Biology Reference
In-Depth Information
B. THE LIPID BILAYER MEMBRANE
Our current understanding of membrane structure, the Fluid Mosaic Model, is based on
a lipid bilayer. A lipid bilayer is essentially two oil/water interface monolayers, (described
above), placed back to back. The first biological membrane to be appreciated, studied, and
characterized is the plasma membrane. We now know that every cell currently living on planet
Earth and probably every cell that has ever existed on our planet is surrounded by a plasma
membrane. The plasma membrane is the barrier that separates the cell interior from the rest
of the external environment. The study of membranes began indirectly in 1665 with Robert
Hooke's discovery of the cork cell and subsequently followed a long and tortuous path. Out-
lined below are only a few early but important steps in this process.
William Hewson: (1773)
The earliest experiments on the plasma membrane were osmotic studies on red blood
cells. The first important investigator was the under-appreciated Englishman William Hew-
son, (
Figure 2.6
) a good friend and colleague of Benjamin Franklin. In fact, in 1770 Franklin
sponsored Hewson for Fellow of the Royal Society. Hewson's work involved microscopic
studies of erythrocyte shape. He noted that upon addition of water, erythrocytes changed
from flat (discord) to spherical (globular). With too much water erythrocyte simply dissolved,
a process now known as hemolysis. Hewson was the first to show osmotic swelling and
shrinking of erythrocytes and from this deduced the existence of a cell (plasma) membrane
as a structure surrounding a liquid protoplasm
[11]
. Unfortunately, however, this important
work on membranes was largely ignored. Hewson was highly respected and made major
contributions to several fields. He has sometimes been referred to as the 'father of hema-
tology'. He is given credit for isolating fibrin and in defining the lymphatic system for which
he was given the Copley medal in 1769.
In 1998 Hewson received renewed interest, but for an entirely different reason. While in
London (1757
1775), Benjamin Franklin stayed at a house at 36 Craven
Street, which is now the home of the Benjamin Franklin House Museum. In 1772 Hewson
also ran an anatomy school at this same location. In 1998, workmen restoring the Franklin
Museum dug up the remains of six children and four adults hidden below the home. Most
of the bones showed signs of having been 'dissected, sawn or cut'. One skull had been drilled
with several holes. The bones were from the same years Franklin lived there. The question
arose, could this have been a very old crime scene? And what did Benjamin Franklin
know about the happenings?
1762 and 1764
e
e
C. H. Schultz: (1836)
Although membranes are too thin to be seen directly by the human eye, or even by light
microscopy (Chapter 1), their presence can be detected as an invisible barrier by plasmolysis
or by staining the outer membrane surface with various dyes. In 1836 C.H. Schultz used
iodine to visualize the erythrocyte plasma membrane. He was then able to estimate the eryth-
rocyte membrane thickness to be about 220
˚
, only a little larger than the currently accepted
measurement of ~50
100
˚
. The importance of this very early date in membrane studies
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