Structure of Membranes - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 14

Under normal conditions, the physical characteristics stay constant, but in the case
of abnormality (due to disease or disordered conditions) perturbations in the physical
characteristics are inevitable. Different order parameters in lipid membranes are also
sensitive to temperature and chemical compositions. Temperature-dependent alter-
ation of physical properties like lipid phase properties, different dynamical properties
etc. vary. The chemical composition-dependent capacitive effects of membranes also
vary. Here, we should mention that plasma membranes are considered to be excellent
insulators and dielectrics with a capacitance of the order of 1

F. Membrane capaci-
tance is a measure of the quantity of charge moving across unit area of the membrane
to produce unit change of the membrane potential (which will be discussed later in
detail).

µ

2.3.2 Biochemical Characteristics of Membranes

A membrane maintains a chemical or biochemical environment inside it which
differs from its interior and exterior environments. Since Robert Hooke's discovery
of a cell in 1665, many further developments have been made in the understanding of
the various properties of the interior and exterior environments of a cell. In the subcel-
lular compartments exist the membrane, cytoskeleton, genetic material, organelles,
etc. while structures outside the cell wall consist of capsules, flagella, fimbriae, etc.
These two structural arrangements exist in different physical and chemical environ-
ments. The membrane's interior region is very different and unique. In the absence
of water molecules and at low dielectric condition [ 20 ] it always maintains a gradient
in most of the biochemical characteristics, compared to the exterior. For example,
the membrane around peroxisomes shields the cell from peroxides. A membrane's
most important selective permeability characteristics couple with the geometric size,
charge, and chemical properties of the atoms and molecules attempting to cross it,
and determine whether the biochemical properties [ 4 ] of both together allow the
diffusion/permeabilization of the particles across the membranes. A membrane's
transport proteins also play a very important role to allow particles to cross through
the membranes. We discuss this issue in detail in Chap. 4 .

2.3.3 Electrical Characteristics of Membranes

Membrane Potential

First measurements of the electrical properties of cell membranes were made on red
blood cells by H. Fricke, and on sea urchin cells by K.S. Cole in 1937, and it was
found that membranes act as capacitors maintaining a potential difference between
oppositely charged surfaces composed mainly of phospholipids with proteins embed-
ded in them. A typical value of the capacitance per unit area C

cm 2

/ A is about 1 µ F /