The Cell Membrane: The Gate Keeper

The Cell Membrane as Gate Keeper

The cell membrane is very important to a cell because it controls what goes into and out of the cell. This is very important because the cell can't just go over to the supermarket when it needs some more oxygen for cellular respiration. The whole cell counts on the cell membrane to spot some oxygen passing by and get it into the cell. On the other hand, sometimes the cell wants to get rid of things, such as toxins that are made during synthesis, and the cell membrane is able to call them a taxi and escort them out of the cell.

The Chemistry of the Cell Membrane

How does this work? The cell membrane is made up of phospholipids and proteins. If you remember, lipids don't like water- they are hydrophobic- or water fearing. The phosphate group that is attached to them are hydrophilic or water loving. Since the whole cell membrane is surrounded by water on both the inside and the outside, this is very important. The lipid portion of the molecule tries to stay as far from the water as possible so it surrounds itself with outher lipids. This leaves the phosphate group facing out, towards the water. If you took a bunch of phospholipids and placed them in a beaker of water, they would assemble themselves spontaneously into a bi-layer membrane.

Well, what happens it that this bi-layer is a very good barrier between the inside and the outside of the cell. Since most molecules are either hydrophobic or hydrophilic, and since all molecules must pass through a hydrophobic and a hydrophilic region of the membrane, almost all of them have a bit of a problem getting through this cell membrane barrier. Only very small molecules such as O2, CO2 and H2O can pass through the membrane whenever they want. Most molecules need help from a group of very diverse proteins. Proteins are scattered throughout the plasma cell membrane like doormen. Each
protein is responsible for letting different molecules into or out of the cell.

Even Proteins Get Paid

Just like a doorman, sometimes the protein will not work without a little tip. They get paid in the form of ATP. When the protein will only help the molecule if it gets a tip, we call this active transport. This isn't because the protein is greedy, it just takes energy to get the molecule through the plasma cell membrane. When the protein does not use ATP, we call this passage passive transport. In this case, the protein allows the molecule through, but it does not take any energy to do so.