R O C Z N I K I F IL O Z O F I C Z N E T o m X X X I , z e s z y t 3 — 1983
JOZEF ZON
ELECTRONIC CONDUCTIVITY IN BIOLOGICAL MEMBRANES
The ionic conductivity across biological m em branes has long been a su- bject of investigation and its significance for basie physiological processes in m any respects has been ascertained. However, the possibility of elec- tronic conduction in these m em branes continues to be a subject of con- siderable controversion. This type of conductivity m ay be realized in the molecular structures of biomembranes by one of the following ways:
.i) tunelling across intra- and interm olecular energy barriers (Eley 1962;
De Vault et al. 1967; Floyd et al. 1971; Chance 1975; Hopfield 1977;
Petrov et al. 1979; Chernavskij et al. 1982),
ii) activated hopping of electrons over these barriers — so called exciton mechanism (Frohlich, Sewell 1959; LeBlanc 1962),
iii) conduction along common energy bands (Szent-Gyorgyi 1941; Bril- louin 1962; Pethig 1977; Ladik 1979).
Semiconductivity of chemical compounds isolated from biological stru
ctures has been reviewed m any tim es (e.g. Gutman, Lyons 1967; Rosen
berg, Postow 1969; Boguslavskii, Vannikov 1970; Meier 1974; Cope 1975;
Ernst 1975; Pethig 1977; Simionescu e t al. 1978; Pethig 1979; Simionescu, Percec 1980; Kryszewski 1980). It has been proved that proteins, lipids, and nucleic acids, when investigated in vitro, m ay behave as electronic conductors. This same m ay be said about more composite biological ma- terials, as macromolecular complexes of some cells and organelles and extensive fragm ents of some tissues.
It is, howevćr, problematic w hether these m aterials may also function
as electronic conductors during normal physiological activity. In order to
approach the answer to the question, it has been resolved to review the
papers describing characteristic features of semicon ductor behaviour
in biological membranes. To make the survey more complete, along with
experim ental results, some theoretical considerations also have been taken
into account.
166 J O Z E F Z O N
An electronic semiconductor is characterized by the following features (Kryszewski 1980, p. 2):
i) electric conductivity in the rangę from 10—10 to 106 mho-m-1, ii) positive tem perature coefficient of conductivity,
iii) non-ohmic and often rectifying action in contact with other conductors, especially metals,
iv) high values of thermoelectric force, the sign of which does not depend on the type of metal brought into contact with the semiconductor, v) photoconductivity,
vi) the dependence of conductivity on the type and amount of impurities.
Usually, as a criterion for semiconductivity of the m ateriał the com- pliance with the reąuirem ent (ii), described by the following formuła is used:
o= c0exp[—Es(2kT)~i] (1)
where:
o — the observed value of conductivity (mho),
<t0 — a constant, dependent mainly on the mobility and effective mass of charge carriers (mho m ~-'),
Ej — the energy gap (J),
k — Boltzmann’s constant (J K—1), T — absolute tem perature (K).
Whereas this type of therm al behavior of conductivity may also be found in dielectrics, it is assumed that the value of Eg in semiconductors should be lower than 3 eV (5,4X10~19 J). Moreover, the other properties listed above should be also ascertained in the m ateriał to classify it as an electronic semiconductor.
One of the earliest suggestions of electron transfer across biological
membranes was made by Lund (1928). He proposed that the membrane
was a structure coupling two redox systems placed on both sides of it (an
electrostenolytic system). The idea was relinąuished for many years, be-
cause it was belived that there a re no molecules in the membranes, which
may function as „wires” carrying electronic currents. A new momentum
was given to the idea, when it has been discovered that m any biologically
active compounds can act as electronic semiconductors. The Lund’s idea
was undertaken by Jahn (1962) who ascribed the role of wires spanning
the biological membranes to carotenoids — molecules possessing systems
of conjugated bonds. The generation of free electrons for the conduction
across the m em branes was ascribed to the action of ATP. The electro-
motive force, driving the electrons along these bonds, was supposed to be
generated due to the differences in the electronic pressure between both
sides of the membrane.
E L E C T R O N IC C O N D U C T IV IT Y I N B I O L O G I C A L M E M B R A N E S 167
The research on the electronic conduction across biological membranes in the solid-state physics perspective began in the middle of the sixties and up to no w has covered all main types of these structures. Paralelly with these efforts, a new field of the research dealing with electronic conduc
tion in models of natural membranes — the bilayer lipid membranes (BLM) — was open in th e late sixties. This new field rem arkably supple- m ents the research done on natural systems, and makes the conclusions drawn on the properties of natural membranes more reliable. Extensive reviews of this area of the study have been given by Rosenberg (1971) and Tien (1972, 1973, 1974, 1979).
This paper has been divided into two main parts. In the first one, the electronic conduction in the membranes physiologically active in the dark are presented; in the other one — the electronic properties of membranes functioning as phototransducers have been described. In the latter case special attention is paid to the photoelectric effects in natural membranes.
1. ELECTRONIC PR O PER TIES
OF THE M EM BRANES PH Y SIO LO G IC A LLY AC TIV E IN THE D A R K
1.1. Secretory cells and erythrocytes
In 1970, Mandel when investigating the relation betw een the voltage and current flowing across the membranes of several types of secretory cells of animals, suggested th at the most adeąuate approach to the naturę . of these processes is to assume an electronic current flowing across semi- conducting m em branes which divide different solutions. Pohl and Sauer (1978) have arrived at sim ilar conclusions when investigating electroste- nolytic effects taking place across the salivary gland of the Lone Star tick.
However, both the research done by the authors mentioned above, and Digby (1964) are not to be directly referred to celi membranes, because the membranes of secretory cells are very thick (up to several hundred nanometers) and composite structures.
The electron flow across the membranes of erythrocytes was discove-
red by Marinov (1979). To oxidize hemoglobin in the interior of these
cells, he exposed the erythrocytes to the action of hydroxylamine. Next,
the erythrocytes were incubated in the solution containing eosine and
NADH, and the suspension was illuminated with the visible light. In the
result, a shift of the Soret band has been found and a new absorption band
in the rangę of 540-580 nm was brought about. These changes were inter-
preted as indicative of the reduction process having taken place inside the
cells. Connected w ith the reduction, the oxidation of complexes of eosine
168 J O Z E F Z O N
with NADH outside the cells was found. Considering both types of reac- tions, ascertained simultaneously on both sides of the erythrocyte mem- branes, the investigator concluded th at transfer of electrons across the membranes took place. In additional experiments, involvement of mem- brane permeable electron carriers has been excluded. The role of the me
dium conducting electrons across the erythrocyte membrane was ascribed to giant protein molecules of spectrin.
1.2. Muscle cells
In the membranes of muscle cells, three types of effects characteristic of semiconductors were discovered: the exponential increase of conducti- vity with rising tem perature; the thermoelectric force generation, and photoconductivity. The conductivity changes, fitting in Eąuation (1), have been found in dried muscle fibres (Lakatos 1962), and in fresh frog mus
cle (Nagy 1970). In the latter case, the exponential changes of conducti- vity took place in the tem perature rangę 289-297 K. When the tem peratu
re was brought above 297 K the conductivity did not change, and above 312 K — its value dropped. The value of Eg, calculated on the assumption of validity of Eq. (1) was about 2 eV. It has not been determined whether the current carriers were electrons or holes.
The generation of differential thermoelectric force in the muscle of the frog was found by Lakatos and Kollar-Morocz (1969). The value of this force was calculated to be 230 ± 40
m>V K—:1. The holes have been found to be the dominant current carriers in the tem peratures below 292 K, and the electrons — above 298 K. The ability to generate the thermoelectric force ceased, when the muscle had been treated by chloroform.
The photoelectrically mediated action of the light on the heart of the frog was demonstrated by Lakatos and Kollar-Morocz (1966). In their experiment, the beating of isolated hearts was stopped by depletion of K+
ions form the bathing Ringer solution. When to this solution Na-eosine was added and an intense light was used to illuminate the hearts, the be
ating recured more freąuently than in the nonilluminated ones. Slight rise of tem perature (of about 1 degree) was accounted not to be significant in causing these changes. Instead, the investigators pointed to the possibi- lity that the action of the light caused an ełectronic excitation of eosine.
In the next step, the electrons were supposed to move along the Jt-bonds
systems of the molecular structures of the membranes, which eventually
lead to the increase of the probability of recurence of heart beating. Simi-
lar type of causal connection was suggested by thes same investigators
(Lakatos, Kollar-Morocz 1967) for the decrease of the threshold of exci-
tation of eosine- and light-treated muscle cells.
E L E C T R O N IC C O N D U C T IV IT Y I N B IO L O G IC A L M E M B R A N E S 169
1.3. Neurons
E. Ernst (1955/1956) was the first who tried toconnect the functions of nerve cells w ith th e properties of semiconductors. Among other similari- ties in behavior of both semiconductors and neurons, he mentions the reactions to tem perature changes and to contaminations. This author saw also essential similarities in functioning of the p-n semiconductor junction and the modulation of the nerve impulse (Ernst 1956). These precursory and simplified analogies draw n by Ernst have been criticized by Liber- man (1958), who put also some suggestions concerning further research on semiconductivity in nervous system.
Blocking of the action potentials in the Pacinian bodies by procaine and thioguanine, he considers as a manifestation of the stopping of the flow of free electrons in the neuron m em branes (Ernst 1966). In a simi- lar way he explained the action of veratrine and novokaine (Ernst 1968).
On the basis of formal sim ilarity between high-frequency oscillations in the Gunn’s diodę, Ernst (1968) proposed that the freąuency modulation in some nerve cells was brought about by a mechanism similar to that one active in the Gunn’s diodę.
The photocurrent and photovoltage generation was found in stained giant axons of Aplysia (Chalazonitis 1964). The sign of the photopotential generated in the m em brane has been shown to depend on the type of dye used for staining, and its concentration in the membrane. The m easure of the influence of light on the m em brane was the rate of the depolarisation taking place in the first second after the beginning of illumination. The rate of depolarisation, dV/dt, was shown to change accordingly to the formuła:
^ = A e x p ( k t ) ' (2)
where:
A and k are constants dependent on the intensity of light at a given wavelength, tem perature, polarisation of the membrane, and the partial pressure of oxygen,
t is the tim e lapse after beginning of illumination.
If the intensity of the light at a given wavelength reached certain va-
lue, rhytm ical nerve firing was brought about. The densities of the sur-
face photocurrent were evaluated to be about 60 mA/m-2. In discussion of
the results of the experiments, the attention was paid to the role of elec-
tronic excitation and transfer of electrons. It has been hypothetizeó, that
after the illumination causing electronic excitation in the dye molecules,
free electrons or holes should be created. These charge carriers, along
170 JÓ Z E F ZO N
with protons, before trapping in the finał acceptors, were thought to de- polarize the membrane. Cope (1968) showed that the Chalazonitis’ results m ay well be interpreted as displaying behavior typical to the generation and decay of free charge carriers in semiconductors, described by the ki- netic Elovich equation.
In 1962, Kirzon et al., observed an exponential drop of impendance in the sciatic nerve of the frog caused by illumination. As a possible expla- nation of the results they obtained, the semiconductivity of the compo- nents of the nerve membrane was suggested. If the neurons were stained either with eosine, or bengal rose, or with neutral red, and then illumina- ted, the firing activity was generated (Lakatos 1969). In spite of the fact th at less than in 50°/o of stained neurons the activity was observed, the acfivity did not occur neither in stained but non-illuminated, or in unsta- ined and illuminated. These results were explained in a similar way as the Chalazonitis’ ones. In earlier experiments (Ludvikovskaya, Pangloleva 1965), the firing of neurons was shown to be generated even in unstained neurons, when they were illuminated with ultraviolet (^=260, 280, 313 nm) or visible (A.=405 nm) light and the tem perature kept between 279 and 283 K. The neurons did not respond to the light when the tem perature wa3 in the rangę of 293 to 295 K.
On the basis of earlier suggestions made by Ernst, Nagy et al. (1978) investigated the relationship between the tem perature and the velocity of the propagation of the excitation in the sciatic neuron of the frog. They found that in tem peratures between 278 and 303 K, the velocity of the spike propagation increased linearly. The increase was not exponential, as demanded by Eq. 1. Therefore, it cannot be regarded as directly connected with possible exponential rise of electronic conductivity.
Finally, the generation of thermoelectric power in the neuron was dis- covered (Lakatos, Kollar-Morocz 1969). The thermoelectric coefficient was evaluated to be 45 ± 15
m-V K ~ I t s value decreased with time, and completely ceased after 9 days of preservation of the nerve.
1.4. Mitochondria
This subcellular structure is the main source of ATP in the cells of aerobic organisms. Before mitochondrion was identified as the „power plant” of cells, A. Szent-Gyórgyi (1941), following earlier suggestion ma
de by P. Jordan (1938), had hypothetized that transport of energy in the
celi in.volves a similar mechanism as that found in some electronically
conducting solids. According to this hypothesis, in insoluble proteins, com-
mon energy bands occur, and the energy transport takes place when elec-
E L E C T R O N IC C O N D U C T IV IT Y I N B IO L O G I C A L M E M B R A N E S
171 trons move along these bands. This suggestion gave rise to numerous works devoted to the investigation of semiconductivity in biological ma- terials.
Vannikov and Boguslavskii (1969) estimated the mobility of free charge carriers in films made of mitochondria. The value of m- was about 5 X10-6 m2 V—1 s~1 in samples hydrated in less than 1%. In protein-lipid extracts from mitochondria, the mobility of electrons was found to be about 2 orders of m agnitude higher (Eley, Pethig 1971). However, if these investigators took into account a correction for the space involved in me- asured conductivity, they concluded that the actual values of m- should be even 10 times higher than previously estimated. The thermoelectric force generation in pressed pellets of m ixture of intact mitochondria with sub- mitochondrial particles was investigated by Eley et al. (1977). They found that in tem peratures below 342 K, electrons are the dominant charge car
riers and holes — above this tem perature.
1.5. Non-photosynthetizing plant cells
Skierczyńska et al. (1970) observed th at immersion of algal celi in w ater caused 508/o reduction of longitudinal impedance of this celi. As the possibility of the influence of outward currents flowing across the celi membrane had been excluded, participation of electronic currents in mem- branes was suggested.To test this possibility, an attem pt at detecting the Hall voltage generation in the wali and membrane of the celi of Nitellop- sis obtusa was made (Skierczyńska et al. 1977). The induction of the ma- gnetic field used in the experim ents ranged from 0.8 to 2.0 T. The currents generated across the wali and celi m em branes were from 1.3 do 2.2
mA. In spite of considerable difficulties, the H all's voltages from 100 to 650 i*V were detected in living cells. No H all’s voltage generation was found in the dead ones.
Changes of resting potentials in plant cells under the influence of light are known phenomenon (cf. Hansen et al. 1973), and these respon- ses are not directly influenced by light. Namely, in evoking them, the metabolic activity dependent upon the influence if light upon the chloro- plasts is involved. This not withstanding, a direct action of light on the plant celi m em brane is possible. It has been demonstrated on rhisoids of some algae, where chlorophyll occurs in extrem ely smali ąuantities (Adrianov 1970). Similar observations were reported earlier by Bose (1907 p. 396) and W aller (1925).
Using ultraviolet light, Bulanda and Pałczyńska (1976/1977) were able to detect a direct action of this light on celi membranes of algal cells.
This action manifested itself in fast changes (<^ 1 sec) of the resting poten-
172 J O Z E F z o n
tial (U ^S.5 mV). The possibility that these changes were brought about by the absorption of ultraviolet by lipids and proteins was considered to be of minor significance.
2. ELECTRONIC PROPERTIES
OF TH E M EM BRANES PHY SIO LO GICA LLY ACTIVE IN THE LIGHT
2.1. Light receptors
Dried rods of the eye of the sheep were shown to be photoconductive and follow the conductivity changes in accord with Eąuation 1 by Rosen
berg et al. in 1961. The value of Eg was calculated to be about 2.3 eV. More rigorous measurements, avoiding the problems connected with using ste- ady currents, were carried out by Trukhan et al. (1970) on pigment epi- thelium of the eye of the frog. Applying high freąuency currents, they showed th at photoconductivity is proportional to the intensity of light and w ater content in the samples. The holes were indentified to be the charge carriers. Their Hall mobility was estimated to reach 1.5 X 10“ 8 m2 V—1 s~l.
Falk and F att (1967) measured the admittance of rod segments in the freąuency rangę from 15 Hz to 60 kHz. In discussing the mechanisms underlaying the observed admittance changes, they saw electronic condu
ction to be an option to the protonie movement along the membranes.
One of the features of the involvement of electrons in a response to an external agent is a very fast change of voltage or current across the mem- brane. The most spectacular changes of this type were observed in the so called early receptor potential (ERP).
ERP was discovered by Brown and M urakami in the retina of the mon- key in 1964 (1964a). This response was found in visual receptors of inver- tebrates (Brown 1965; Brown, Gage 1966; Crawford et al. 1967; Hagins, McGughy 1967) and other vertebrates (Brown, Murakami 1964b; Pak 1965; Cone, Cobbs 1969). ERP consists of three phases of potential chan
ges, the last one being identical with the a-wave of electroretinogram.
The first two phases, the positive and negative one, have been shown to be independent of the ionic current, flowing across the membrane, on the following grounds:
i) they are not changed by anoxia (Brown, Murakami 1964a),
ii) they occur even if the ionic environm ent of the membranes has been changed (Pak 1965; Brindley, Gardner-Medvin 1966; Crawford et a.l.
1967),
iii) the lowering of tem perature does not abolish the positive phase, but
it reversibly changes the negatire one (Pak, Cone 1964; Pak, Ebrey
1965; Brown, Gage 1966). However, inereasing the tem perature of re-
E L E C T R O N IC C O N D U C T IY IT Y I N B I O L O G I C A L M E M B R A N E S 173
tina above 321 K or cooling to 188 K and subseąuent thawing it (Ha- gins, McGughy 1967), abolishes the photoresponse of retina. This da- maging action of high or Iow tem peratures is believed to be caused by damages of molecular organisation of the membranes,
iv) fixation with glutharaldehyde or formaldehyde abolishes only the third, ion-dependent phase, but only slightly changes the course of the first two ones (Brindley, Gardner-Medvin 1966; Hagins, Mc Gughy 1967; Arden et al. 1968),
v) the fast positive phase does not vanish even in dehydrated pigment cells (Brown, Gage 1966),
vi) the responses to light of retina and of Silicon photocell having identi- cal passive electrical properties as retina are indistinąuishable (Arden et al. 1966).
The mentioned above evidence for non-ionic naturę of the first two phases of ERP have lead some investigators (Brown, Gage 1966; Brown, Crawford 1967; Crawford et al. 1967) to consider first stages of photorece- ption in term s of solid state electronics.
2.2. Chloroplasts and chromophores
The first authors to raise the idea that energy conversion and transport may involve conduction of electrons along common energy bands were Móglich and Schón (1938). Many experiments have been carried out to explore this new, based on solid state physics, perspective in biophysics of photosynthesis.
The conductivity changes in dried chloroplasts following Eq. 1 and photoconductivity have been reported by m any investigators (Arnold, Sherwood 1957; Arnold, McLay 1958; Ichimura 1960; Zvalinskii, Litvin 1967; Litvin, Zvalinskii 1971). Another approach was chosen by McCreć (1965) to investigate photoconductivity in chloroplasts. Using a condenser method, allowing the detection of currents 103 times weaker than those found in morganic photoconductors, he was not able to find photocondu- ctivity either in dried chloroplast layers, or in monolayers of chlorophyll and proteins. A ttem pts to find photoconductivity in dried green algae or higher plant leaves were also unsuccessful. On the basis of these results, McCree concluded that even if photoconductivity occurs in plant materiał, it cannot be regarded as an efficient mechanism of light energy conver- sion in photosynthesis.
Zvalinskii and Litvin in 1967, using a sensitive electrometer, found
th at the action spectrum of photoconductivity and absorption spectrum
coincide w ith each other in visible region. In the infra-red, the photocur-
ren t was also generated w ith maxima lying at 950, 1040, 1260, and
174 J O Z E F z o n
1550 nm. This coincidence of spectra and the occurence of maxima in in- fra-red action spectrum, point to the chlorophyll molecules and aggrega- tes of them, as playing a crucial role in photoconductivity of chloroplasts.
In a more rigorous study (Litvin, Zvalinskii 1971), it has been found that the action spectrum of photoconductivity coincides with the resultant absorption spectrum of main photosynthetic pigments present in chloro
plasts. Both the dark and the conductivity under illumination followed Eq. 1 in tem peratures between 253 and 293 K. The specific dark conducti- vity of chloroplasts was calculated to be of the order of 10—11 to 10—10 mho m—:1, and E g^l.72 eV. The drift mobility was estimated to be between 1X10~5 and 1X 10~3 m2 y r ^ s r 1.
The generation of free charges in chloroplasts and the membranes of photosynthetic bacteria has also been investigated, using measurements of dielectric losses, e", at microwave frequencies. Blumenfeld et al. (1970) showed that e" in leaves and chloroplasts extracted from sorghum and broad bean parallels the absorption spectrum of chlorophylls, a and b, and probably, fS-carotene. Although the action spectra of both native le- aves and extracted chloroplasts were qualitatively similar, the action spectra of native leaves were 5—7 times stronger. The microwave photo- conductivity signal depends on the functional state of photosynthetic mem
branes. When chloroplasts are fresh and intact, the response is biphasic, and if they are old or damaged (e.g. by heating), the response in monopha- sic (Blumenfeld et al. 1974; Deryabkin et al. 1978). To ascertain the pos- sible electron involvement in generation the observed dielectric losses, a model system was used. Is consisted of a suspension of photoconducting ZnO particles, illuminated with the light, X=380 nm. The photoconduc- tivity signal of these electronically conducting particles and chloroplasts was alike (Deryabkin et al. 1978).
Bogomolni and Klein (1975) combined the microwave photoconducti- vity measurements with Faraday rotation measurements on films of dried intact and broken chloroplasts from spinach and algae as well as on the chromophores from photosynthetic bacteria. They found that photocon- ductivity signal was generated by both negative and positive carriers. The negative ones seemed to be released therm ally from the prim ary accep- tors, the positive carriers having been identified with the movement of di- m eric chlorophyll cation radicals. The Hall mobilities of charge carriers and their density were calculated to be the order of 1X 10—4 m2 V-1s—1 and 1019 m—3, respectively. The data obtained in these measurements seemed to give support for the tunneling or hopping mechanism of charge migra- tion, instead of conduction along common energy bands.
The microwave conductivity in reaction centers of photosynthetic bac
teria has been shown (Skachkov et al. 1980) to consist of two components
E L E C T R O N IC C O N D U C T IV IT Y I N B IO L O G IC A L M E M B R A N E S 175
characterized by different rates of rise. The first one, conceivably having its cause in the m igration of electrons between ąuinones, builds up very fast (<^ 1 s) and th e other one develops slowly (<Ę=J20 s). The amplitudę of the fast response depends strongly on the level of hydration, and rises with its increase. The investigators, having in mind the dependence of micro- wave conductivity on the hydration degree, suggested that the presence of water influences the mobility and population of electrons in conduction bands of proteins in reaction centers. On the other hand, the slow compo- nent was interpreted as reflecting the accumulation of mobile electrons in electron transport chains and/or changes in the charge distribution in the photosynthetic apparatus.
The emission of the so called delayed light, discovered by Strehler and Arnold in 1951, is interpreted by some authors as a manifestation of elec
tron m igration and trapping in energy bands of semiconductive photosyn
thetic structures. Tollin and Calvin (1957) investigated the dependence of the delayed light intensity on tem perature, down to 133 K. Because the free radical recombination and emission from triplete states had been excluded, the authors explained their results in term s of electron trapping and subseąuent recombination of electrons and holes, giving rise to lu- minescence. Similarily, enzymatic reactions have been also excluded, be
cause their rate at Iow tem perature was incompatible w ith th e delayed light intensity (Tollin et al. 1958; Calvin 1958; Litvin, Shuvalov 1968). If an external electric field had been applied, th e intensity of delayed lumi- nescence rose about 50 tim es (Arnold, Azzi 1971; Arnold 1972). This increase was interpreted as caused by electric field enhancement of the rate of detrapping of electrons and their recombination. The depth of perticular charge traps localized in the forbidden energy band, was stu- died by measuring th e tim e or tem perature distribution of the intensity of lelayed light (Litvin, Shuvalov 1968; Arnold 1977).
The whole process leading to the generation of the delayed lumines- cence is believed to consist of the following stages (Arnold, Sherwood
1959; Arnold 1965; 1977):
i) translocation of electrons to the conduction band,
ii) trapping them on the energy levels lying close to the bottom of the conduction band,
iii) therm ally evoked release of trapped electrons to the conduction band, iv) reversion of electrons to the valence band, connected with their re
combination with holes and light quanta generation.
This relatively simple picture of the mechanism of the generation of
delayed luminescence has been supplemented by the processes of trapping
of holes, creation of radicals by the energy released on the traps, and fi-
nally, generation of chemically stable compounds (Calvin 1958; Arnold,
176 J O Z E F z o n
Azzi 1968). Although this generał idea of semiconductor processes invol- vement in the generation of delayed luminescence may essentially be cor
rect, it is considered to be an oversimplif ication. Malkin’s review (1977) discusses other mechanisms of the delayed light generation, competing with the semiconductor approach. Another group of experiments revealing the electron transfer in the membrane present the investigations of the shifts in absorption spectra (electrochromism) of chloroplast pigments (Jungę, W itt 1968; Witt, Zickler 1973). Their study suggests this electron transfer probably does not take place perpendicularily to the membrane surface (Jungę, W itt 1968) and is very fast (<! 20 ns) (Wolf et al. 1969).
These fast electron movements and following much slower relaxation pro
cesses m ay well be compatible with the semiconductor model of photosyn- thesis.
To find in which direction the electrons are transfered in illuminated chloroplasts, Fowler and Kok (1974) investigated the electric field gene
ration in suspension of chloroplasts. It has been found that the chloro
plasts charge negatively on the illuminated side and positively on the dark one when illuminated with a very short pulse of light. The rise time of the photopotential was shorter than 1 ns. Similar results have been obta- ined by Nobel and Mel (1966) who investigated the changes of mobility cf chloroplasts caused by their illumination. They found that continuous illumination of chloroplasts charges them negatively and causes about 15% increase in their mobility. Uhese authors argued that the involvement of some mechanisms of solid state physics seems very probable in causing these effects, sińce they did not occur when membranes of chloroplasts have been damaged.
Fast (<C 10~2 s) and positive charging of the inside of chloroplasts was described by Bulychev et al. (1971, 1972) and Vredenberg and Bulychev (1976), who used microelectrodes to measure these changes. Finally, it is worth mentioning here that a response similar to ERP was detected in leaves of the gout weed (Arden et al. 1966). This m ay be understood as another evidence for close connection between the displacement and flow of electrons in pigmented celi constituents and their natural respon- ses to light.
3. CONCLUDING REM ARKS
Biological membranes m ay be regarded as ultrathin, double layers composed of solid particles (mainly proteins) and liąuid crystals (lipids).
The iesults of investigations reviewed above, show that in aggregates of molecules building up the biological membranes free electrons m ay occur.
Taking into account the occurence of such semiconductor properties as:
E L E C T R O N I C C O N D U C T IV IT Y I N B IO L O G IC A L M E M B R A N E S 177
positive tem perature coefficient of conductivity, the values of forbidden energy gap and conductivity lying in the rangę characteristic of semicon- ductors, m easurable Hall effect and photoconductivity, one m ay depict living organisms as systems penetrated by extrem ely ramified, ultrathin and highly differentiated membraneous semiconducting materiał.
Regarding the discrepancy between the complexity of biological mem- branes and methodology used for detecting the electronic currents in the- se structures, it m ust be said that the results reviewed in the paper may also be subject to interpretation in term s of ionic conductivity and po- larization phenomena. However, in most of the experiments mentioned above, the attention was paid to exclude these possibilities. Moreover, experiments with models of biological membranes (Rosenberg 1971, Tien 1979), often carried out to obtain cheap and efficient photocells (Calvin 1974, 1978), m ake the electronic conductivity in biomembranes a very plausible possibility.
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ELEKTRONOWE PRZEW ODNICTW O W BŁONACH BIOLOGICZNYCH
S t r e s z c z e n i e
W artyk u le dokonano przeglądu tak ogólnych argum entów , jak i danych uzyska
n y ch ek sp erym en taln ie, św iad czących o elek tron ow ym p rzew od n ictw ie w błonach biologicznych. G łów nym i racjam i teoretyczn ym i św iad czącym i za zaangażow aniem przew od nictw a elek tron ow ego w błon ach są: i. w łańcuchach biochem icznych reak cji redoks zachodzi transfer elek tron ów m ięd zy m olekułam i, czasam i na znaczne odle
głości, jak m a to na p rzykład m iejsce w elektrostenolizie, 2. w ysoce uorganizow ane łtru k tu r y su pram oleku larn e błon, je śli sam e są badane in v itro i w yk azu ją w ted y w ła sn o ści półp rzew odnik ow e, p ow in n y rów nież w yk azyw ać te w łasn ości w w arun kach fizjologicznych, 3. je st praw dopodobne, że w elektronow ym przew od n ictw ie p rzy
n ajm n iej jed en z p od an ych m ech an izm ów je st czynny: a) tu n elow an ie, b) ekscytony, c) przew od nictw o w zdłuż pasm w sp óln ych energii.
E L E C T R O N I C C O N D U C T IV IT Y I N B I O L O G I C A L M E M B R A N E S 183
W p ła szczy źn ie ek sp erym en taln ej n a stęp u ją ce e fek ty m ogą być u w a ża n e za te, k tó r e w sk a z u ją na praw d opod obne p rzew od n ictw o elek tro n o w e w błon ach : 1. w y k ła d n iczy w z ro st p rzew od n ictw a w raz z tem p eraturą, 2. e fek ty fo to elek try czn e i zm ia
n y term iczn e p rzew od n ictw a w sk azu jące, ż e w artość Eg p ow in n a być m n iejsza niż 3 eV , 3. e fe k ty term oelek tryczn e, 4. bardzo szyb k ie reak cje e lek try czn e na p u ls św ia tła, k tó r e n ie zgadzają się z m ech anizm em jon ow ym ty c h zm ian, 5. m ierzaln e napięcia H alla.
P o m ija ją c m itoch ondria, ch lorop lasty i recep tor w zroku, fizjologiczn a rola p rą
dów ele k tro n o w y ch w b łon ach n ie je st jasn a w dalszym ciągu. J e st tak że w ą tp liw e czy p e w n e zja w isk a , k tóre w y ja śn ia n o w k ategoriach p rąd ów elek tron ow ych , n ie m ogą b yć w y ja ś n io n e p op rzez p rą d y jonow e. W zw iązk u z tym bardzo w a żn y w gląd m ożna uzysk ać p rzez bad ania nad elek tro n o w y m p rzew od n ictw em u k ład ów m ode
lo w y ch zn a n y ch p od n a zw ą p o d w ó jn o w a rstw o w y ch (czasam i: czarnych) b łon lip id o
w y ch (BLM).
