Structure of the Cell Surface
By J. F. DANIELLI, PH.D., D.Sc., F.R.S.
A personal view of the growth of ideas about the structure of the plasma membrane.
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T HE PROBLEM of the basic structure of
the plasma membrane was essentially
solved by about 1940. At that time there was
adequate evidence that the membrane was a
bimolecular lipoid leaflet with adsorbed protein layers on both surfaces, and a basis had
been established for calculating the rates at
which different molecular species could permeate by simple diffusion.' Since then it has
been relatively simple to distinguish between
permeation by simple diffusion and permeation by more complex processes.
That permeation by more complex processes
was of great physiological importance was
also quite clearly established by 1940.* For
example, calculation showed that molecules
such as glucose, fats, and proteins could not
possibly enter cells by simple diffusion at the
rates necessary for physiological processes.
The characteristics of the more complex processes which had been established were:
1. Certain molecular species entered certain
cells far more rapidly than could be explained
by simple diffusion, e.g., chloride with red
cells; glucose with human red cells.
2. The Qlo values for the rates of the special permeation processes were abnormal.
3. The rates of the special permeations were
strikingly dependent on pH.
4. Many of the permeation mechanisms
could be specifically poisoned by certain cations, and the most toxic cations varied with
the mechanism considered.
5. Only a small part of the total surface
area (less than 1 per cent) was concerned in
any particular special mechanism.
6. The stereochemical requirements for per-
meation, e.g., by sugars, were very precise.
In addition, it was known that some, but
not all, of the special permeations involved an
energy contribution by the cell, i.e.:
7. In certain cases the direction of net
transport involved utilization of chemical free
energy for transport.
Many of the above characteristics of permeation were strikingly similar to those of
enzymes. This led Davson2 and me' (quite
independently) to suggest that the areas of
the cell surface concerned in these special
processes were essentially similar to the active
centers of enzymes. In accord with the prevalent biochemical view that the essential function of an enzyme was to reduce the energy
of activation for a specific chemical change,
the function of the active patches of a cell
membrane was to reduce the energy of activation for permeation of the membrane.
Once this picture of the plasma membrane
and its permeation had been established, attention for the subsequent twenty years
focussed mainly on the special mechanisms of
permeation. The main new generalizations
which have emerged are:
1. In many (but not all) cases the special
mechanisms become saturated at high concentrations of the transported species and exhibit
Michaelis-Menten kinetics.
2. Where two species of molecule are structurally related they may compete for the same
membrane sites.
3. Some physiologically active molecules,
such as insulin and the cardiac glycosides,
complete or activate special membrane-transport processes.
In order to assist in clarifying thought
about the nature of the permeation phenomenon in these special processes, I suggested in
1954 that the special permeation processes
From the Department of Zoology, Kings College,
London, England.
*For the sake of brevity, consideration of pinocytosis and related phenomena has been omitted.
Circulation, Volume XXVI, November 1962
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DANIELLT
1164
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which do not involve an energy contribution
by the cell should be called facilitated diffusion and should be clearly separated from
active-transport phenomena in which an energy contribution is essential.3 It is not by
any mneans clear whether the nmoleeular miechanism of facilitated diffusion is wholly different from that of active transport. An alternative possibility is that active transport
consists of a facilitated-diffusion unit, backed
up by an energy-transfer mechanism. That
the energy-transfer inechanism, or pump, may
be distinct from the facilitator has recently
been strongly emphasized by several authors.4
In general, two basic hypotheses have been
considered for the molecular mechanism of
the phenomena of special permeation. The
first, put forward by Osterhout around 1930,
is that there is a carrier or shuttle in the menmbrane.3 On the whole this theorv has been
favored by the majority of workers in this
field. An alternative hypothesis, that a stereochemically specific, hydrogen-bonding pore
extends through the thickness of the membrane, is equally compatible with most of the
available data.3 The main obstacle to permeation of the plasma membrane by molecules
such as glueose, for example, is the requiremnent that all the hydroffen bonds between a
glucose molecule and water must be broken
before the molecule can enter the lipoid layer
of the nembrane. The "carrier"' hypothesis
provides for this by postulatinfg the formation
of alternative hydrogen bonds with the earrier. The "pore" hypothesis provides for
this by postulating a series of hydrogen-bonding sites in the pore.
Very careful studies have been made of the
kinetics of permeation for some types of facilitated diffusion and of active transport, e.g..,
for sugars penetrating red cells, by Lefevre,6
Wilbrandt,8 9 and Widdas,7 and their colleagues. But in many cases it is impossible
to distinguish between the "pore' and "'carrier'" hypotheses by studies of kinetics.
Stein'0 11 has recentlv brought forward a new
hypothesis, which may well constitute a third
gfleneral alternative. This hypothesis is that
the active site in a membrane facilitates the
formation of dimers between a pair of molecules, so that they mutually satisfv one
another's hydrogen-bonding capacity. Though
recent, there is much kinetic evideniee in favor
of this hypothesis.
The study of the permeability of bacteria
has added additional data. Mitchell has adduced striking evidence for a liaison between
transport and assimilation in bacteria and, in
particular, has demonstrated that often the
enzyme responsible for the first step in metabolism of a compound is an integral part
of the bacterial plasma membrane.12 This has
led to the hypothesis that the elizymne in questioil meay be not only an enzyme but also the
transporting mechanism. On the other hand,
Cohen and Monod have demonstrated that
with galactosides, for instance, the inechanism
responsible for entry into the cell is "distinct
froin and independent of the homologous
metabolic enzymnes. "'s3 However this difference may be resolved, it is apparent that all
the different approaches to the special mechanisms postulate the presence of moleeules in
the plasma membrane whose function would
be partially or wholl- meaninl(ess other than
in a membrane.
Pharmacologists and physiologists have
long postulated the existence of other special
molecules in memubranes-the receptors. It is
possible, however, to put forward a theory
which, if correct, would provide an essential
unity for the main classes of special mechanisms we have considered. In the standard
representation of the conversioni of a substance Si to S2 in the presenice of a catalvst E,
Si
ES
S2
the directions of movement froni the transition state Es are not only formally significant
but also vectorially significant, representing
different directions of movement out of the
activated complex. MIitchell12 has pointed out
that, if a catalyst is appropriately oriented
in a iiembrane, this vectoral factor will result in the presence of S, on one side of the
membrane and S2 on the other.
Circulation, Volume XXVI, November
1962
SYMPOSIUM ON THE PLASMA MEMBRANE
feahave considered is passage through a transition state
in which an activated complex is formed between a diffusible small molecule component
S and a second component E, which may be
in a membrane. Then we can write
Now let
us
generalize that the
ture of all the special processes
enzyme:
S
ES
S1
ES
facilitated
diffusion:
common
we
S2
*
S2
Cd
activeS
transport:
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receptor:
~-S2
lq~
II=
1
2
S1ES
where d+ is an energy donor.
Thus, all these phenomena can be given a
similar formal representation. Does this mean
that, at the molecular level, all these phenomena are special examples or differentiations of
the same fundamental phenomenon? This
question, and indeed most of the problems of
the molecular biology of the cell membrane,
can only be resolved by isolation in a relatively pure state of the main molecular components responsible for each specific activity,
followed by precise study of their physical
and chemical properties. Just as the correct
picture of the general structure of the membrane arose from the separation of lipoidal
and protein components and their study in
isolation and in combination, so will the correct picture of the special structures of these
membranes arise from the isolation and detailed investigation of the special components.
The isolation of these special components,
however, constitutes a task of considerable
difficulty for two reasons. The first is that
even the membranes constitute only a small
fraction of the total cell volume, anid the special components of the membrane are likely
to be only minor fractions of the membrane.
The second major difficulty is that many of
the most important components cannot readily be identified except when actually in the
membrane of a cell. The main approaches to
Circulation, Volume XXVI, November 1962
1165
isolation of these membrane components appear to be as follows: (a) characterization
through special chemical reactivities; (b)
characterization by enzymic activities; (e)
characterization by antigenic properties; (d)
characterization by direct tagging.
I believe that one of the most successful
approaches will be the development of reagents which will permit direct tagging. Thus,
dinitrofluorobenzine, a specific inhibitor of
human red-cell glucose facilitator, dibenamine
and related compounds reacting with epinephrine receptors, and alkylating derivatives of
cardiac glycosides may be cited as providing
points of entry for an appropriate program.
In conclusion, I should like to comment on
one aspect of the nature of scientific endeavor.
In making an appreciation of work in this
field, it is interesting to observe that critical
discoveries, or hypotheses, commonly arise
more than once, independently. Within my
experience, there have been at least three such
instances in this field. First, the concept of
the lipoid layer of the plasma membrane as
a bimolecular leaflet was originally proposed
by Gorter and Grendel in a paper published
in a Dutch journal in 1925. I was quite
unaware of this work when putting forward
the same hypothesis in 1934,3 anid did not
encounter the work of Gorter and Grendel
until 1939. Then the concept that the membrane was composed also of adsorbed protein
layers arose in my mind in 1934 from studies
of the effect of proteins upon interfacial tension and was deduced at about the same time
by F. 0. Sehmitt from x-ray studies of myelin.'4 lastly, that the active centers in membranes responsible for facilitated diffusion
and the like are essentially enzyme-like was
deduced by Davson and me from studies of
facilitated diffusion and active transport in
red-cell membranes over the period 193919411 and was rediscovered quite independently by Monod and his colleagues in studies
on bacteria around 1955.
The conclusion to which one is forced is
that there is commonly a degree of inevitability about scientific discovery: within a
shortish period, if one person does not put
1166
DANIELLI
forward a hypothesis or discover a fact, another will. No scientist can afford to be arrogant about the degree of originality he
achieves.
References
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1. DAYSON, H., AND DANIELLI, J. F.: The Permeability of Natural Membranes. Cambridge, Cambridge University Press, 1943.
2. DAVSON, H., AND REINER, J. M.: An enzyme-like
factor in the cat erythrocyte membrane. J.
Cellular Comp. Physiol. 20: 325, 1942.
3. DANIELLI, J. F., AND D_AxSONx, H.: Theory of permeability of thin films. J. Cellular Comp.
Physiol. 5. 495, 1934.
4. HOKIN, M. R., AND HOKIN, L. E.: Synthesis of
phosphotidic acid by brain microsomes. J.
Biol. Chem. 234: 1381, 1959.
5. OSTERHOUT, W. J. V., KAMERLING, S. E., AND
STANLEY, W. M.: Kinetics of penetration. J.
Gen. Physiol. 17, 445, 469, 1934.
6. LEFEYvRE, P. G.: The evidence for active transport of monosaccharides across the red cell
membrane. Symposia Soc. Exp. Biol. 8: 118,
1954.
Sugar transport in the red blood cell: Strueture-activity relationships in substrates and
antagonists. Pharmacol. Rev. 13: 39, 1961.
7. WIDDAS, W. F.: Facilitated transfer of hexoses
across human erythrocyte membrane. J.
Physiol. 125: 163, 1954.
8. WILBRANDT, W.: Secretion and transport of nonelectrolytes. Symposia Soc. Exp. Biol. 8: 136,
1954.
9. WILBRANDY, W., AND ROSENBERG, T.: Enzymatic
processes in cell membrane penetration. Intern. Reev. Cytol. 1: 65, 1962.
10. STEIN, W. D.: Dimer formation and permeation
of red cells by glycerol. Nature 191: 352,
1961.
Dimer formation and permeation of red cells
by glueose. Nature 191: 1277, 1961.
11. STEIN, W. D., AND DANIELLI, J. F.: Structure
and function of red cell permeability. Disc.
Faraday Soc. 21: 238, 1956.
12. MITCHELL, P. D.: Structure and function in micro-organisms. Biochem. Soc. Symposia 16:
73, 1959.
Chemi-osmotic coupling. Nature 191: 144, 1961.
13. COHEN, G. N., AND MONOD, J.: Bacterial permeases. Bacteriol. Rev. 21: 169, 1957.
14. SCHMITT, F. O., AND BEAR, A.: The ultrastructure of the nerve axon sheath. Biol. Rev. 14:
27, 1939.
. . . we can be sure that it is by these hands and the
brain above them that man has come into his vast freedom.
He has literally taken the world with them, both because
he had them to use and because he had to use them.-Homer
Smith. Kamongo. New York, Viking Press, 1956.
Circulation, Volume XXVI, November
1962
Structure of the Cell Surface
J. F. DANIELLI
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Circulation. 1962;26:1163-1166
doi: 10.1161/01.CIR.26.5.1163
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