The Cell Membrane
By: Greg Geibel
http://www.people.virginia.edu/~rjh9u/cellmemb.html
Structure
The fluid mosaic model of the membrane (shown above) shows the membrane is composed of a number of proteins which are similar to shifting tiles. The spaces between the tiles are filled with fluid-like phospholipids. The phospholipid consists of hydrophilic heads, which point towards the outside environment and the cytoplasm. The hydrophobic tails repel the water and point in. Thus, the phospholipids form a bilayer that acts like a barrier between the cell and the environment. The phospholipid bilayer also contains cholesterol, which makes the bilayer stronger, more flexible and more permeable. There are a number of important proteins in the plasma membrane which will be discussed later.
Function
The purpose of the membrane is to control what goes in and out of the cell. The items that go in are highly regulated. It also communicates with other cells for example with receptors on the surface or cell to cell adhesion. Proteins that are found in the bilayer are receptor proteins, which deal with communication, recognition proteins and transport proteins that regulate the movement of water and soluble molecules through the membrane. In order to regulate the transport of molecules, there are two types of proteins in the cell: carrier proteins and transport proteins. The two types of transport proteins are channel and carrier protein. Transport is either active or passive. Active transport is moving molecules against the concentration gradient and energy is required in the form of ATP. Passive transport is moving molecules down the concentration gradient and no energy is required. Examples of passive transport are diffusion, which moves from high concentration to low concentration and osmosis, which is the diffusion of water molecules. Finally, the cell membrane is important for the connections between cells. There are four different types of these connections. Desmosomes attach cells together like "glue". A tight junction consists of fusing the cells together. A gap junction consists of pairs of channels fused. Finally, plasmodesma consists of binding plants together.
Works with other Organisms
The cell membrane recieves signals through receptors from the outside environment and transmits a signal to organells within the cell. This process is extremely important because it regulates changes in gene regulation.
Bibliography
Teresa and Gerald Audesirik. Biology, Life on Earth. Prentice Hall, New Jersey, 1999.
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