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The life of plants is driven by the coordinated processing of

a vast number of chemical reactions.

All chemical reactions are accompanied by a redistribution of electrical charge in the reacting molecules.

In the electrochemical process the overall reaction is divided into two conjugated reactions. An electron is released from a molecule in one location, where as in another site it is added to another molecule. The site of the system where the oxidative process occurs is usually called the anodic site (anodic), and the site where reduction proceeds is called cathodic site (cathode).

Introduction

The scheme of electrochemical oxiation of hydrogen by oxygen may be written as the following pair of conjugated reactions :

H2 – 2e → 2H+ released of electrons at anodic site

½ O2 + 2H- + 2e → H2O addition of electrons at cathodic site

Inasmuch as the anodic and cathodic

sites are separated spatially and

processes at these sites are

conjugated, electrochemical

systems must include at least

two phase with different types

of conductivity.

Electrochemical processes depend on electrical fields. The electrochemical potential, total work required to bring particle i from a vacuum into a phase :

µi = µi + ziFΦ

where,

ziFΦ : electrostatic work required to bring a charge ziefrom infinity in a vacuum to inside phase

F : Faraday number, the quantity of electricity associatedwith 1 mol of elementary charge, 96500 C

Electrochemical processes enable the direct transformation of chemical energy into work. In chemical reactions the total change in energy of a reacting system is transformed into heat and, thus, into chaotic movement of molecules.

Scheme of an electrochemical systems in engineering (A) and in

biology (B). In the former case the electromotive force arises across the gap in an electron conductor, in the latter it arises across a phase with ionic conductivity.

Electrochemistry in Engineering and Biology

Panel A represent the scheme of an electrochemical engineering device such as, for example, a battery. Metals are used as phases with electronic conductivity, and ionic conductors are usually electrolyte solution. Electromotive force, ΔE, is a difference in electrical potentials arises between two electrodes and can be used to perform work.

Panel B represent the scheme of an bioelectrochemical device. The electron conductor is continous, whereas the phase with ionic conductivity is split.

Nature designed biochemical mechanisms with the conductive medium, insulator for the separation of aqueous phases and the electronic conductor to provide the transfer electrons between the points where they are released and accepted.

Phospolipid membranes, which surround cells and subcellular structures such as chloroplast and mitochondria, play the role of insulators. The function of an electron conductor in biological systems is performed by electron transfer chain.

Thermodynamics of Electrochemical Systems

The expression for the equilibrium electrode potential in the case of a redox reaction proceeding with the participation of m protons and n electrons is

The redox potential of a system measure its capability to display electron donor or acceptor properties, or to function as a reducing or an oxidizing agent. A system with high positive redox potentials may act as an oxidant toward a system with less positive potential and vice versa.

As the values of redox potentials are specified against the SHE, they characterize the reducing “power” of redox system compared to that of molecular hydrogen at 1 atm and 25oC in acid solution.

Redox systems, involved in biochemical processes,

present example of several types of substances. Among them are a variety of quinoid compounds, derivatives of isalloxazine, derivatives of nocotinoicacid, metalloporphyrins, and iron-sulfur protein.

Biological Redox Systems

These substances are present in plants primarily as

p-benzo and p-naphtho-quinones with various side chains. The molecules of these compounds do not change their overall configuration during redox reactions.

The Quinones

Present a somewhat different

type of quinoid structure. Their ability to undergo redox conversion is due to the ease in reorganizing their system of conjugated bonds.

The representatives of the flavins are riboflavin (vitamin B2), flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD).

Isoalloxazines (Flavins)

This significant group of substances capable of undergoing

redox conversions is represented mainly by NAD+ (nicotinamide adenine dinucleotide) and NADP+ (nicotinamideadenine dinucleotide phosphate).

A molecule of NADP+ differs from that of NAD+ by the presence of an additional phosphate group linked to one of the two five-member ribose cycles.

Derivatives of Nicotinic Acid

It contain in their active center several

iron atoms (usually between one and four) linked to the protein body via sulfur bridges.

They play a definite role in photosynthetic electron transfer, nitrogen fixation, the reduction of nitrates and nitrites, and other processes. The function due to changes in the valences of their iron atoms.

The Iron-Sulfur Proteins (Ferredoxins)

Sequence of redox component along transfer electron

due to flow of electrons from a high energy level to a lower is called electron transfer chain (ETC).

The mitochondrial ETC is often referred to as the respiratory chain because the step of biological oxidation of organic matter and reduction of oxygen occurs here.

Arrangement of the Electrochemical Processes

Electrons are delivered to the ETC from the reduced

components of the Krebs cycle.

The first link of the ETC consists of enzymes known as the pyridene-dependent dehydrogenases.

The next link of the chain, to which NADH releases the transfer electrons, is the flavoprotein NADH dehydrogenase.

The ubiquinones are found farther down the chain.

The subquent path of electrons along the chain begins at the potentials of ubiquinone and continues down to the equilibrium potential oxygen.

The Respiratory Chain

The process of photosynthesis involves the oxidation

of water followed by the released of oxygen and the generation of highly substances, whch are then used to carbon dioxide.

Simultaneously, ATP is produced by the energy of electron flow down the ETC.

The electron transfer chain is arranged more intricately than that in mitocondria, but the basic principle is quite similar.

Electron Transfer at Photosynthesis

The coupling of electron flow to the synthesis of ATP is

provided by the arrangement of thylakoid bags as closed containers whose internal volume is separated from the outer medium by a membrane.

The components of the electron transfer chain are localized across the membrane asymetrically in quite a definite fashion.

The ETC embedded the membrane plays the role of an electronic conductor.

Electron s enter the ETC and leave it as the result of anodic or cathodic electrochemical reactions at the membrane-water interface.

Coupling of Electron Flow with The Synthesis of ATP

The path of electron flow along the ETC embedded in the

membran begins at the level of NADH and terminates in the reduction oxygen.

As a result an electrochemical potential gradient of protons arises across the membrane.

The gradient is the form in which energy is stored across the membranes of chloropasts and then used to produce ATP.

Electronchemical proton gradients comprise two components, one due to the difference in hydrogen ion concentration and the other due to the difference electrical potential.

In biological electrochemical systems such as

chloroplasts and mitocondria, energy is stored in the membrane structure as a transmembrane gradient of proton electrochemical potential.

The energy stored as the transmembrane gradient of electrochemical potential drives the processes of ATP synthesis.

Synthesis of ATP in mitochondria is driven by the inward flow of protons through ATPase, whereas in chloroplasts the ATPase is oriented oppositely and is driven by outward proton flow.

The redox potential of a system depends upon the

ratio between the number of molecules in an oxidized state and those in a reduce state.

In large systems this ratio, i.e., the degree of oxidation, varies in a continous manner.

The situation changes, however, for a small system consisting only a few particles.

In considering biological objects, we have dealt with small, nonstatistical systems.

Is The Notion of Redox Potential Applicable to Small Systems?

The energy-transducing complexes embedded in the

membranes of mitochondria or chloroplasts contain only a few molecules that develop redox properties and provide the translocation of electrons through the membrane.

In analyzing, the usual approach based on a statistical background can hardly be regarded as justified.

Hypothesis by Ludwig Boltzmann, the time average state of a sufficiently large number of molecules.

The redox potential of a small system is imposed by

the redox potential of the macrosystem with which a small system is in equilibrium.

The relationship between the redox potential and the probability that the molecule will be found in one of two conjugated redox states, i.e., to be either a donor or an acceptor of electrons, gives a key to understanding why electron transfer chains are designed from a large set of components that overlap a wide range of potential levels in small steps.