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Biological Membranes.

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Biological Membranes
By Werner Gross"]
Membranes are essential components of living cells. They act as barriers and pumps, as enzyme
supports, and also as a means of protection. Their chemical makeup from proteins, lipids, carbohydrates, ribonucleic acids, and water is well known, but there is as yet no generally accepted
modelfor their structure. I t is also by no means certain that all membranes have the same structural
plan.
1. Introduction
Membranes are structures of universal occurrence in living
nature. All cells, even those of the most primitive microorganisms, are enclosed by a plasma membrane. Since the
biochemical reactions taking place in the cytoplasm require
a constant environment, the intracellular space must be separated from the extracellular space. The cell organelles, such
as nuclei, mitochondria, and vacuoles, are also enclosed by
membranes. The reaction spaces (compartments) separated
in this way by membranes differ qualitatively and quantitatively in the substances that they contain; it must therefore be concluded that all membranes possess selective
permeability. However, further generalizations appear to
be possible only within limits, in view of the morphological
and chemical differences between the membranes.
The plasma membranes of liver cells and of erythrocytes
have been investigated in some depth, whereas less is known
at present about the membranes of the microorganisms.
The investigations on function and on structure are closely
linked. Results of functional studies led to certain requirements regarding the membrane architecture, and morphological results contributed to an understanding of functional processes. Morphological views and biochemical and
physiological studies have contributed in a similar manner
to the current picture of biological membranes. A number
of monographs and reviews on this rapidly growing field of
research have been published['- 13].
Physical methods of investigation (IR, NMR, and ESR
spectroscopic and X-ray analysis) have been used to a particularly large extent in the study of the components of
membranes and their interactions. Since lipids and proteins
do not usually give ESR signals, these membrane components must be labeled with synthetic organic radicals such
as 3-maleimido-2,2,5,5-tetramethylpyrrolidin-l-oxyl~'41
or
nitroxide" ' 1 . Spin labeling can be introduced into thiol and
amino groups of the proteins by means of the first of these
radicals, and into fatty acids and cholesterol by the second.
Increasing use is also being made of optical rotatory dispersion and circular dichroism measurements[161,and investigations based on differential thermal analysis[171have
shown that phase transformations occur in phospholipids
even in physiological temperature ranges[".
As a result
of these methods, important conclusions have recently
"'.
[*] Priv.-Doz. Dr. W. Gross
Institut fur vegetative Physiologie der Universitat
6 Frankfurt (Main), Ludwig-Rehn-Strasse 14 (Germany)
388
been drawn concerning the behavior of individual membrane components, interactions between similar and different molecules, and their phase. By refined preparation
methods and the use of new detergents for solubilization,
it has been possible to isolate membranes and membrane
fragments that yielded more reliable analytical results. This
has led to a considerable increase in knowledge of the
pattern of structural components in biological membranes.
Growing numbers of investigations are being carried out
on microorganisms, which offer the following advantages
over animal cells. (a) Some microorganisms have no intracellular structures that could lead to serious contamination
during the isolation of their plasma membrane, apart from
mesosomes. (b) The concentration of certain membrane
proteins can be varied by induction and repression of the
protein synthesis. This offers the possibility of labeling
these proteins by incorporation of radioactive amino acids
and isolating them'*']. (c) The composition of certain membranes can be modified experimentally. Thus a mutant of
E. coli that requires fatty acids for growth and that incorporates them into its membrane has been isolated[' '].
In yeast, the pattern of the mitochondria1 lipids depends
on the partial pressure of oxygen during growth[221.The
plasma membrane of Mycoplasma laidlawii is rich in
lipids[23];its composition is determined by the nature of
the culture medium. Extreme membrane compositions
have been found for halophilic bacteriaLz4!
2. Properties of Biological Membranes
It has been assumed since the turn of the century that a
submicroscopic lipid layer surrounds the cell and separates
the cytoplasm from the extracellular space. This assumption
is based in part on the work of O v e r t ~ n [on~ ~
the
] ability of
lipophilic and hydrophilic substances to penetrate into
the cell. The dependence of the rate of penetration of a
substance on its lipid solubility was later confirmed by the
classic experiments carried out by Collander[261on the cells
of Chara plants. Determinations of the erythrocyte surface
area and the area that was required by the lipids extracted
from the membrane showed that the area occupied by the
lipids was twice as large as the erythrocyte surface area.
Gorter and Grendel"'] concluded from their results that
the erythrocyte surface is formed by a bimolecular lipid
layer.
The views on the molecular arrangement of the lipids
within biological membranes were derived from what is
Angew. Chem. internat. Edit. 1 Vol. 10 (1971) 1 N o . 6
known about the behavior of such substances at interfaces.
During the 1930’s, Danielli and Dauson[281described a
membrane model in which the membrane is assumed to
consist of lipids and proteins. Their model was modified
more than two decades later by Robertson‘291on the basis
of electron-microscopic observations (concept of the “unit
membrane”).
The surface tensions found for cells in aqueous media are
small (around 1 erg/cm2). The surface tension of a droplet
of fat is 70 times as great. Absorption of proteins on such
droplets of fat causes a considerable decrease in the surface
tension, so that it seems reasonable to assume that proteins
are present on the cell surface. Though this is correct, such
a conclusion does not necessarily follow. A bimolecular
lipid layer should behave differently from a droplet of fat,
since compensation of the opposing tensions occurs in a
bimolecular layer, and the difference may be very small[301.
Since the surface tension is not constant, but depends on
the pressure, elastic properties can be ascribed to the cell
surface. This elasticity points to the presence of proteins,
since a pure lipid layer would not be elastic[311.
Strong arguments in favor of the existence of a substantially
continuous lipid layer are provided by investigations on the
electrical properties. Biological membranes have a high
electrical resistance. For plasma membranes, the value
varies between lo3 and l o 5ohm.cm2.After damage to the
membrane or during stimulation, the conductivity is increased by a few orders of magnituder321,since the ion
permeability in the affected regions of the membrane is
changed. (For conflicting interpretations of the electrical
phenomena, see r331.) Synthetic membranes have an even
higher electrical resistance, which can be reduced by two
to four orders of magnitude by substances such as the
“excitability inducing material” (EIM)[*],the cyclodepsipeptides, and the polyene antibiotics. Thus EIM causes a
decrease in resistance from 10’ to lo4 ~ h m . c m ’ [ ~ ~
The
].
electrical capacity of the cells is about 0.5 to 1.0 pF/cm2.
Plant cells and microorganisms are additionally surrounded on the outside by a cell wall, whose chemical composition is different from that of the cell membranel8I.A clear
separation of the cell wall and the cell membrane is possible
in gram-positive bacteria. If the cell wall of B. rnegaterium
is dissolved in an isotonic sucrose solution by lysozyme,
spherical protoplasts are formed[351.When the osmolarity
is reduced, the protoplasts burst, and the remaining membranes can be isolated. It has been shown conclusively by
permeability investigations on such protoplasts that the
membrane, and not the wall, forms the osmotic barrier
between the intracellular and the extracellular spaces. It was
shown in the case of isolated plasma membranes of E . c d i
that such membrane preparations are biologically intact.
These membranes, which are closed to form vesicles, can
accumulate certain amino acids in the interior of the vesic l e ~ [ ~After
~ ] . the removal of the cell contents had been
achieved in a number of cases, several functions could be
definitely assigned to the plasma membrane. In the case of
erythrocytes, it was possible to remove the cell contents by
reverted h e m o l y ~ i sThe
~ ~ ~cell
~ .contents were squeezed out
of giant nerve fibers and replaced by solutions of known
[*] EIM
IS probably
a protein.
Angew. Chem. inzernaf. Edit.
Vol. 10 (1971) ,INo. 6
composition. The action potential was recognized in this
way as a membrane function[381.
3. The Ultrastructure of Biological Membranes
Since the 1950’s, increasing use has been made of the electron microscope in the investigation of biological memb r a n e ~ [ 39-45].
’ ~ ~ The plasma membrane of an erythrocyte
is 80-1WA thick. Imaging in the electron microscope
after fixation with osmium tetroxide or potassium permanganate shows a trilaminar structure (Fig. I),
i.e. two
outer dark lines, in which the fixing medium has deposited,
Fig. 1. Surface of a human erythrocyte. Electron micrograph after
fixation with permanganate (from [29], p. 3). The trilaminar structure
of the plasma membrane is clearly visible. Magnification: 280000 x .
separated by a central light zone. This sandwich structure
has been found for several membranes, and this was therefore postulated to be a general structural principle. This
concept of the “unit membrane” is a further development
of the Danielli-Davson model (the differences between the
two models will be discussed in Section 7).
A favored object for investigation was the myelin sheath
of the nerve fibers, though myelin probably occupies a
special position in its chemical composition. The birefringence of the myelin sheath was discovered as early as
the 1930’s, and it was assumed to consist of alternating
lipid and protein layer^[^^*^^]. It is now known that the
myelin sheath develops from the plasma membrane of the
Schwann cell (Fig. 2), the mesaxon being wound up in a
Fig. 2. Development of the myelin sheath (highly schematized). The
myelin sheath develops from the plasma membrane (gray) of the
Schwann cell. 1=axon; 2 = mesaxon; 3 = Schwann cell. Left: initial
stage of development; right: intermediate stage.
spiral and the individual layers then coalescing. It was
found from the thickness of the repeating units that there
is probably no other material enclosed between the lamellas. X-ray diffraction studies have confirmed these assumptions concerning the myelin structure.
More recent findings have cast doubt on the postulate of a
general structural principle of biological membranes.
389
SjOstrandf4*’ showed with the aid of high-resolution electron micrographs that several membranes, including those
of the endoplasmic reticulum, exhibit a granular ultrastructure. Further conclusions follow from the picture of
the inner mitochondria1 membrane[’]. Liver cell membranes exhibit a globular structure after treatment with
ethylenediaminetetraacetic acid. Plan views show a characteristic hexagonal pattern, which suggests that they are
built up from subunits[491.
Despite the ease with which biological membranes can be
imaged, conclusions concerning the actual structure on
the basis of electron micrographs are associated with some
uncertainty ; this situation has frequently been discussed
in detail. Little is known about the reactions of the fixing
medium with the membrane components. It would be
premature to take it as certain that OsO, reacts only with
proteins or that the original protein conformation could
not be
The fixation does not necessarily “freeze
in” the morphological situation. Crosslinks produced by
the fixing agent between proteins could change the position of the latter within the membrane. Removal of water
can also lead to changes in state or influence the interactions
between the membrane components. Considerable differences were discovered on measurement of the thicknesses of various membranes. However, the determinations
were complicated by the fact that the membranes cannot
always be clearly delimited and that the results depend on
the fixing method. New paths are now being followed with
the freeze-etching technique. The electron-microscopic
investigations have contributed considerably to presentday views on membrane architecture. However, the question of the importance to be placed on individual items of
evidence must go unanswered for the present.
4. Membrane Components
In order to determine the chemical composition, it is
necessary to obtain pure membranes. Isolation is facilitated
by the use of labeling enzymes (see Section 4.1.2). The components of biological membranes are proteins, lipids, water,
ribonucleic acids, and covalently bound carbohydrates.
Scheme 1 illustrates the relations between the combined
components.
/
Lipoproteids
Proteolipids
I
\
\
Lipids
Proteins
I
I
L
Glycoproteins
\
[GI ycolipids]
Carbohydrates
/
Scheme 1. Position of the combined membrane components. Lipoproteins and proteolipids differ in their solubility. The glycolipids belong
to the group of lipids.
4.1. Proteins
Classification of the membrane proteins is difficult. Despite
fluid transitions, three groups can be distinguished ; these
are transport proteins, enzymes, and structural proteins.
390
4.1.1. Transport Proteins
These are mostly proteins that bind substances having low
molecular weights in a specific manner. In some cases they
have been broken down into several binding components.
If gram-negative bacteria are transferred from a sugar solution of high osmolarity into a salt solution of low osmolarity, about 5% of the membrane proteins pass into solution[”’. Some of these solubilized proteins exhibit enzymatic activity, while others have characteristic binding
specificities without catalytic function in uitro. Some of
these “dumb” proteins are probably involved in transport
. Tese
II proteins, which have recently been
very intensively studied, are listed in Table 1. Their molecular weights are around 30000.
Table 1. Membrane proteins that are probably involved in transport
processes. The proteins were isolated almost exclusively from microorganisms.
Membrane protein
Sugar transport:
M-protein (galactosides)
Heat-stable protein
P-Galactoside permease
Galactose-binding protein
Galactose-binding protein
Galactose carrier
Glucose-6-phosphate-binding protein
Glucose-binding protein
L-Arabinose-binding protein
L-Arabinose-binding protein
Molecular weight
Ref.
31000
9400
PO1
-
35000
-
[551
~561
C571
i5si
E591
1601
32000
35000
[6f1
E621
c631
Amino acid transport:
Leucine-binding protein
Leucine-binding protein
Leucine-binding protein
Arginine-binding protein
Tryptophan-binding protein
Histidine-binding protein
Phenylalanine-binding protein
Ion transport
Sulfate-binding protein
Calcium-binding protein
Phosphate-binding protein
>z
34000
36000
37000
~ 4 1
-
WI
-
m
PSI
~ 5 1
C671
t681
-
1691
32000
250W28000
-
~701
1711
r721
4.I .2. Enzymes
Many enzymes are components of membranes or are
associated with them. They are either located exclusively
in the membrane (marker enzymes) or they are at least
more highly concentrated in the membrane than in the
adjoining parts of the cell.
Contamination of the membranes cannot be ruled out in
every case. Some of the enzymes have been isolated only
with difficulty. Even lipid extractions led to a loss of activity, but this could be partly reversed by addition of phospholipids. Cytochrome oxidase, succinate dehydrogenase,
and P-hydroxybutyrate dehydrogenase can be reactivated
in this way.
Benedetti and E r n r n e l ~ t ~investigated
*~~
liver cell membranes and found 24 enzymes, including a Na’-K+stimulable adenosine triphosphatase, a p-nitrophenyl phosphatase, and a phosphodiesterase. Enzymes were also
located histochemically in some cases1731.
4.1.3. Structural Proteins
A protein obtained from the membrane of erythrocytes has
been described in detail as a structural proteinr7*! This
protein could be resolved into fractions by electrophoresis
Angew. Chem. internal. Edit. 1 Vol. I0 (1971)
No. 6
at pH 9. Criddle er nl.[751have prepared a structural protein
from bovine heart mitochondria that is insoluble in water
and accounts for about 40% of the total protein. Though
several arguments indicate that it is homogeneous, this has
not yet been proved. This structural protein forms complexes with other mitochondrial proteins. Woodward and
M ~ n k r e s [characterized
~~]
a mitochondrial structural protein from Neurospora. The mitochondrial structural proteins are probably essential to the full activity of the respiratory chain. Structural proteins have also been detected in
other membranes, and their apparently universal occurrence suggests a general function.
4.2. Lipids
Several synopses of the lipid composition of membranes
and cell organelles have been published (e.g.[771).
The
ratio of the quantity of lipids to the quantity of proteins
varies considerably from one membrane to another. Thus
the lipid content of myelin“ is about 70%, that of the
mitochondria“ is only about 30%, and that of the endoplasmic reticulum is about 50%. The lipid composition also
In the membranes of the mitochondria and
microsomes, phosphatidylcholine and phosphatidylaminoethanol account for about 80% of the total lipids, whereas
their content in myelin is only 25%. Cardiolipin has been
detected only in mitochondrial membranes. However, the
cholesterol content of myelin is much higher than that of
the membranes of mitochondria and microsomes. In the
case of liver mitochondria and microsomes, it corresponds
to ca. 5% ofthe total lipids[”! Bacterial membranes contain
little or no cholesterol. Cholesterol can be incorporated
into the cell membrane of Mycoplasma laidlawii if it is
provided in the culture medium. Data on the lysophosphatide content of the membranes should be regarded with
caution, since the results may be influenced by autolysis.
Table 2. Lipid and fatty acid composition of an “average erythrocyte”
(mammal).
Compound
Interesting recent observations indicate that lipids are
involved in syntheses. Thus a lipid fulfils a catalytic function in the biosynthesis of the peptidoglycan of the bacterial
cell
4.3. Water
Little attention has been paid so far to water as a membrane
component, and the literature data are scanty. Water,
which constitutes about 30-500/, of the membrane
volume, is essential for a number of membrane functions,
such as ion permeability and associated electrical phenomena[82*83?Since little is known about the role of water
either in synthetic or in biological membranes, it has been
disregarded in most membrane models. However, it must
have an influence on other membrane components.
4.4. Ribonucleic Acids
RNA has been detected in several bacterial membranes.
The maximum RNA content in liver cell membranes is
around 10 pg/mg of protein[49! RNA has also been found
in other animal membrane fractions, though adsorption
of RNA on the membranes during preparation cannot be
entirely ruled out.
4.5. Carbohydrates
Amount
Lipids:
Cholesterol
Phospholipids
Phosphatidylchohe
Phosphatidylaminoethanol
Phosphatidylserine
Phosphatidylinositol
Phosphatidic acid
Sphingomyelin
Gangliosides
Other glycolipids (sulfatides, cerebrosides)
Fatty acids:
Palmitic acid
Stearic acid
Oleic acid
Linoleic acid
Arachidonic acid
Other fatty acids
27%
60%
31%
27%
12%
3%
2%
25%
6.5%
6.5%
30%
16%
11%
10.5%
10%
16.5%
The lipid composition of erythrocytes depends on the
species[79! It is ihstrated in Table 2 for an “average
erythrocyte”. The most abundant lipid is cholesterol, and
[*] The inner and outer mitochondrial membranes differ in their
composition. The composition of myelin depends on the state of maturity.
Angew. Chem. internat. Edit.
this is followed by the group of phospholipids. The phosphatidylcholine content of erythrocytes varies considerably
from species to species. It is scarcely detectable in sheep
erythrocytes, while its value in pig erythrocytes is 25% and
in rat erythrocytes 50%. Glycolipids seem to occur exclusively in plasma membranes. Even within the classes of
lipids, the fatty acid composition varies, and the fatty acid
pattern of biological membranes fluctuates over a wide
range. It also depends on the temperature and the diet[801.
In human erythrocytes, five fatty acids make up more than
3/4 of the total fatty acid content.
Vol. I0 (1971) J N o . 6
The carbohydrates present in the membrane are bound
either to lipids (glycolipids)or to proteins (glycoproteins).
The glycoproteins are localized mainly in the outer layer
of the plasma membrane.
5. Interactions between the Membrane Components
The fact that membranes can be isolated shows that they
are stable structures, and do not collapse like a lipid film
when removed from their natural environment. Investigations with organic solvents and with mild detergents
indicate that covalent bonds between lipids and proteins
are of no importance. On the contrary, stabilization appears
to be due to ionic bonds, hydrogen bonds, London-van
der Waals forces, and hydrophobic bonds.
Phosphatidylcholine (a-lecithin) may be taken as an example of the structure of membrane lipids. Phosphatidylcholines from liver mitochondria and from the endoplasmic reticulum have approximately the same fatty acid
compositionrscJ.Close packing of the moIecules within the
39 I
membrane is possible in the physiological pH range. In
erythrocyte membranes, the removal of the polar groups
of the phospholipids by phospholipase C (the arrow points
to the cleavage site) does not lead to breakdown of the
membrane structure, so that ionic bonds between proteins
and lipids cannot be entirely responsible for stabilizing the
structure[84!
hydrocarbons cannot form myelin figures at 25°C; this
becomes possible only when the temperature is raised.
The myelin figures can be recognized under the optical
microscope as coiled or layered structures. They are
birefringent. Electron-microscopic investigations have
shown that they consist of a series of bimolecular lipid
layers, which show no tendency to coalesce (Fig. 3). Water
is intercalated between the lipid lamellas, and the thickness
of the lamellas varies between 80 and 120A, depending
on the water content. In some cases, very homogeneous
structures with a molecular weight of a few million appear,
According to the "unit membrane" concept, the proteins
should be spread on a central bimolecular lipid film. The
p-conformation of the membrane proteins should allow
closer contact, and hence stronger interactions, between
protein and lipid. However, the absence of the IR band at
1630cm-' indicates the absence of proteins with the
pleated sheet structure. Optical rotatory dispersion and
circular dichroism studies indicate, on the contrary, that
the membrane proteins probably have a globular structure
and a high content of a-helix['6*85-88].
The hydrophobic character of many membrane proteins
is shown by their poor water-solubility and their solubility
in organic solvents. Since these membrane proteins do not
differ in any obvious respect from water-soluble proteins
in their amino acid compositions, their lipophilic nature
can hardly be due to the primary structure, but must result
from higher structures such as the a-helixrsgl.NMR studies
on erythrocyte membranes have shown that the signals of
the methylene protons of the hydrocarbon chains in the
membranes are strongly influenced by protein^[^'.^'^,
probably as a result of hydrophobic bonds between the
two components. In the case of phosphatidylcholine, the
signals of the paraffin chains disappear from the NMR
spectrum on addition of cholesterol. The mobility of the
chains appears to be restricted[92! Differential calorimetric
studies on mixtures of phosphatidylcholine and cholesterol
have further confirmed this assumption[191.
6. Myelin Figures (Liposomes)
When hygroscopic phospholipids such as phosphatidylcholine come into contact with water, tubular or spherical
bodies known as myelin figures (and special forms as
liposomes) are spontaneously produced. The physicochemical properties of the phospholipids have been discussed in detail by a number of author^[^^.^^! The formation of myelin figures depends on the method, the species
of phospholipid, and the temperature. The paraffin chains
melt above a critical temperature, and this results in the
coexistence of crystalline order of the polar groups and a
more disordered liquid phase of the paraffin chains (mesophase). The lipids are in a liquid-crystalline state. The critical temperature is low in the case of phospholipids with
short-chain unsaturated fatty acids. Myelin figures are
therefore formed at room temperature only by phospholipids that pass into a liquid-crystalline phase at low
temperatures. Phospholipids with long-chain saturated
392
Fig. 3. Electron micrograph of myelin figures from phospholipids after
fixation with osmium tetroxide (taken from 1951). Magnification :
825000 x .
while in others the figures are heterogeneous and the
diameters ofthecylinders differ.A relatively stable phospholipid dispersion in which the liposomes are roughly equal
in size can be obtained by ultrasonic treatment.
Their structure and behavior make liposomes suitable for
use as models of biological membranes ; several research
groups[96-I''
ha ve carried out extensive studies on these
particles. Since the liposomes enclose water during their
formation, they can be used for permeability studies. Emux
experiments with anions and cations have shown that
sufficiently small liposomes have no pores ; anions and
cations permeate at different rates. Observations made on
synthetic membranes of identical or similar chemical
composition have added to the picture of the permeability
propertiesf"'. '"1.
After fixation with OsO,, myelin figuresexhibit a trilaminar
structure similar to that of biological membranes. This
trilaminar structure is even more pronounced after addition
of proteins to the lipid lamellas. It is often assumed that
the hydrophilic groups of the phospholipids are oriented
outward in the bimolecular film, while the hydrocarbon
chains of both layers of molecules are turned inwards, but
it is possible that the orientation of the molecules is exactly
the opposite. This question cannot be answered by an
electron micrograph alone.
7. Membrane Models
In 1935, Danielli and Davson1281proposed a membrane
model based mainly on the results of functional and physicochemical studies on plasma membranes. This model is
Angew. Chem. internat. Edit. J Vol. 10 (197s) J NO. 6
shown in Fig. 4a. A central lipid layer is separated on both
sides from the water-filled compartments, the intracellular
space and the surrounding medium, by protein layers.
These views of Danielli and Davson determined the picture
of the ultrastructure of biological membranes for a number
of years.
lipids from the mitochondria1 membranes were incomplete.
Protein bridges could penetrate the lipid layer and connect
the two protein layers, with the result that the collapse of
the membrane is prevented. The structure made up from
subunits as seen in the electron micrograph could be confined to the protein layer, or it could be an artefact. The “unit
This model was modified in three points by R o b e r t ~ o n ’ ~ ~ . ~ ~ ~ ,
mainly on the basis of electron-microscopic evidence. The
number of lipid layers was limited to two; instead of being
globular, the proteins were assumed to be spread on the
lipid layer, and the membrane was assumed to have an
asymmetric structure. Robertson also suggested that this
concept was generally valid (“unit membrane”). These
views are illustrated schematically in Fig. 4b. In the center
exterior
Fig. 5. Membrane model proposed by Sjijstrand (p. 10 of [48]) lo explain
electron-microscopic findings for cytoplasmic membranes. Construction of the membrane from globular proteins, between which the lipids
are interposed.
lioid
interior
exterior
membrane” model has recently been opposed by “subunit”
models, according to which the membrane structure can
be described as a two-dimensional aggregate of lipoprotein
l o 3 *‘04]. To illustrate this type of approach, Sjostrand’s membrane
is shown in Fig. 5.
The answers to the questions which of the models is
preferable and what modifications are necessary will have
to await the outcome of further investigations.
8. Functions of Biological Membranes
8.1. Membranes as Barriers and Pumps
--protein
interior
Fig. 4. Membrane models (after [29]).
(a) Danielli-Davson membrane model;
(b) Robertson’s membrane model (‘‘unit membrane”). The proteins are
in the 6-conformation and are different from one another.
is the lipid film, about 50 &
. thick, which consists of two
layers of molecules. The hydrophobic parts of the molecules
are turned toward the inside of the membrane, while the
hydrophilic groups are directed outward, and form bonds
with the spread proteins.
According to recent investigations, however, the p-conformation of the proteins no longer appears to be tenable.
It was also difficult to reconcile several observations with
Robertson’s concept. Thus, thickness determinations on
various membranes showed considerable differences. The
typical trilaminar picture could be obtained even for
mitochondria1 membranes whose lipids had been removed
by extraction. The membranes do not collapse despite the
extraction of the lipids. It appeared that the rapid restoration of the membrane after lesions could be readily explained by the incorporation of preformed structural
elements as subunits. Electron-microscopic plan views of
biological membranes often show structuring that suggests
that they are built up from subunits.
Results that initially seemed convincing were thus collected.
However, a number of arguments have not been able fully
to stand up to critical reexamination. The extractions of
Angew. Chem. internal. Edit. / Vol. 10 (1971) J No. 6
Membranes on the one hand are permeability barriers
between two spaces, and on the other they must allow the
passage of substances between these spaces in both directions. Thus the plasma membrane must not be equally
permeable to all substances, but must be able to select
among the substances offered to it for passage. The passage
of some substances through the membrane is hindered,
while that of others is facilitated. According to the “unit
membrane” concept, the osmotic barrier for hydrophilic
substances is the quasi-liquid lipid phase in the interior
of the membrane. The greater ease of penetration of lipophilic substances has already been mentioned.
In the use of Fick’s law to describe the diffusion of particles
through biological membranes, difficulty is encountered in
the determination of the diffusion constant1*].This led to
the use of another parameter, the diffusion coefficient, to
describe the diffusion. Some substances passed through the
membrane much more rapidly than one would expect in
the case of diffusion. In these cases, the rate of entry of a
substance tends toward a maximum with increasing concentration (facilitated diffusion, active transport)“ ”]. Since
the investigations by O ~ t e r h o u t [ ’ ~it’ ~is, usually assumed
that mobile carriers within the membrane are responsible
for translocations of this nature. The essential features of
[*] The diffusion distance and the concentration profile are not
accurately known for biological membranes.
393
the transport are structure specificity, ability to be inhibited
and activated, and saturation behaviorr30*
'06
'1.
8.5. Membranes as Protection
Absorption and secretion in metazoa are the responsibility
of highly specialized cells such as those of the intestinal or
renal epithelia. High molecular weight compounds or
particles can pass through the membrane by pinocytosis
or phagocytosis.
The plasma membrane protects the cell when its environmental conditions change. By exclusion or neutralization
of certain substances, interference with intracellular biochemical reactions is avoided. Enzymes that are dangerous
to the cells are enclosed by membranes (in e.g. lysosomes)
and withheld from their substrates in this way. Many
additions could be made to the list, but the functional
versatility of membranous structures is clear from the
points mentioned.
~
8.2. Conduction of Stimuli
Biological membranes can be stimulated by electrical,
mechanical, chemical, and thermal stimuli. The stimulation
is associated with electrical phenomena that occur as a
result of the movement of ions (K', Na', and C1-) through
the membrane. This involves localized changes in the
permeability of the membrane. K + , Na+, and C1- are
unevenly distributed in the intracellular and extracellular
spaces. This uneven distribution is probably due to the
difference in the concentrations of macromolecular polyelectrolytes on the two sides of the membrane (Donnan
distribution) and essentially to the ion pumps. Membranebound adenosine triphosphatases that can be stimulated
by Na+ and K + are evidently involved in the active transport of
The potentials that arise on porous membranes as a result
of the different penetration rates of the ions can be described
by the Nernst equation or, if the salt concentration on both
sides of the membrane is the same, by Planck's formula.
Hodgkin and Katz[1141have published a formula for the
determination of the resting potential of biological membranes in which the permeability coefficients of K + :Na+:
C1- are assumed to be related as 1:0.04:0.45. At the instant
of stimulation, a radical change occurs in the permeability
of the membrane, and the ratio of the permeability constants can now be expressed as 1:20:0.45. The action
potential can thus be attributed to an extreme increase in
the sodium permeability.
8.3. Membranes as Receptors
The membranes are the sites of action of many hormones
that influence the permeability or act on the membrane
enzymes. The membrane enzymes then liberate a transmitter substance (second messenger), which intervenes in
the metabolism. The surface structures of the cells play a
decisive role in immunochemistry. The contact inhibition
of growing cells must also be initiated by the cell membrane.
This contact inhibition is lacking in tumor cells["51.
8.4. Membranes as Enzyme Supports
Reference has already been made to the large number of
membrane-bound enzymes. Thus the components of the
respiratory chain are bound to the mitochondria1 membrane, and a high degree of order is achieved in the sequence
in this way. Protein synthesis in animal cells takes place
on the membranes of the endoplasmic reticulum, and on
the plasma membrane in bacteria. The division of the cell
into reaction spaces (compartments) by membranes ensures
that many metabolic sequences can proceed undisturbed.
394
Received: November 20,1970 [A 820 IE]
German version: Angew. Chem. 83,419 (1971)
Translated by Express Translation Service, London
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