Biology Reference
In-Depth Information
It is evident that living cells, and therefore the membrane that surrounds them, are very
small, making their study all the more difficult. It is now believed that if living cells are found
elsewhere in the universe, they too will be small. The size of a living cell is limited by
a universal constant, the rate of solute diffusion in water.
1
Diffusion rate
¼
K
ð
T
=
m
Þ
2
Where K is a constant that is related to the size of the solute, T is the temperature in
o
K and
m is the molecular weight of the solute. Even diffusion of water (a tiny solute) in water is
quite low:
Diffusion Distance for Water in Water
Time
Distance
0.1 sec
10
m
m
1 sec
100
m
m
100 sec
1 mm
Diffusion works well over very short distances but is far too slow to be effective over long
distances. This limits the size of cells. Diffusion is further discussed in Chapter 9. Coupled
with the small sizes associated with membranes are the rapid times involved in membrane
dynamics. Size domains spanning the
mto
˚
sto
ns, necessitates use of combinations of biophysical instrumentation discussed in Chapter 14.
m
range and time domains ranging from
m
E. BASIC COMPOSITION OF MEMBRANES
A striking feature of biological membranes is that while at first glance all biological
membranes may appear to be quite similar in size, structure and basic composition, they
support very different functions. The conundrum of how membranes can be similar, yet
different, forms the central theme of this topic. The structural similarity of all membranes
is to a large extent due to the polar lipids that comprise the lipid bilayer (Chapters 4 and
5) while biochemical diversity is due to the proteins that reside in the bilayer (Chapter 6).
As a general rule, the more biochemical functions a particular membrane supports, the
higher its protein content will be.
Table 1.2
demonstrates this concept. The myelin sheath
TABLE 1.2
General Composition, Expressed as Percent by Weight, of Three
Very Different Mammalian Membranes.
Membrane
Lipid
Protein
Myelin Sheath
80%
20%
Plasma Membrane
50%
50%
Mitochondrial
Inner Membrane
25%
75%
