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Alexander G. Volkov and Courtney L. Brown
Department of Chemistry, Oakwood College
7000 Adventist Blvd.
Huntsville, AL 35896, USA

(May, 2004)


Green plants: Electrochemical interfaces

All processes of living organisms that have been examined with suitable and sufficiently sensitive measuring techniques generate electric fields. The conduction of electrochemical excitation must be regarded as one of the most universal properties of living organisms. It arose in connection with the need for the transmission of a signal about an external influence from one part of a biological system to another. The study of the nature of regulatory relations of the plant organism with the environment is a basic bioelectrochemical problem. It has a direct bearing on the tasks of controlling the growth and development of plants.

The synchronization of internal biological processes and their responses with the environment is associated with the phenomenon of excitability in plant cells. The basis for excitability is the extreme sensitivity of the protoplasm to chemical effects. Plants are comprised of cells, tissues, and organs which possess the ability to become excited by altering their internal condition in response to environmental factors, referred to as irritants; this excitability can be monitored.

According to Goldsworthy, bioelectrochemical signals resemble nerve impulses. These signals exist in plants at all levels of evolution. The inducement of nonexcitability after excitation and the summation of subthreshold irritations were developed in the vegetative and animal kingdoms in protoplasmatic structures prior to the morphological differentiation of nervous tissues. These protoplasmatic structures merged into the organs of a nervous system and adjusted the interfacing of the organism with the environment.

Action potentials are signals caused by the depolarization of cellular membrane potentials. Action potentials in plants have been studied in detail in the giant cells of Chara and Nitella. These plants possess many of the properties associated with the action potentials in animal cells, such as the all-or-nothing law, threshold potential, and a refractory period. In higher plants, action potentials have been researched in detail in many species, and these same electrophysiological properties have been found.

The plasma membrane allows for cells, tissues, and organs to transmit electrochemical signals over short and long distances. Excitation waves or action potentials in higher plants are possible mechanisms for intercellular and intracellular communication in the presence of environment changes.

Plants respond to irritants at the site of the applied stimulus, but excitation can be dispersed throughout the entire plant. Waves of excitation travel from the top of the stem to the root and from the root to the top of the stem, but not at identical rates. The speed of propagation is dependent on factors such as the chemical treatment, intensity of the irritation, mechanical wounding, previous excitations, or temperature.

Electrical impulses arise under the impact of various chemical compounds such as herbicides, plant growth stimulants, salts, and water. Physical factors such as electromagnetic or gravitational fields, mechanical wounding, and temperature effects also elicit electrical impulses.

Bioelectrochemical signals are the most rapid technique of long distance signal transmission between plant tissues and organs. Plants respond quickly to changes in light intensity, osmotic pressure, and temperature. They also rapidly respond to cutting, mechanical stimulation, water availability, and wounding. These responses can be discovered in distant parts of the plant directly after the stimulus occurs. Action potentials activate the membrane enzymatic systems that realize biochemical reactions, accelerate the production of ethylene, increase the concentration of the proteinase inhibitor, and modify the rate of production of polysomes and proteins.

The speed of propagation of bioelectrical signals with values from 0.05 cm/sec to 40 m/sec adequately sustains swift long-distance communication. This also allows the rapid response feature to be observed in plants. The speed of propagation of action potentials depends upon the varying types of induced stress.

Action potentials in soybean
Fig. 1. The action potentials in soybean have been induced by irradiation with light at 450 nm (A), 470 nm (B), 671 nm (C), and 730 nm (D) wavelengths (nm = billionth of a meter).
Plants generate different types of extracellular electrical events in response to environmental stress. In higher plants, action potentials may be the information carriers in intercellular and intracellular communication in response to environmental changes. A potential pathway for the transmission of this electrical signal might be the phloem sieve-tube system, since it represents a continuum of plasma membranes. A phloem is an electrical conductor of bioelectrochemical impulses over long distances. Phloem consists of two types of conducting cells: the characteristic type known as sieve-tube elements and another type called companion cells. Sieve-tube elements are elongated cells that have end walls perforated by numerous minute pores through which dissolved materials can pass. Such sieve-tube elements are connected in vertical series, known as sieve tubes. Although their nuclei disintegrate before the element begins the conductive function, sieve-tube elements are alive at maturity. Companion cells, which are smaller, have nuclei at maturity and are alive. They are adjacent to the sieve-tube elements, and are believed to control the process of conduction in the sieve tubes. This sieve-tube apparatus, representing a continuum of plasma membranes, is a potential pathway for electrical pulses to travel.

Electrical potentials have been measured at the tissue and whole plant level. At the cellular level, electrical potentials exist across membranes, and thus between cellular and specific compartments. Electrolytic species such as potassium, calcium, hydrogen, and chloride ions are actively involved in the establishment and modulation of electrical potentials.

The generation of electrochemical responses induced by blue and red photosensory systems was observed in soybean plants (Figure 1). A phototropic response is a sequence of the four following processes: reception of the directional light signal, signal transduction, transformation of the signal to a physiological response, and the production of directional growth response. Experimentation reveals that irradiation of soybean plants with light of wavelength of 450 ± 50 nm (nm = billionth of a meter) induces action potentials with duration times of about 0.3 ms (millisecond) and amplitudes around 60 mV.

Atmospheric electrochemistry: effects of the electric double layer of the earth and of acid rain

The existence of ions in the atmosphere is the fundamental reason for atmospheric electricity. The voltage between the earth's surface and the ionosphere is approximately 40 kV, which induces an electrical current of approximately 2000 A with a current density around 5 pA/m2. The Earth is an "electrode" immersed in a weak gaseous "electrolyte," the naturally ionized atmosphere. The earth's surface absorbs anions, and has a negative charge. The electrostatic field strength at the earth's surface is around 110-220 V/m, and depends on time of a day. Usually it is about 110 V/m, but at 7 p.m., Greenwich time, the electrostatic field strength is nearly 220-250 V/m around the surface of the Earth. Oceans, lakes, and rivers cover a significant part of the Earth, and their surface is also charged negatively against the atmosphere. Electrical polarity in soybean, potato, tomato, and cacti coincides with the electrical field of the "electric double layer" of Earth: negative in roots and positive on the top of the plants. Atmospheric change of the electrostatic field strength at 7 p.m., Greenwich time, does not induce action potentials or change in the variation potential of soybean or potato plants.

Action potentials in soybean
Fig. 2. The potential difference between two silver/silver chloride electrodes in the stem of soybean 1 (a), 15 (b), and 48 (c) hours after spraying 0.1 milliliter of 0.05 M sulfuric acid onto the plant.
Chemical reactions involving aerosol particles in the atmosphere are derived from the interaction of gaseous species with liquid water, and associated with aerosol particles and dissolved electrolytes. For example, the generation of nitric acid from nitrogen oxides takes place at the air/water interface of seawater aerosols or in clouds. Clouds convert between 50% and 80% of sulfur dioxide to sulfuric acid. This process contributes to the formation of acid rain. Acid rain exerts a variety of influences on the environment by greatly increasing the solubility of different compounds, thus directly or indirectly affecting many forms of life. Acid rain is the most serious environmental problem, impacting agriculture, forestry, and human health.

Acid rain has a pH below 5.6. Sulfuric and nitric acids are the two predominant acids in acid rain. Roughly 70% of the acid in acid rain is sulfuric acid, and nitric acid contributes about 30%. Spraying the soybean with a relatively dilute aqueous solution of sulfuric acid does not induce action potentials or deviations in the variation potential; however, spraying the plant with 0.1 milliliter of stronger acidic solution or depositing drops of aqueous solution of sulfuric or nitric acid on leaves induced action potentials in the soybean (Figure 2). The duration of single action potentials, after treatment by nitric and sulfuric was 0.2 second and 0.02 s, respectively.

The cells of many biological organs generate an electric potential that may result in the flow of an electric current. Electrical impulses may arise spontaneously, or result from stimulation. Once initiated, they can propagate to the adjacent excitable cells. The change in transmembrane potential creates a wave of depolarization, or action potential, that affects the adjoining, resting membrane. When the phloem is stimulated at any point, the action potential is propagated over the entire length of the cell membrane and along the phloem with a constant voltage. The propagation of each impulse is followed by the absolute refractory period, during which the fiber cannot transmit a second impulse. The high sensitivity of the protoplasm and all cell organelles to any natural and chemical effect is the basis for excitability. The integral organism of a plant can be maintained and developed in a continuously varying environment only if all cells, tissues, and organs function in concordance. Plants are continuously balancing with the external world.

Insect-induced bioelectrochemical signals in potato plants

Action potentials in potato plant
Fig. 3. The action potentials in the stem of a potato plant with Colorado beetles on the young terminal leaves. Distance between electrodes: (a) 3.5 cm; (b) 12 cm; (c) 22 cm; (d) 20 cm. The reference silver/silver chloride electrode was inserted in the stem between the cotyledons. The working silver/silver chloride electrode was inserted into the stem (a, b, c) or the tuber (d).
Time interval between action potentials
Fig. 4. The dependence of the time interval between the positive and negative peaks of action potential induced by Colorado potato beetles on the distance between electrodes. The electrodes were inserted in the potato tubers and different parts of the plant stem.
Volkov and Haack were the first to afford a unique opportunity not only to investigate the role of electrical signals induced by insects in long-distance communication in plants, but also to confirm the mechanism by which electrical signals can directly influence both biophysical and biochemical processes in remote tissues.

Action and variation potentials were measured in potato plants in the presence of leaf-feeding larvae of the Colorado potato beetle. The larvae consumed the upper leaves of the potato plants. After 6-10 hours, action potentials, with amplitudes of 40 ± 10 mV, were recorded every 2 ± 0.5 hours during a 2-day test period. The variation potential decreased from 30 mV to a steady state level of 0 ± 5 mV. Figure 3 shows the positive spikes and negative humps that appear during measurement of electrical potential difference between two reversible silver/silver chloride electrodes. The action potential induced by the Colorado potato beetle in potato plants propagates slowly; hence, the speed of propagation can be measured with two silver/silver chloride electrodes. The action potential propagates from plant leaves with Colorado potato beetles down the stem, and to the potato tuber (Figure 3). The speed of propagation of the action potential does not depend on the location of the working electrode in the stem of the plant, in the tuber, nor does it depend on the distance between the working and reference electrodes. The speed of propagation of the action potential, induced by the Colorado potato beetle, can be determined from the slope of the dependence of the time interval between peaks of the distance between electrodes (Figure 4); in this case it is approximately 0.05 cm/sec.

Green plants interfaced with a computer through data acquisition systems can be used as fast biosensors to: monitor the environment; detect effects of pollutants, pesticides, and defoliants; predict and monitor of climate changes; and facilitate the direct and fast control of conditions influencing the agricultural harvest. The use of new computerized methods provides opportunities for detection of fast action potentials in green plants in real time.

Studying the electrochemistry in green plants allows us to learn more about the processes of life through the observation of excitation waves. Green plants are a unique canvas for studying signal transduction, the foundation to discovering and improving biosensors, and essential to the development of alternative sources of energy.


This appendix presents further aspects of bioelectrochemistry of green plants in a somewhat more technical style.

Effects of pesticides, pollutants, uncouplers, and protonophores

Effect of DNP on action potentials
Fig. 5. The potential difference between two silver/silver chloride electrodes in the stem of soybean 25 (A) and 75 (B) hours after adding 50 milliliter of 10-3 M 2,4-dinitrophenol (DNP) to soil.
2,4-Dinitrophenol (DNP) is a member of the aromatic family of pesticides; many exhibit insecticidal and fungicidal activity. DNP is considered highly toxic to humans, with a lethal oral dose of 14 to 43 mg/kg. Environmental exposure to DNP occurs primarily from pesticide runoff to water. DNP is used as a pesticide, a wood preservative, and to manufacture dyes. DNP is an uncoupler, and has the ability to separate the flow of electrons and the pumping of hydrogen ions for ATP synthesis. The mechanism of action of DNP is believed to inhibit the formation of ATP by uncoupling oxidative phosphorylation. Although the interaction of proton-conducting ionophores with photosynthetic electron transport has been extensively studied during the past decade, the mode of action of protonophores remained uncertain. Electrochemical measurements in real time are required for a better understanding of the molecular mechanism of the action of protonophores.

Most protonophoric uncouplers widely used in photosynthesis research, are oxidized by the manganese cluster of the Photosystem II oxygen-evolving complex in chloroplasts and inhibit photosynthetic water oxidation. Oxidized uncouplers can be reduced by the membrane pool of plastoquinone, leading to formation of an artificial cyclic electron transfer chain around Photosystem II involving uncouplers as redox carriers. Protonophores inhibit the Hill reaction in chloroplast and cyanobacterial membranes. Inhibition of the Hill reaction by uncouplers reaches maximum when the pH corresponds to the pK values of these compounds.

DNP induces fast action potentials and decreases the variation potential to zero in soybeans. The addition of aqueous DNP to the soil induces fast action potentials in soybeans. After treatment of soil by an aqueous solution of DNP, variation potential, measured between two silver/silver chloride microelectrodes in a stem of soybean, slowly decreases from 80-90 mV (negative in a root, positive on the top of the soybean) to zero during a 48-hour time frame. The duration of single action potentials, 24 hours after treatment by DNP, varies from 3 sec to 0.02 sec (Figure 5). The amplitude of action potentials is about 60 mV. The maximum speed of action potential propagation is one m/sec. After two days, the variation potential stabilized at zero. Fast action potentials were generated in a soybean, with amplitude of about 60 mV, 0.02 sec duration time, and a speed of 2 m/sec, after treatment. Fromm and Spanswick studied the inhibiting effects of DNP on the excitability of willow by recording the resting potential in the phloem cells. In willow, 10-4 M DNP rapidly depolarized the membrane potential by about 50 mV.

Electrical activity of excitable membranes

 HH equivalent circuit
Modified HH equivalent circuit
Fig. 6. The Hodgkin-Huxley (HH) equivalent circuit for an axon (a) and the modified HH circuit for sieve tubes in phloem (b).
Nerve cells in animals and phloem cells in plants share one fundamental property: they possess excitable membranes through which electrical excitations, in the form of action potentials, can propagate. These propagating excitations are modeled theoretically as traveling wave solutions of certain parameter dependant nonlinear reaction-diffusion equations coupled with some nonlinear ordinary differential equations. These traveling wave solutions can be classified as single loop pulse, multiple loop pulses, fronts and backs, or periodic waves of different wave speed. This classification is matched by the classification of the electrochemical responses observed in plants. The experimental observations also show that under influence of various pathogens, the shapes and wave speeds of the electrochemical responses undergo changes. From the theoretical perspective, the changes in the shapes and wave speeds of the traveling waves can be accounted for by appropriate changes in parameters in the corresponding nonlinear differential equations.

Hodgkin and Huxley formulated a membrane model that accounts for potassium, sodium, and ion leakage channels in squid giant axons (Figure 6a). The membrane resting potential for each ion species is treated like a battery. A variable resistor models the degree to which the channel is open.

Fromm and Spanswick found that electric stimulation of the plant is followed by ion shifts that are most striking in the phloem cells. While the content of potassium and chloride was diminished after stimulation, the amount of cytoplasmic calcium increased slightly. These displacements lead to the conclusion that calcium ion influxes, as well as potassium and chloride ion effluxes are involved in the propagation of action potentials. The main difference between the propagation of action potentials in animals and plants is that in an axon there is the potassium/sodium ion transmembrane transport, but in phloem cells the potassium/calcium ion channels are involved in this process (Figure 6b).

Measuring of action and variation potentials in plants

Figure 7 is an example of the experimental configuration for the measuring of extracellular action potentials and variation potential. All electrochemical measurements are conducted at constant temperature inside a Faraday cage mounted on a vibration-stabilized table in a laboratory. A microcomputer with multifunction data acquisition board is interfaced, through a Simultaneous Sample and Hold interface with nonpolarizable, reversible silver/silver chloride electrodes to record the digital data. The multifunction data acquisition board provides high resolution and a wide gain range. It features continuous, high-speed data acquisition; a single channel can be sampled at any gain up to 333 ksamples/sec. Each analog input channel has its own instrumentation amplifier with differential inputs. The output of every instrumentation amplifier is routed to a track-and-hold amplifier. In track mode, the outputs of the track-and-hold amplifiers follow their inputs. When put into hold mode, the track-and-hold amplifier outputs simultaneously freeze holding the signal levels constant. The digitized data includes negligible time skew (less than 5 × 10-8 sec) between channels.

Experimental set-up
Fig. 7. The experimental set-up for measuring electrical signals in green plants.
As a fundamental rule of sampled data systems, the input signal must be sampled at a rate greater than twice the highest frequency component in the signal; that is, fs/2 > fa, where fs is the sampling frequency and fa is the maximum frequency of the signal being sampled. This rule is known as the Shannon sampling theorem, and the critical sampling rate is called the Nyquist rate. A violation of the Nyquist criterion is considered undersampling, and results in aliasing. Theoretically, the recovery of information about signals with frequencies at or below the Nyquist frequency is impossible. Due to this limitation, all data should be collected with high-speed data acquisition system.

Ksenzhek and Volkov described the preparation of silver/silver chloride electrodes from Teflon coated silver wire. Silver/silver chloride electrodes are sensitive to changes in temperature; therefore, the temperature should remain constant. Both silver/silver chloride electrodes are identical, and are referred to as a reference and working electrodes. The reference electrode (negative) is generally inserted in the stem or in a root of a soybean. On the other hand, the upper working electrode (positive) is inserted in the stem or a leaf. Following insertion of the electrodes, the plants should be allowed to rest until a stable potential difference is obtained between the working and reference electrodes. The insertion of the electrodes in plants induces action potentials across the stem and slow fluctuations of the variation potential. After approximately one to two hours, the variation potential stabilizes.

The automatic measurements of the extracellular and intracellular electrical potential difference can be effectively used in plant electrophysiology to study the molecular interfacial mechanisms of ion transport, the influence of external stimuli on plants, and for investigating the bioelectrochemical aspects of the interaction between insects and plants.

Green plants interfaced with a computer through data acquisition systems can be used as fast biosensors to: monitor the environment; detect effects of pollutants, pesticides, and defoliants; predict and monitor of climate changes; and facilitate the direct and fast control of conditions influencing the agricultural harvest. The use of new computerized methods provides opportunities for detection of fast action potentials in green plants in real time.

Electrochemistry of photosynthetic systems

Photosynthetic systems

Schematic model of PS I and PS II
Fig. 8. A schematic model of the electron transport chain with most of the light-harvesting pigment-protein complexes omitted.
Annually, mankind consumes about 4 × 1017 kJ of energy. This rate is rising rapidly; in fact, it is doubling every 20 years. At present, the known fossil fuel reserves can only yield approximately 5 × 1019 kJ of energy. This deficiency makes exploring alternative sources of energy fundamentally important. Harvesting solar energy is an obvious possibility. On Earth, the sun contributes nearly 5 × 1021 kJ of energy per year; however, 3 × 1018 kJ of this energy is converted into chemical energy by photosynthetic plants and microorganisms. In water-oxidizing photosynthesis, photosystems I (PS I) and photosystems II (PS II) are utilized in a series (Figure 8). The electron transfer starts in both photosystems vectorially across the membrane. Light energy is harvested by the photosynthetic pigment systems where the electronic structure of excited-state chlorophyll donates an electron to a primary acceptor pheophytin, the first component of an electron transport chain. The electron is enriched with the energy of light that was absorbed. In the process of electron transport the energy is captured in two ways. The first involves coupling a proton pump mechanism that is coupled to the sequential redox reactions in one part of the electron transport chain, establishing a proton gradient across the thylakoid membrane. The electrochemical energy of the proton gradient is then drives ATP synthesis via the ATP synthase enzymes embedded in the membrane. The second energy capture occurs when an acceptor molecule such as the enzyme NADP is reduced to NADPH, which in turn, is used to reduce carbon dioxide in the Calvin cycle.

Structure and composition of the oxygen-evolving complex in vivo

The redox map of photosynthesis in green plants can be described in terms of the well-known Z-scheme proposed by Hill and Bendall. The diagram in Figure 9, often called the Z scheme because of its overall form, outlines the pathway of electron flow between two photosystems and the energy relationships in the light reactions. The main advantage of the currently accepted Z-scheme, depicted in Figure 9, lies in the specific mechanism of charge transfer at the stage of water oxidation, which is a multielectron reaction mechanism involving no unknown intermediates.

Membrane structure
Fig. 9. The electron transfer during photosynthesis in higher plants. The abscissa shows the midpoint redox potential at pH 7.0. Light is absorbed in two sets of antenna chlorophyll molecules, and the excitation energy is transferred to the reaction center chlorophyll a molecules of photosystem II (P680) and photosystem I (P700) forming (P680)* and (P700)*. The latter two initiate electron transport. Abbreviations: LHC is light harvesting complex; Z and D are tyrosine residues; Cyt b559 is cytochrome b559 of unknown function; Pheo is pheophytin; QA, QB and PQ are plastoquinone molecules; Fe2S2 represents the Rieske iron-sulphur center; Cyt f stands for cytochrome f; PC is plastocyanin; Ao is suggested to be a chlorophyll molecule; A1 is possibly vitamin K; FNR is ferredoxin NADP oxidoreductase; x stands for inhibitors (DBMIB: dibromo-3-methyl-6-isopropyl-p-benzo-quinone; DCMU: 3-(3',4'-dichlorophenyl)-1,1-dimethylurea).
The molecular organization of a thylakoid membrane is shown in Figure 8. The spectral characteristics of photosystem II indicate that the primary electron donor is the dimer of chlorophyll P680 with absorption maxima near 680 and 430 nm (billionth of a meter). Water can be oxidized by an oxygen-evolving center composed of several chlorophyll molecules, two molecules of pheophytin, plastoquinol, several plastoquinone molecules, and a manganese-protein complex containing four manganese ions. The oxygen-evolving complex is a highly ordered structure in where of polypeptides interact to provide the appropriate environment for cofactors such as manganese, chloride and calcium, as well as for electron transfer within the complex. Figure 10 shows the electronic equivalent circuit of PS I and PS II.

Manganese binding centers were first revealed in thylakoid membrane by electron paramagnetic resonance (EPR) methods, and it is now understood that four manganese ions are essential for oxygen evolution during water photo-oxidation. Plastoquinone (PQ) acts as a transmembrane carrier of electrons and protons between reaction centers of two photosystems in the case of noncyclic electron transfer and may also serve as a molecular "tumbler" that rotates between one-electron reactions and two-electron reactions. Pheophytin is an intermediate acceptor in photosystem II. The direct formation of P680 pheophytin ion radical pairs was revealed by experiments on magnetic interactions between Pheo- and PQ- in the EPR spectra.

Water oxidation to molecular oxygen is a multielectron process that proceeds with surprisingly high efficiency. The water oxidation reaction can proceed upon illumination at 680 nm (billionth of a meter), a wavelength of light that excludes one-electron mechanisms using hydroxyl and oxygen radicals (Figure 11). For a three-electron reaction, an oxidant stronger than the cation-radical P680+ is needed. A synchronous 2:2-electron pathway of the reaction is energetically possible if the energy of the binding of the two-electron intermediate is about -40 kJ/mol. This value corresponds to the energy of two hydrogen bonds forming between hydrogen peroxide and the catalytic center. For this case a molecular mechanism can be proposed (Figure 12), and will be discussed below. The synchronous four-electron oxidation of water to molecular oxygen (Figure 13) is also energetically possible.

One-electron mechanisms of water oxidation are likely to be operative in some model systems with a low quantum efficiency, but two- or four-electron reactions cannot occur due to kinetic limitations. The intermediates formed in these systems would be highly reactive and could enter into side reactions of hydroxylation, oxidation, and destruction of chlorophyll and other components of the reaction center.

 Electronic equivalent circuit
Fig. 10. Electronic equivalent circuit of Photosystems II and I.
 Energy diagram
Fig. 11. Energy diagram for possible routes of the reaction 2H2O <==> O2 + 4H+ + 4e-. Gm is the reaction energy at pH 7.


Molecular mechanism of oxygen evolution in vivo

 2:2-electron mechanism
Fig. 12. Proposed 2:2-electron mechanism of water photooxidation.
 Four-electron mechanism
Fig. 13. Proposed four-electron mechanism of water photooxidation.
Membrane-bound P680 enters an excited state upon illumination. In dimers and other aggregated forms of chlorophyll the efficiency of triplet states is low, and it is the singlet-excited states that undergo photochemical transformations. In several picoseconds (trillionth of a second), an electron is first transferred to pheophytin, and then to plastoquinone QA. From plastoquinone QA, the electron transfers to another polypeptide-bound plastoquinone QB, in thylakoid membrane (Figure 9). This electron transfer results in an oxidized pigment and a reduced acceptor. The cation radical P680+ successively oxidizes four manganese ions, which drives the production of molecular oxygen. Formation of a cation radical of chlorophyll or oxidation of manganese ions is accompanied by dissociation of water bound to the reaction center and ejection of protons. The synchronous multielectron process that describes all four oxidizing states of the oxygen-evolving complex was proposed earlier. The transfer of electrons in a 1:1:1:1 series from a manganese cluster to the electron-transport chain is accompanied by the ejection of 1:0:1:2 protons and the evolution of molecular oxygen.

Protons are released from reaction centers either by regulators of proton distribution, or by a hydrogen bond transfer through the hydration shell of manganese ions. The hydration sphere of manganese is known to contain water molecules that rapidly exchange protons with bulk water. The presence of divalent cations at the interface between two immiscible electrolyte solutions facilitates strong adsorption of water molecules belonging to the second hydration shell of ions. Thus, a portion of coordinatively bound water enters the compact part of the electric double layer, which changes its differential capacity at the interface. In the case of multivalent ions with small radii, the electric field near a cation is large. This can disturb the microstructure of the adjacent intrathylakoid space and bring about dielectric saturation effects.

Manganese ions play a particularly important role in the evolution of dioxygen during photosynthesis. Although there are several manganese pools in chloroplasts, only one is involved in water oxidation. The manganese ions associated with oxygen evolving complex of the chloroplast can perform a number of functions:

  • Mn-polypeptide complex is a redox intermediate that protects the reaction center from redox and radical destruction;
  • Mn-clusters are redox buffers facilitating accumulation of four holes in the reaction center of photosystem II, which are needed to ensure water photooxidation;
  • Hydrated multivalent manganese cations bring water to the reaction center so that rapid proton exchange and transport through the hydration shell of manganese ions in the zone of water oxidation;
  • Multivalent manganese ions induce dielectric saturation effects in the polar region of the reaction center of photosystem II, which reduces the reorganization energy of the medium during charge transfer.


Brief definitions of biological terms which are not appropriate for inclusion in the Electrochemistry Dictionary.

all-or-nothing law: a statement concerning the reaction of phloem. A stimulus of any intensity that produces one of two responses: a response of invariable strength or no response at all.

ATP: adenosine triphosphate. A coenzyme of fundamental importance found in the cells of all organisms; it provides a means of storage of energy for many cellular activities.

axon: The threadlike extensions on a neuron, or nerve cell which conducts nerve impulses. A neuron usually has one long axon, which branches only at its termination.

biosensor: a device that detects, records, and transmits information regarding a physiological change or process.

Calvin cycle: a light independent set of reactions occurring in the stroma of the chloroplast that uses NADPH and phosphorous from ATP to produce glucose.

chorophyll: any of a group of green pigments found in photosynthetic organisms.

chloroplast: plastid containing chlorophyll.

companion cell: in flowering plants, a cell closely associated in origin, position and function with a cell of a sieve-tube. A companion cell has a dense protoplasm and prominent nucleus. A companion cell and a sieve-tube cell are formed by longitudinal division of one parent cell. It appears to regulate the activity of a sieve-tube.

cotyledon: embryonic leaf in seed-bearing plants.

cyanobacterium: a photosynthetic bacterium of the class Coccogoneae or Hormogoneae, generally blue-green in color and in some species capable of nitrogen fixation. Cyanobacteria were once thought to be algae. Also called blue-green alga. the protoplasm of a cell excluding the nucleus.

cytoplasm: the protoplasm of a cell excluding the nucleus.

dedrite:A long extension of a neuron with thin, treelike branches. It receives nerve signals and transmits them to the main body of the cell

DNA: a long linear polymer found in the nucleus of a cell, shaped like a double helix; associated with the transmission of genetic information.

excitability: the disposition of a tissue or living cell to respond to a stimulus or change in the environment.

excitation: the act of producing or increasing stimulation; the immediate response of a cell or a tissue to a stimulus or change in the environment.

grana: stacks of thin layers in a chloroplast in which the green pigment chlorophyll is contained.

impulse: the electrochemical transmission of a signal along a phloem or a nerve fiber that produces an excitatory or inhibitory response at a target tissue.

ionophore: any of a group of organic compounds that facilitate the transport of ions across the cell membrane.

intercellular: between or among cells.

intracellular: describes a processes occurring, or substance found inside the cell.

ionosphere: the upper region of the Earth's atmosphere, between 50 and 400 km above the Earth's surface. It contains free electrons originating from the ionization caused by ultraviolet radiation and x rays from the sun. The ionosphere reflects radio waves, and also radiation from outer space.

irritant: a material causing irritation, especially physical irritation.

messenger RNA: the template for protein synthesis; the form of RNA that carries information from DNA in the nucleus to the ribosome sites of protein synthesis in the cell.

mitochondrial: a spherical or elongated organelle in the cytoplasm of nearly all plant cells, containing genetic material and many enzymes important for cell metabolism, including those responsible for the conversion of food to usable energy.

morphology: the form and structure of an organism or one of its parts.

neuron: a nerve cell; it receives and conducts electrical impulses from the brain. It consists of a cell body, an axon, axon terminals, and dendrites.

nucleus: central part of a living cell.

organelle: part of a cell that is the structural and functional unit.

pathogen: any disease-producing agent (especially a virus or bacterium or other microorganism).

pheophytin: a chlorophyll in which the central magnesium atom has been replaced by two hydrogen atoms. It acts as an early electron acceptor in photosystem II.

phloem: a type of vascular tissue, in flowering plants it contains sieve-tubes and companion cells supported by ground tissue and also flowers. The phloem conducts food materials synthesized by the leaves, to all parts of the plant.

phosphorylation: biochemical reaction in which an organic substrate, such as sugar, is combined with a phosphate ion by enzyme reaction. The phosphorylation of some organic substances produce high-energy bonds in the product, for example ATP; these products are highly reactive under enzymatic activation.

photosynthesis: the complex process in green plants, in which complex organic compounds are synthesized from carbon dioxide and water using energy obtained from the sunlight.

phototropic response: a tropism in response to light. For example, the growth curvature of a stem to light is an example of phototropism.

plasma membrane: the semipermeable membrane that encloses the cytoplasm of a cell.

polysome: a cluster of ribosomes connected by a strand of messenger RNA and functioning as a unit in protein synthesis.

protonophore: any of a group of organic compounds that facilitate the transport of hydrogen ions across the cell membrane.

protoplasm: the jelly-like granular material which comprises the living content of cells; it is a complex mixture of organic and inorganic substances in a state of continuous chemical change.

refractory period: a time period when the state of a membrane is such that it cannot conduct an impulse; this occurs immediately after an impulse has passed.

resting potential: a membrane potential characteristic of a non-conducting, excitable cell, where the inside of the cell is more negative than the outside.

ribosome: an organelle in the cytoplasm of a living cell.

sieve tube: a long tube-like structure consisting of blast cells arranged in a longitudinal row, with the cells interconnected by sieve plates.

sieve plates: in flowering plants the end walls of a blast cell are pierced by numerous pores through which no substance can pass strands of protoplasm. Sieve-tube plates connect blast cells in a series to form a sieve-tube.

subthreshold: (usually a stimulus that is) not strong enough to be perceived or to produce a response.

threshold: alternative term for minimal stimulus. A stimulus to with intensity just sufficient enough to provoke a response.

thylakoid: a circular, fluid-filled sac that forms grana. Photosynthetic pigments are located in thaylakiod membrane.

tropism: an involuntary orienting response.

uncoupler: a wide range of chemically unrelated compounds that inhibit mitochondrial ATP synthesis but are often observed to stimulate the rate of electron transport.

variation potential: the potential difference between phloem cells.

vascular: relating to the tubes which carry blood or liquids in animals and plants.


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  • Interfacial Catalysis, A. G. Volkov, M. Dekker, New York, 2003.

  • Plant Electrophysiology: Excitation Waves and Effects of CCCP on Electrical Signaling in Soybean, A. Labady, D'J. Thomas, T. Shvetsova, and A. G. Volkov, "Bioelectrochemistry" Vol. 57, pp 47-53, 2002.

  • Soybean Electrophysiology: Effects of Acid Rain, T. Shvetsova, J. Mwesigwa, A. Labady, S. Kelly, D'J. Thomas, K. Lewis, and A. G. Volkov, "Plant Science" Vol. 162, pp 723-731, 2002.

  • Waves of Excitation and Action Potentials in Green Plants, A. G. Volkov, T. Shvetsova, and V. S. Markin, "Journal of Biophysics" Vol. 82, p 218a, 2002.

  • Electrochemistry of Soybean: Effects of Uncouplers, Pollutants, and Pesticides, A. G. Volkov and J. Mwesigwa, "Journal of Electroanalytical Chemistry" Vol. 496, pp 153-157, 2001.

  • Green Plants as Environmental Biosensors: Electrochemical Effects of Carbonyl Cyanide 3-chlorophenylhydrazone on Soybean, A. G. Volkov, A. Labady, D'J. Thomas, and T. Shvetsova, "Analytical Science" Vol. 17, pp i359-i362, 2001.

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  • Electrochemical Signaling in Green Plants: Effects of 2,4-Dinitrophenol on Resting and Action Potentials in Soybean, J. Mwesigwa, D. J. Collins, and A. G. Volkov, "Bioelectrochemistry" Vol. 51, pp 201-205, 2000.

  • Green Plants: Electrochemical Interfaces, A. G. Volkov, "Journal of Electroanalytical Chemistry" Vol. 483, pp 150-156, 2000.

  • Plant Electrophysiology: Pentachlorophenol Induces Fast Action Potentials in Soybean, A. G. Volkov, D. J. Collins, and J. Mwesigwa, "Plant Science" Vol. 153, pp 185-190, 2000.

  • Plant Energetics, O. S. Ksenzhek and A. G. Volkov, Academic Press, San Diego, 1998.

  • Action Potentials of Higher Plants not Possessing Motor Activity, A. M. Sinukhin and V. V. Gorchakov, "Biophysics" Vol. 11, pp 966-975, 1996.

  • Insect Induced Bioelectrochemical Signals in Potato Plants, A. G. Volkov and R. A. Haack, "Bioelectrochemical Bioenergetics" Vol. 35, pp 55-60, 1995.

  • Action Potentials in Maize Sieve Tubes Change Phloem Translocation, J. Fromm and T. Bauer, "Journal of Experimental Botany" Vol. 45, pp 463-469, 1994.

  • Evidence for Two Pathways of Phloem Loading, W. Eschrich and J. Fromm, "Physiologia Plantarum" Vol. 90, pp 699-707, 1994.

  • Growth Acceleration of Bean Sprouts by the Application of Electrochemical Voltage in a Culturing Bath, Mizuguchi, Y. Watanabe, H. Matsuzaki, Y. Ikezawa and T. Takamura, "Denki Kagaku" Vol. 62, pp 1083-1085, 1994.

  • Characteristics of Action Potentials in Willow (Salix viminalis L.), J. Fromm and R. Spanswick, "Journal of Experimental Botany" Vol. 44, pp 1119-1125, 1993.

  • Electric Signaling and Systemic Proteinase Inhibitor Induction in the Wounded Plant, D. C. Wildon, J. F. Thain, P. E. H. Minchin, I. R. Gubb, A. J. Reilly, Y. D. Skipper, H. M. Doherty, P. J. O'Donnell, and D. J. Bowles, "Nature" Vol. 360, pp 62-64, 1992.

  • Characteristics of Action Potentials in Helianthus-Annus, T. Zavadzki, E. Davis, H. Dziubinska, and K. Trebacz, "Physiologia Planetarium" Vol. 83, pp 601-604, 1991.

  • Control of Phloem Unloading by Action Potentials in Mimosa, J. Fromm, "Physiologia Plantarum" Vol. 83, pp 529-533, 1991.

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  • The Role of Electricity in Plant Movements, P. J. Siomons, "New Physiologist" Vol. 87, pp 11-37, 1981.

  • Action Potentials and Rapid Plant Movements, T. Sibaoka, in "Plant Growth Substances 1979, Proceedings of the 10th International Conference, Madison, Wisconsin, July 22-26, 1979" pp 462-469, F. Skoog (editor), Springer, Berlin, 1980.

  • Action Potentials in Lipinus angustifolius L. shoots. V. Spread of Excitation in the Stem, Leaves and Root, T. Zavadzki, "Journal of Experimental Botany" Vol. 31, pp 1371-1377, 1980.

  • Resting Membrane Potential and Action Potential of Nitella expansa Protoplasts, S. Abe, J. Takeda, and M. Senda, "Plant Cell Physiology" Vol. 21, pp 537-546, 1980.

  • Electrical Diffuse Layers and Their Influence on Photosynthetic Processes, J. Barber, J. Mills, and A. Love, "FEBS Letters" Vol. 74, pp 174-181, 1977.

  • Electro-Osmotic and Biopotential Measurement on Phloem Strands of Nymhoides, D. S. Fensom and D. C. Spanner, "Planta" Vol. 88, pp 321-331, 1969.

  • Action Potentials in the Reproductive System of Plant, A. M. Sinukhin and E. A. Britikov, "Nature" Vol. 215, pp 1278-1280, 1967.

  • Excitable Cells in Mimosa, T. Sibaoka, "Science" Vol. 137, pp 226-228, 1962.

  • Plant Autographs and Their Revelations, J. C. Bose, Macmillan, New York, 1927.

  • Transmission of Excitation in Plants, J. C. Bose, "Nature" Vol. 119, pp 48, 1927.

  • The Nervous Mechanism of Plants, J. C. Bose, Longmans-Green, New York, 1926.

  • Transmission of Stimuli in Plants, J. C. Bose, "Nature" Vol. 115, pp 457, 1925.

  • On the Electromotive Properties of Dionaeva in the Excited and Unexcited States, J. Burdon-Sanderson, "Philosophical Transactions" Vol. 179, pp 417-449, 1888.

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  • On the Mechanical Effects and on the Electrical Disturbance Consequent on Excitation of the Leaf of Dionaeva Muscipula, J. Burdon-Sanderson and F. J. M. Page, "Proceedings of the Royal Society of London" Vol. 25, pp 411-434, 1876.

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