed.
[i] Endocytosis and Exocytosis[/i]
Cells use yet another type of transport tomove large particles through the cell membrane. In exocytosis, wasteproducts that need to be removed from the cell are placed in vesiclesthat then fuse with the cell membrane, releasing their contents intothe space outside the cell. Endocytosis is the opposite of exocytosis:the cell membrane engulfs a substance the cell needs to import and thenpinches off into a vesicle that is inside the cell.
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There are two kinds of endocytosis: in [b]phagocytosis[/b] the cell takes in large solid food particles that it then digests. In [b]pinocytosis[/b], the cell takes in drops of cellular fluid containing dissolved nutrients.
