Introduction - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 4

Both prokaryotic and eukaryotic cells have membranes which primarily separate
the interior of a cell from its exterior, selectively regulate the movement of molecules
across them, and most importantly, maintain an electric potential difference between
the interior of a cell and its exterior. Membrane properties seem to be robust and
simple but they reflect the states of a membrane which are, in turn, results of many
very complicated processes taking place inside, across, and outside membranes.
Understanding of those processes requires thorough analysis of the membrane con-
stituents, electrical environment inside and outside the cell, mechanical membrane
properties, dynamical processes taking place in the cell, and many specific as well
as non-specific effects due to sources inside and outside the cell.
The fluid contents of a cell are known as the cytoplasm. The cytoplasm is
hugely important because it provides the medium in which fundamental biophysical
processes such as cellular respiration take place. Its properties are somewhat different
than those of dilute aqueous solutions. The contents must be accurately known for in
vitro studies of enzymatic reactions, protein synthesis, and other cellular activities.
Typical constituents of the cytoplasm are ionic and bio-molecular. Most of the trace
ions are positively charged; however, the cytoplasm does not have an overall electric
charge, thus the difference is made up of the other constituents such as proteins,
bicarbonate (HCO 3 ), phosphate (PO 3 4 ), and other ions which are for the most part
negatively charged, a few of which are significantly electronegative.
Most cells maintain a neutral pH and their dry matter is composed of at least 50 %
protein. The remaining dry material consists of nucleic acids, trace ions, lipids, and
carbohydrates. A few metallic ions are found which are required for incorporation
into metallo-proteins but these ions such as iron(II) (Fe 2 + ) are typically found in
nano-molar concentrations.
There is experimental evidence for the existence of two phases of the cytoplasm.
These are the so-called liquid and solid phases, sol and gel , respectively. In the solid
phase, the major constituents of the cell are rendered immobile while in the liquid
phase, the cytoplasm's viscosity does not differ significantly from water. Diffusion
in the cytoplasm is affected mainly by macromolecular crowding. In the solid phase,
diffusion is slowed by a factor of three relative to diffusive movement in water. Such
properties of the cytoplasm seem to be regulated in some sense by the cytoskele-
ton, but the manner in which this regulation is accomplished is largely unclear. It is
believed that it involves the tangling and detangling of a mesh of various protein
filaments. However, once the cell has acted to organize itself, the transition to a solid
phase can allow it to expend relatively minimal energy to maintain its organization.
Contrary to early perceptions, the cytoplasm is not a viscous soup-like amorphous
substance but a highly organized, multicomponent, dynamic network of intercon-
nected protein polymers suspended in a dielectrically polar liquid medium.
A variety of solute molecules are contained within cells. The cellular fluid
(cytosol) has a chemical composition of 140 mM K + ,12mMNa + ,4mMCl − , and
148 mM A − where 1 mM stands for a concentration of 10 − 3 mol/L. The symbol A
stands for protein. Cell walls are semipermeable membranes which permit the trans-
port of water easily but not solute molecules. We can apply the osmotic pressure