Structure of Membranes - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 11

Trans-membrane osmotic pressure, an electrostatic imbalance between the outer and
inner regions, various membrane constituents—all of these factors play important
independent and collective roles in characterizing the membrane's barrier properties.
The barrier properties are due to a combination of different physical effects, (e.g.,
electrical, mechanical, geometrical etc.) and chemical effects (e.g., chemical species
concentrations, the value of pH, etc). Due to the various dynamics continuously
occurring inside and outside the membrane, its barrier properties also are subject to
change. Furthermore, they are strongly time-dependent functions. Different phys-
iological conditions cause perturbations in the membrane barrier properties, often
temporarily—but there can also be permanent changes occurring mainly due to var-
ious chronic diseases, aging, and other physiological changes on a longer timescale.
A single observation of the membrane using a specific technique cannot always
find key clues, since effects originate from many sources. Traditional biological
approaches, therefore, need to go beyond simple observations and descriptive char-
acterization, and instead explore physical, chemical, as well as engineering technolo-
gies to be used in membrane science. All these techniques, when combined, have
improved our understanding of the membrane seen as just a barrier. The applications
of science and technology to date have made it possible to enable tracing the origins
of many static and dynamic processes taking place inside and near membranes. Such
approaches help us not only understand the membrane itself, but may also help find
avenues for further development of scientific approaches to improved drug discovery
using membrane-based technology. The reader will be exposed to this crucial issue
via the chapters that follow.
2.2 Membrane Constituents
Lipids are the primary components of a biological membrane. Two layers of lipids
make a bilayer, and the lipids align in such a way that the head groups point in outward
directions in both lipid monolayers. Electrolytes and water molecules are expelled to
the exterior of the membrane, while the inner membrane layer stays inside a cellular
compartment.
The membrane structure looks deceptively simple, but complex static and dynam-
ical phenomena take place in this crucial cellular component. We describe many of
these phenomena in later chapters. Here we only wish to mention that the mem-
brane constituents such as proteins and hydrocarbons, together with lipids contin-
uously form, break, and translocate between different complex structures (e.g., ion
channels, defects, etc.) inside and across membranes, which can be responsible for
changing the membrane's insulating properties. As a result, materials such as ions,
water molecules, other small molecules, etc. may pass through membranes.