INFLUENCE OF RADIATING ON ELECTRON TRANSPORT IN
CHLOROPLASTS. MATHEMATICAL MODEL.
A.N.Nasibova, R.D.Gasimov, L.A.Aliyev, RJ.Khalilov
Institute of Radiating Problems of National Academy of Sciences of Azerbaijan
ABSTRACT
This work is dedicated to the mathematical model of light stages of photosynthesis which describes the processes of electron transport in chloroplasts. The diffusion of plastocyanine and plastoquinone is taken into account. A chloroplast is taken as a system of granal and intergranal thylakoids. Influence of radiating on kinetics of electron transport in chloroplasts is analysed.
1.INTRODUCTION
Ecological problems are very actual problems of modern life. This problems influence badly on wildlife, including plants. Some ecological factors influence the process of photosynthesis. Photosynthesis is one of the important processes in the nature. Due the photosynthesis plants acquire the energy of sunlight, accumulate organic formations, enrich the atmosphere with oxygen, providing necessaiy conditions for preservation and development of life on the Earth.
Processes of photosynthesis in higher plants take place in chloroplasts - energy creating organells of a vegetative cell. There are thylakoids under a shell of a chloroplast. The ATP - synthesis and electron transport complexes of the photosynthesis apparatus are located in membranes of thylakoids.
The scheme of the chloroplast model is shown on fig. 1.The processes of electron transport taking peace in granal and intergranal thylakoids are considered. Those thylakoids are in the form of coaxially located cylinders of radiuses a and b.
Fig. 1. T h e s c h e m e o f c h lo r o p la s t and lo c a tio n o f e le c tr o n tra n sp o rt c o m p le x e s in th y la k o id s .
The external cylinder of radius b includes a granal thylakoid (a cylinder of radius a) which continuously transforms into an intergranal thylakoid. The motionless complexes of PS 2 (photosystem 2) are uniformly distributed in membranes of the granal area A ; the motionless complexes PS 1 are uniformly distributed in
membranes nosed to strome (the areai?); the citochrome b /f complexes are uniformly distributed in areas A
and B.
For the description of electron transport processes let’s introduce the variables: P+700 (r, t) = [P+70o] - the local concentration of oxidized reaction centers PS 1; Pc (r, t) = [Pc] - the local concentration of oxidized plastocyanine diffusing in thulakoids; Q (r, t) = [Q] - the local concentration of oxidized plastoquinone diffusing in membranes. All these functions are defined as local concentrations of the corresponding components in the vicinity of the point r at the moment t
Let's think that the PS 1 - complexes containing the reaction centers P700, are rigidly fixed in the membrane of intergranal thylakoids. Plastocyanine and plastoquinone can diffuse in lateral direction in a membrane and in a thylakoid. In this case the system of equations, describing dynamics of oxidation - reduction transformations of electron transport, has the following form:
d[P7m\
= L,KP7oo[P700] - Kpc[P [Pc] (1)dt
d\Pc]
A H ’ = Dpc V2 [Pc] - 2 Kqct bf + KPc [P+70o ] [Pc ] (2)ct
® = Dq V 2 [Q] + KqCT bf - L2 CT FS 2 KHo [H+0] / 2 (3)ct
Where Li and L2 - numbers of quantums of light influencing the reaction centers PS 1 and PS 2 in unit of time; & bf and & Fs2 densities of b/f complexes and PS 2 respectively; Dpc Dq - are the diffusion coefficients of plastocyanine and plastoquinone; KP70o, KPc, KHo - effective constants of speeds of corresponding reactions; [H+0] - local concentration of ions of hydrogen in interthylakoidal space.
The numerical realization of this model gives the adequate description of kinetics of main stages of electron transport on which the regulation of transportation of electrons between two photosystems is carried out. Thus having defined the system of differential equations ( 1 - 3 ) and having set the initial and boundary conditions, it is possible to investigate the behaviour of our model system by numerical integration of the given system. Here the influence of some radiation factors on the process of photosynthesis is taken into account.
We compare the obtained results with the results of our experiments. We have investigated the kinetics of photoinduced redox - transformations of the fibers transporting in the electron transport chain of the energy transforming organells of plants. Investigationof kinetics of photoinducedoxidation - reduction transformations of different plants in the electron transport chain by the EPR - spectroscopy method gives a lot of information about its functional organization and interaction of two photosystems. The main advantage of the EPR - spectroscopy is that it’s possible to observe the state of the reaction centers P700 by meansof the EPR 1 signal not only in suspensions of chloroplasts , but also in intact leaves of higher plants. This opportunity has appeared after Baynert and Kok [2] have found that the oxidation of P700 centers is carried out together with appearance of free radicals in chloroplasts which give the single EPR 1 signal.
Our experiments have shown that the irradiation of leaves by UV- light results in change of non monotonic kinetics of oxidation - reduction transformations P700. As it is seen in fig.2 the UV - irradiation catalizes the EPR 1 signal to the stationary level under the action of white light. This can be a consequence of 1) reduction of inflow of electrons from PS 2 to P700due to inhibition PS 2; 2) due to activation of outflow of electrons from PS 1.
Fig.2. The kinetics of light - induced changes of EPR 1 signal, obtained by the P700 centers of intact leaves Vida faba\ 1 - control; 2 - 5 min.,3 - 15 min. and 3 - 2 5 min. UV - B irradiation ,| and [ is turning the light on and off respectively.
The UV - irradiation changed the form of non monotonic kinetic effect and the outflow on to the stationary level is activated when the white light is turned on. The direct proportionality between the quantity of the effect and the doze of irradiation is obtained. Probably it is not connected with inactivation of noncyclic an electron transport for the reaction to turning the M350 nm light on stimulating both systems didn’t change. It can be assumed that the acceleration of growth of EPR signal induced by UV- irradiation after turning on the white light is caused by activationof reactions on the accepting part PS 1.
As it is seen in fig.2 the decreament of EPR 1 signal of tested and UV - irradiated leaves when switching
X 707 nm —> white light and the quantity of the stationary level arising as a result of following reoxidation of the P700 centers differ.
2.CONCLUSION
Comparing our theoretical and experimental data we find out its big similarity and conclude that the radiating factors influence a lot on speed of processes of photosynthesis.
3. REFEREN CES
1. Photosynthesis. T .l/ Red.Qovinji M.M.:the World,1987.
2. Kok B. Partial purification and determination of oxidation - reduction potential of the photosynthetic chlorophyll complex absorbing at 700 mp // Biochim/ Biophys. Acta. - 1961. V.48.P.527.
3. Dubinskiy A .Y., Tihonov A.N. // Biophys. 1995. T. 40. P. 365-371.
