In the previous article, we ended with the impermeability of the cell membrane not allowing most elements and molecules in or out of the cell, resulting in certain cell death. In Part Two, we will take a closer look at the cell membrane to see how some of these hurdles have been overcome.
Some small non-polar molecules are able to pass through the bi-lipid cell membrane. These include oxygen (O2) and carbon dioxide (CO2). Other small uncharged polar molecules are also able to diffuse across the cell membrane. These include water, urea and ethanol. On the other hand, cell membranes are highly impermeable to charged molecules (ions), no matter how small they are.
Two key elements for cell survival are sodium ions (Na+) and potassium ions (K+) neither of which can pass through the cell membrane without help.
Cell membranes contain what are known as ion channels. These are small membrane bound proteins that have an affinity for specific ions. When they come in contact with their specific ion, it readily pulls the ion into the cell and releases it. Researchers have discovered over 100 different membrane bound ion channels and believe there are still more to be discovered.
A common ion channel is one that allows sodium ions to pass into the cell. The ion channels allow the sodium to pass into the cell but not to pass back out of the cell. It is a one-way passage. It wouldn’t take long for the amount of sodium ions to build up to a lethal concentration and kill the cell.
While the cell membrane contains an ion channel for sodium ions, it does not have channels for potassium ions which are also necessary for cell function. Without potassium ions, the cell will soon cease to function and will die.
Somehow, if evolution were true, the first cell (given it was able to overcome all the incredible odds so far) needed to create a way to get the potassium ions into the cell while at the same time eliminating the excess sodium ions.
Miraculously, there is a membrane bound protein that does just that. This protein molecule is known as a Plasma Membrane Sodium – Potassium Ion Pump and is found in virtually all animal cells. Its function is to pump potassium ions into the cell and excess sodium ions out of the cell.
The sodium-potassium ion pump protein extends beyond both the outer and inner layers of the cell membrane, folding itself into seven sections. When the seventh section or leg of the protein is folded inside the cell, it uses the energy of adenosine triphosphate (ATP) to pick up 3 sodium ions (1). Once the sodium ions are attached, the leg of the protein folds itself so that it is now outside the cell. Now, the protein no longer has an affinity for the sodium ions and releases them outside the cell (2). Now the protein develops an affinity for potassium ions and picks up 2 of them (3). Once the potassium ions attach to the leg of the protein, it causes the leg to fold and return to the inside of the cell. It now loses its affinity for the potassium ions and it releases them inside the cell (4) and the process then repeats itself over and over again.
An interesting side note is that molecular researchers are discovering that many proteins have multiple functions depending on their shape. Folded one way, the protein will perform one function and folded another way a different function.
Another membrane bound ion pump is the hydrogen ion (H+) pump found in many types of cell membranes including bacteria. This ion pump is very important for cell survival as the hydrogen ion or proton, is an important energy supply for a variety of cell functions. The hydrogen ion pump is also found in the membranes of mitochondria within the cell and are very important for the mitochondrial function as well.
Thoughts for consideration:
How did the first sodium ion channel evolve?
How did more the first sodium-potassium ion pump evolve?
What are the odds of both of these membrane bound protein structures evolving simultaneously with the evolution of the first cell?
What about the evolution of the more than 100 other protein ion channels?
Or are these marvelous molecular membrane structures a testament to a Creator God whose wisdom is infinite. Even when we study the microscopic world of molecular biology, we see God’s handiwork, which far exceeds anything man could ever think of. And if man could never have dreamed of the molecular details we see in the ‘simple’ cell, how can anyone believe that it happened by random chance and all at the same time?