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Sodium-potassium pumps and calcium ion channels are examples of transmembrane proteins. This means that they are proteins that span through the cellular membrane, effectively connecting the inside of the cell with the outside of the cell. These transmembrane proteins have evolved over time to allow very specific chemicals to pass through the cell membrane. These particular proteins allow polar ions to pass through since these type of species would normally not be able to pass through the membrane.
Transmembrane proteins are composed of several subunits, but the portion spanning the actual membrane consists mostly of alpha helices. Alpha helices are a common peptide type of secondary structure that consist of right handed spiral coils that form a linear tube type of shape with a network of stabilizing hydrogen bonds within the coils. Alpha helices are popular for transmembrane proteins because they can shift slightly to accommodate allowing the ions to pass though the protein, thus passing from one side of the cell membrane to the other side.
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