Structure of Membranes - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 12

Fig. 2.1 A simplified membrane structure is presented here [ 2 ]. Two lipid monolayers are the pri-
mary components of a membrane. Near lipid head groups ( light blue color ) are seen the cholesterol
molecules residing in the hydrophobic region. Globular proteins ( in red ) are shown to reside across
the membrane. Alpha-helical proteins have both hydrophilic and hydrophobic parts. Membrane
curvature and thickness change, which is also schematically diagrammed here. For simplicity, we
did not include the presence of different ion channels, or other complicated membrane protein
structures
2.3 Characteristics of Membranes
2.3.1 Physical Characteristics of Membranes
A membrane's primary role is to serve as a barrier as well as a transporter. It naturally
serves as a compartment to ensure a controlled transport of material and information
between the cell's inner and outer regions. A membrane maintains a constant osmotic
pressure profile and a fairly constant (average) geometric thickness, which excludes
other material such as electrolytes, water molecules, etc. from its vicinity. A specific
back-to-back arrangement of lipids of different types, a possible presence of choles-
terol, various types of membrane proteins and hydrocarbons, etc., taken together
form the structure of a layer membrane with a certain approximately constant thick-
ness. Membranes with a well-organized structure are characterized by observable
geometric and physical properties, e.g., liquid crystalline structure [ 15 ], mechanical
stiffness [ 1 ], capacitive effects [ 1 ], etc.
A membrane's mechanical rigidity is one of the most fundamental physical prop-
erties that have been investigated (Fig. 2.1 ). Suppose we increase the osmotic pressure
of a cell. The cell will try to swell but this is prevented because the surface area of
the plasma membrane is nearly fixed, i.e., an elastic stress will be built up inside
the membrane and if this is too great the cell will burst—a condition called lysis.