Structure of Membranes - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 10

shared equally. This end of the molecule has a charge and is therefore attracted to
water. Biomembranes also compartmentalize areas of different metabolic activity in
the cell, and regulate the flow into and out of cells and cell compartments. Finally,
membranes are sites of key biochemical reactions.
The key functions of cell membranes can be summarized as follows:

• They are a selectively permeable barrier between two predominantly aqueous

compartments.

• They allow compartmentalization of the various structures in the cell.

• They enable the formation of a stable and fluid medium for reactions that are

catalyzed.

• They provide a flexible boundary between the cell or an organelle and its surround-

ing medium.

• They maintain an electric potential difference, participate in signal transmission

to the actin cycloskeleton (via integrins), and provide adhesion forces for the cells
to their substrates (controlled by membrane elasticity).

• They enable mass transport (via ion channels).

The fluid mosaic model of Singer and Nicolson [ 23 ] views the membrane as
a fluid bilayer of amphipathic complex lipids with proteins embedded in it and
spanning it. The relative abundance of proteins in a membrane varies from species
to species, and it correlates with metabolic activity. For example, the mitochondrial
wall contains large amounts of protein (52-76 %) and smaller amounts of lipids (24-
48 %), facilitating its high metabolic activity. Conversely, the inactive membrane of
the myelin sheath in neurons contains only 18 % proteins and 79 % lipids.
A double layer of phospholipid molecules with a variety of embedded proteins
makes up the plasma membrane of a cell (see Fig. 2.1 ). This plasma membrane does
not resemble the surface of a fluid or even the interface between two fluids. The
reason for this is that it has an essentially fixed surface area, i.e., there are only a
fixed number of phospholipid molecules and proteins which, when packed together,
make up the membrane. Each lipid molecule or protein has a preferred surface area
so, unlike the surface of a fluid, the plasma membrane is, for practical purposes,
inextensible.
The various components of membranes are subject to rapid movements. Rapid
lateral movement of lipids is characterized by a diffusion constant of approximately
10 − 8 cm 2 s, while those of proteins range between 10 − 10 and 10 − 12 cm 2 s. On the

other hand, flip-flop movements across the membrane are slow, of the order of 10 − 5 s.
Indeed, the phospholipids of the membrane may undergo a phase transition from a
gel phase to a liquid crystal phase. This may take place as a result of changing the
ambient temperature, external pressure, or even membrane composition.
A cell membrane regulates transport of materials and signals across it, between
internal and external regions with different physiological states. The barrier proper-
ties of membranes can be both specific and non-specific, that is, the mode of barrier
action is not identical in all cases; membrane constituents play important roles.

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