Cell Membrane 2

The lipid bilayer of the cell membrane which I described in a previous post allows only a few types of molecules through it, such as water. Because of the biochemical qualities of the interior fat layer of the membrane, charged particles have more trouble getting through than neutral ones. Water is able to go through with relative ease. This is not fully understood, since it does have some polarity. In any case, the membrane needs to have other ways to get ions, atoms and molecules from the outside in and vice versa.

It does this by specific proteins which regulate these things individually or in sets. Above is a picture of a protein complex that regulates potassium, (link HERE) from the Research Collaboratory for Structural Bioinformatics Protein Data Bank (RCSB PDB). The gray band in the picture is the cell membrane. Several proteins sit right in the membrane and act as a filter against everything but potassium. Then, the channels below leave potassium in or not depending on the cell's needs. The levels of ions such as potassium and sodium are finely regulated to fit to the metabolic needs of the cell. There are large differences between the levels of ions on the inside and outside of the cell.

The pictures at left from RCSB PDB are channels as viewed from the top. (2019 Update: links were retrieved April 2009. The entry at left is now HERE and is illustrated in a different manor.)

There are various channels depending on what is transported.