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Cellular membranes are mostly composed of lipids, but depending on which part of the body the cell is located, it can also contain a very large amount of protein. Membrane proteins have various functions. The major two, which I will talk about, are transport and receptors. Also, there are two major types of membrane proteins: integral proteins, which are located through the membrane itself (similar to a nail hammered into a sheet of wood); and the second type are peripheral proteins, which are located only on the surface of the membrane.

1) Transport:

Transporting molecules across a cell membrane is crucial to the survival of the cell and the entire organism. Some small particles may be small enough to cross the cell's membrane without the use of a transport protein. An example of this would be oxygen entering the red blood cells traveling through the alveoli (small sacs) in the lung. This type of transport is very simple; the concentration of oxygen in the alveoli is higher than the concentration in the red blood cell, so the oxygen simply diffuses into the blood. This, unfortunately, does not take place for all molecules required by the cell. Larger molecules, such a glucose (sugar), can only enter the cell through a membrane with the help of a carrier protein. A carrier protein is an integral protein that has an open end at the outside of the cell which a glucose molecule can fit into. When the glucose binds to the protein, the protein changes shape (called a "conformational change") which closes the end of the protein exposed to the outside of the cell, and opens the end of the protein exposed to the interior of the cell. The glucose molecule then simple exits the protein and is now inside the cell, ready to be used. Note that this process does not use any ATP and is therefore consider passive transport. This is because glucose is moving down a concentration gradient (there is a higher concentration of it outside the cell compared to inside).

If something is needed to be moved against the concentration gradient (there is a lower concentration outside the cell than inside) then ATP is required and this process is called active transport. For this, again, a transport protein is needed. In many cells, the sodium concentration is very high outside the cell and very low inside the cell, and potassium is low outside the cell and high inside the cell. In order to maintain these gradients, cells need to pump sodium out while pumping potassium in. Clearly, this is

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