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Functions Of Cell Membranes.

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Functions Of Cell Membranes A cell membrane other wise known as a Plasma Membrane is a thin molecular layer that surrounds all living cells. The plasma membrane separates the cell from its surroundings, protects it from changes in the chemical and physical environment, and regulates the traffic of molecules into and out of the cell. The cell membrane moves materials in and out of the cell in several ways. Diffusion - Certain small molecules are able to move through the membrane by diffusion. This is a process in which molecules randomly move from an area where there are many of them (high concentration) into an area where there are fewer. Diffusion is a fairly slow process since there is no energy used to push the molecules, and it only works if the molecules are small enough to pass through tiny pores in the membrane. How the molecules pass through the membrane is also affected by whether they are fat soluble or water soluble. Some of the membrane proteins form channels that help water soluble molecules pass through the hydrophobic lipid interior of the membrane. ...read more.


A cell uses two methods to move such substances from one side of the plasma membrane to another, known as passive transport and active transport. Both of these processes involve proteins in the plasma membrane. Passive transport is accomplished by diffusion, the spontaneous movement of a substance from a region of greater concentration to a region of lesser concentration. The difference between the concentration of a substance in two different areas is known as a concentration gradient. Diffusion moves molecules down a concentration gradient in a manner that does not require the cell to expend energy. Water, oxygen, carbon dioxide, and a few other small molecules diffuse directly across the plasma membrane by passing between phospholipid molecules. Substances that cannot pass directly through the plasma membrane diffuse into or out of cells with the aid of hollow, channel-like proteins in a process known as facilitated diffusion. These channel proteins are shaped so that only one substance, or a small group of closely related substances, can pass through each type of protein. This specificity enables a cell to control precisely the molecules that travel in and out of the cell. ...read more.


The second mechanism, exocytosis, is a reversal of endocytosis. A sac inside the cell containing proteins and other molecules moves toward the outer edge of the cell until it touches the plasma membrane. The membrane of the sac then joins with the plasma membrane, and the contents of the sac are released from the cell. Most of the proteins released by animal cells, such as hormones and antibodies, exit the cells where they are made through exocytosis. In multicellular organisms, the plasma membrane also plays a critical role in communication between cells. Proteins embedded in the plasma membrane act as receptors, binding to hormones and other molecules sent as signals from other cells. In animal cells, certain membrane proteins also act as markers that help the immune system distinguish the body's own cells from foreign cells. These marker proteins help trigger the immune reaction that protects humans and other animals from disease-causing organisms such as bacteria, viruses, and fungi. These markers also play a role in the rejection of transplanted tissues and organs. In certain types of cells, the plasma membrane has a wide variety of additional functions. Some membrane proteins are involved in holding neighboring cells together. In bacteria, plasma membrane proteins participate in photosynthesis and other reactions supplying the cell with energy. ...read more.

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