ABSTRACT
A living cell is essentially a bag filled with charged objects. Besides
the charged macromolecules (DNA, RNA and proteins) and the
membranes (that also contain some charged lipids) there are lots
of small ions. These ions are mostly cations, positively charged ions,
compensating the overall negative charges of the macromolecules:
5-15mM sodium ions, Na+, 140 mM potassium ions, K+, as well as smaller amounts of divalent ions, 0.5mM magnesium, Mg2+, and 10−7 mM calcium, Ca2+. Here mM stands for millimolar, 10−3
moles of particles per liter. There are also small anions, mainly
5-15 mM chloride ions, Cl−. We know the forces between those charged object; in fact, basic electrostatics is even taught in school.
But even if we consider, for simplicity, themacromolecules as fixed in
space, a cell contains a huge number of mobile small ions that move
according to the electrostatic forces acting on them which in turn
modifies the fields around them and so on. This problem is far too
complicated to allow an exact treatment. There is no straightforward
statistical physics approach that can treat all kinds of charge-charge
interactions occurring inside a cell. In other words, we have not yet
a good handle on electrostatics, the major interaction force between
molecules in the cell. And that despite many years of hard work. The
current chapter tries to give you a feeling of what we understand
well and what not. Hopefully, this makes you rather critical when
you encounter in the future electrostatics problems in biophysics.
