Why Bacteria Hate Magnets
Bacteria are single-celled organisms that are surrounded by phospholipid membranes.
The purpose of the membrane is two-fold. First, it physically contains a cell's
organelles and other cellular machinery (proteins) that are needed for survival. Second,
it maintains a separation between the intracellular and extracellular salt solutions in
which the cells exist. A simple diagram is shown below which illustrates the make-up of
these salt solutions. Note that the concentration of potassium (K+) ions is higher inside
the cell than outside, and that the opposite is true of sodium (Na+) and chloride (C1-)
ions. This separation of ions across the bacterial cell wall is essential, and is
maintained by the impermeable phospholipid membrane. If all of the charges (+ and -) on
the inside and the outside of the cell are summed (separately), you would find that there
is a net negative charge on the membrane's intracellular surface. In other words, the
inside of the cell is more negative than the outside of the cell.
Ion Chammels and the
regulation of cellular pH
As stated earlier, different channel proteins transport different ions across biological
membranes. On such ion is the proton, or positively chargen hydrogen atom (H+). The flow
of protons through ion channels in bacterial cell membranes is used to control the pH of
the intracellular solution. The regulation of cellular pH is crucial for the survival of
biological cells. This is true because if the pH is too high or too low, the structural
integrity of intracellular proteins is compromised. This, in turn, makes the protein
incapable of performing its normal duties, most of which involve catalyzing cellular
reactions that are needed to keep the cell alive. The bottom line is that a cell that is
unable to control its pH is a dead cell.
The pH of any solution (including biological ones) is directly related to the
concentration of protons, or positively charged hydrogen atoms, in the solution. The
higher the concentration of (H+), the lower the pH, and vice versa. A pH of 7 is neutral,
and most cells cannot tolerate having an intracellular pH that is very far from this
value. Therefore, bacteria (and other organisms) have developed ways of controlling
their pH. This occurs in one of two ways. First, there are intracellular molecules
called buffers that bind protons if their concentration gets too high and release protons
if their concentrations get too low. The buffer molecules are fine-tuned, however, and are
easily saturated. When this happens, (when the concentration of protons gets very high)
they can simply be transferred accross the cell membrane via ion channels.
The effect of magnets on ion channel behavior
As we discussed earlier, the direction of flow of ions through protein channels is
affected by both the electrical and chemical potential that exists across the cell
membrane. If bacteria, for example, are placed in an environment where large electrical
fields exist, the electrical potential across their cellular membrance will be affected.
The presence of a strong magnetic field is a good example of such an environment. The
polarized regions of a large magnet will create highly unphysiological electrical
potentials in the bacteria's environment. This potential will overwhelm any existing
potentials in these very small cells, and they will no longer have control over the
movement of ions across their membranes.
The separations of charges across the membrane creates two separate driving forces of the
ions. First, because the inside of the cell is more negatively charged than the outside,
there is an electrical driving force. In this case, if the membrane was punctured,
positively charged ions (cations) would be attracted into the cell and negatively charged
ions (anions) would be repelled from the interior of the cell. Second, the separation of
the charge creates a chemical driving force. In this case, ions would want to flow through
the puncture down their concentration gradient. For example, both sodium and chloride ions
would flow from outside (where they are highly concentrated) to inside the cell (where
their concentration is lower). The opposite is true of potassium ions which are more
concentrated inside the cell.
Of course, movement of the ions across bacterial membranes does not occur
via gaping holes. Rather, it ocurs with the aid of proteins that are embedded in the
cell membrane. These proteins span the entire membrane, and thus face the extracellular
solution on one side of the cell and the intracellular solution on the other. The proteins
exist in both "close" and "open" conformations, and the movement of
the ions between these two states is regulated by the bacterial cell. when closed, no ions
are allowed through the membrane. When the protein is "open", it forms a small
cylindrical hole in the membrane through which ions can pass. Usually, cations and
anions flow through different protein channels. Also, some proteins are able to select
among different ions of a particular charge. For example, some channels allow sodium but
not potassium ions to pass through their pore. The diagram to the right illustrates the
flow of ions through protein channels.
The flow of ions across cell membranes is coupled to many important cellular processes,
therefore, bacterial cells become very "sick" when they lose the ability to
regulate the ionic currents through protein channels. One of the deadliest scenarios is
when the flow of protons is disturbed. In this case, the destruction of the protons'
electrochemical gradient equals the destruction of the ability to expel them from the
cell. When the hydrogen ion concentration rises, then, the cell cannot release the
ions to the environment, and the pH is lowered to a level that is not tolerable. Death
ensures.
