Flash photolysis-electron spin resonance studies of Photosystem I. A fast reduction component of P-700+

A 300 μs decay component of ESR Signal I (P-700⁺) in chloroplasts is observed following a 10 μs actinic xenon flash. This transient is inhibited by treatments which block electron transfer from Photosystem II to Photosystem I (e.g. 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU), 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), KCN and HgCl₂). The fast transient reduction of P-700⁺ can be restored in the case of DCMU or DBMIB inhibition by addition of an electron donor couple (2,6-dichlorophenol indophenol (Cl₂Ind)/ascorbate) which supplies electrons to cytochrome f. However, this donor couple is inefficient in restoring electron transport in chloroplasts which have been inhibited with the plastocyanin inactivators, KCN and HgCl₂. Oxidation-reduction measurements reveal that the fast P-700⁺ reduction component reflects electron transfer from a component with E_m = 375±10 mV (pH = 7.5). These data suggest the assignment of the 300-μs decay kinetics to electron transfer from cytochrome f (Fe²⁺) to P-700⁺, thus confirming the recent observations of Haehnel et al. (Z. Naturforsch. 26b, 1171–1174 (1971)).     

Transient kinetics of the electron transfer between P-700, plastocyanin and cytochrome f in chloroplasts suspended in fluid media at sub-zero temperatures

The kinetics of redox changes of P-700, plastocyanin and cytochrome f in chloroplasts suspended in a fluid medium at sub-zero temperatures have been studied following excitation of the chloroplasts with either a single-turnover flash, a series of flashes or continuous light. The results show that: (1) The kinetics of reduction of P-700⁺ and those of oxidation of plastocyanin are consistent with a bimolecular reaction between these two components as previously suggested (Olsen, L.F., Cox, R.P. and Barber, J. (1980) FEBS Lett. 122, 13–16). (2) Cytochrome f shows heterogeneity with respect to its kinetics of oxidation by Photosystem I. (3) In contrast to the situation when plastoquinol is the electron donor, reduction of cytochrome f by electrons derived from diaminodurene occurs with sigmoidal kinetics that shows a good fit to an apparent equilibrium constant of 12 between the cytochrome and P-700. (4) The rate of electron transfer from plastoquinol to Photosystem I depends on the redox state of the plastoquinone pool. (5) In relation to current ideas about the lateral heterogeneity of Photosystem I and Photosystem II in the thylakoid membrane, the results are consistent with the function of plastocyanin as a mobile carrier of electrons in the intrathylakoid space.     

The oxidation-reduction potential of P-700 in chloroplast lamellae and subchloroplast particles

The oxidation-reduction potential of P-700 has been determined in chloroplast lamellae and in subchloroplast particles by measuring the magnitude of flash-induced absorption changes at 820 or at 703 nm (due to the oxidation of P-700) in the presence of known concentrations of potassium ferro- and ferricyanide. A midpoint potential of about +490 mV was determined in chloroplast lamellae and in particles prepared with digitonin (D-144) or Triton (TSF-1). A lower potential was determined with Photosystem I particles obtained after harsher treatments with Triton or a mixture of detergents. The potential is even lower in chlorophyll-protein complex I particles prepared with sodium dodecyl sulfate (about +430 mV). Very similar values were determined from oxidized minus reduced spectra measured with a double-beam spectrophotometer. Titrations were also made with D-144 and TSF-I particles, with 66% glyeerol in the buffer, at 21 °C and at 77 °K. P-700 was found to be half-oxidized at ferricyanide/ferrocyanide ratios of about 60 at 21 °C and of about 1 at 77 °K. This shows that the redox equilibrium is largely perturbed by the cooling process.     

Evidence that the intermediate electron acceptor,A2, in Photosystem I is a bound iron-sulfur protein

Absorption changes accompanying the formation of light-induced P-700⁺ were investigated in a highly enriched Photosystem I preparation where an intermediate electron acceptor preceding P-430 could be detected. In an enriched Photosystem I particle, light-induced reversible absorption changes observed at 700 nm in the presence of dithionite resembled those previously seen at 703 nm and 820 nm [9], thus indicating the presence of a backreaction between P-700⁺ and A^?₂. After this same Photosystem I particle was treated to denature the bound iron-sulfur centers, the photochemical changes that could be attributed to P-700 A₂ were completely lost. These results provide evidence that the intermediate electron acceptor, A₂, is a bound iron-sulfur protein. Additional studies in the 400–500 nm region with Photosystem I particles prepared by sonication indicate that the spectrum of A₂ is different from that of P-430.     

Cytochrome f and plastocyanin kinetics in Chlorella pyrenoidosa. I. Oxidation kinetics after a flash

P-700, plastocyanin and cytochrome f redox kinetics were measured after one flash, using dark-adapted Chlorella in the presence of hydroxylamine and 3(3,4-dichlorophenyl)-1,1-dimethylurea. Plastocyanin becomes increasingly oxidized with a half-time of 70 μs, then undergoes reduction with a half-time of 7 ms. Cytochrome f oxidation has a sigmoidal time-course and a half-time of 100 μs. Its reduction exhibits a half-time of 4 ms. These results are interpreted in a linear scheme:

An equilibrium constant of 2 between cytochrome f and plastocyanin (PC), which contrasts with the large equilibrium constant between PC and P-700 is computed.The presence of cytochrome b₆ in a cyclic path around Photosystem I is confirmed under these conditions.     

Light-induced oxidation-reduction reactions in a cell-free preparation from the blue-green alga Nostoc muscorum: The role of cytochrome f, cytochrome b558, C550, and P700 in noncyclic electron transport

1. Cytochrome f (λ_max = 554 nm, E_m = +0.35 V) and cytochrome b₅₅₈ (λ_max = 558 nm, E_m = +0.35 V) were photooxidized by Photosystem I and photoreduced by Photosystem II in a cell-free preparation from the blue-green alga Nostoc muscorum. The steady-state oxidation levels of both cytochromes were affected by noncyclic electron acceptors and by inhibitors of noncyclic electron transport. These results are consistent with the hypothesis that the mechanism of NADP reduction by water involves a Photosystem II and a Photosystem I light reaction operating in series and linked by a chain of electron carriers that includes cytochrome f and cytochrome b₅₅₈.2. Phosphorylation cofactors shifted the steady-state of cytochrome f to a more reduced level under conditions of noncyclic electron transport but had no effect on cytochrome b₅₅₈. These observations suggest that the noncyclic phosphorylation site lies before cytochrome f (on the Photosystem II side) and that cytochrome f is closer to this site than is cytochrome b₅₅₈.3. A Photosystem II photoreduction of C550 at 77 °K was observed, suggesting that in blue-green algae, as in other plants, C550 is closely associated with the primary electron acceptor for Photosystem II. A Photosystem I photooxidation of P700 at 77 °K was observed, consistent with P700 serving as the primary electron donor of Photosystem I.     

Orientation of Photosystem-I pigments: low temperature linear dichroism spectroscopy of a highly-enriched P700 particle isolated from spinach

The linear dichroism of Photosystem I particles containing 10 chlorophylls per P700 has been investigated at 10 K. The particles were oriented by uniaxial squeezing of polyacrylamide gels. The oxidation state of P700 was altered either by incubation of the gels with redox mediators or by low temperature illumination. The Q_Y transitions of the primary electron donor P700, of the remaining unoxidized chlorophyll in P700⁺ and of a chlorophyll molecule absorbing at 686 nm, which presumably corresponds to the primary electron acceptor A₀, are all preferentially oriented perpendicular to the gel squeezing direction. The Q_Y transition of the chlorophyll forms absorbing at 670 and 675 nm appear tilted at 40 ± 5° from this orientation axis. This orientation of the various chlorophylls is compared to that previously reported for more native Photosystem I particles.Abbreviations PSI Photosystem I - P700 primary electron donor of PSI - A₀ primary electron acceptor of PSI     

Functional homogeneity of P-700 in cyclic and non-cyclic electron transport reactions in thylakoids

The photoinduced turnover of P-700 (the reaction center chlorophyll a of photosystem I) in higher plant thylakoids was examined at room temperature by observation of the kinetics and amplitude of the transmission signal at 700 nm. The concentration of P-700 functional in cyclic and non-cyclic electron transfer reactions was compared. For the cyclic reactions mediated by N-methylphenazonium-p-methosulfate, 2,3,5,6-tetramethylphenylenediamine, 2,6-dichlorophenolindophenol and N,N,N′,N′-tetramethylphenylenediamine and non-cyclic reactions utilizing either methylviologen or NADP⁺ as acceptor, the illuminated steady-state concentration of P-700⁺ was shown to be similar. The data support the concept of a homogeneous pool of P-700 that is capable of interaction in both cyclic and non-cyclic electron transfer reactions and are consistent with previous data obtained in vivo.The amplitude and kinetics of the P-700 signal were found to be very dependent upon the composition of the reaction medium and differences were noted for turnover in the cyclic and non-cyclic reactions. Specifically, at white light saturation, the addition of low concentrations of divalent cations, such as Mg²⁺ or Ca²⁺, had no effect on the signal amplitude during the cyclic reactions, but, in confirmation of previous work, caused an attenuation of the signal amplitude during non-cyclic flow. At low light intensities, the divalent cations caused a similar reduction in redox level of P-700 in the steady-state during non-cyclic flow and also reduced the rate of P-700 photooxidation in the cyclic reactions. The concentration of divalent cation that reduced the signal amplitude of P-700⁺ during non-cyclic flow was compared with that required for the stimulation of the variable component of fluorescence, and it was shown to be similar with half maximal effects at 1 mM Mg²⁺. The observations confirm that divalent cations control non-cyclic electron transport by an activation of Photosystem II in addition to regulating the distribution of excitation energy between the two photosystems.     

Species dependence of the redox potential of the primary electron donor p700 in photosystem I of oxygenic photosynthetic organisms revealed by spectroelectrochemistry

The redox potential of the primary electron donor P700, E(m)(P700/P700(+)), of Photosystem I (PSI) has been determined for 10 oxygenic photosynthesis organisms, ranging from cyanobacteria, red algae, green algae to higher plants, by spectroelectrochemistry with an optically transparent thin-layer electrode (OTTLE) cell to elucidate the scattering by as much as 150 mV in reported values of E(m)(P700/P700(+)). The E(m)(P700/P700(+)) values determined within error ranges of ± 1-4 mV exhibited a significant species dependence, with a span >70 mV, from +398 to +470 mV vs. the standard hydrogen electrode (SHE). The E(m)(P700/P700(+)) value appears to change systematically in going from cyanobacteria and primitive eukaryotic red algae, then to green algae and higher plants. From an evolutionary point of view, this result suggests that the species believed to appear later in evolution of photosynthetic organisms exhibit higher values of E(m)(P700/P700(+)). Further, the species dependence of E(m)(P700/P700(+)) seems to originate in the species-dependent redox potentials of soluble metalloproteins, Cyt c(6) and plastocyanin, which re-reduce the oxidized P700 in the electron transfer chain.     

Properties of the low-temperature Photosystem I primary reaction in the P-700-chlorophyll a-protein

The Photosystem I primary reaction, as measured by electron paramagnetic resonance changes of P-700 and a bound iron-sulfur center, has been studied at 15°K in P-700-chlorophyll a-protein complexes isolated from a blue-green alga. One complex, prepared with sodium dodecyl sulfate shows P-700 photooxidation only at 300°K, whereas a second complex, prepared with Triton X-100, is photochemically active at 15°K as well as at 300°K. Analysis of these two preparations shows that the absence of low-temperature photoactivity in the sodium dodecyl sulfate complex reflects a lack of bound iron-sulfur centers in this preparation and supports the assignment of an iron-sulfur center as the primary electron acceptor of Photosystem I.     

The study of the primary photoprocesses in Photosystem I of chloroplasts Recombination luminescence,chlorophyll triplet state and triplet-triplet annihilation

The dependence of the delayed luminescence of Photosystem I on the state of the reaction centers has been studied. Light flash induces a charge separation in the centers: $\text{P-700 · P-430}\textcolor{red}{}\text{P-700}{}^{\mathrm{+}}\text{· P-430}{}^{\mathrm{?}}$. Dark recombination of charges is accompanied by the recombination luminescence with $\text{τ}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{? 20}\text{ms}$.If the centers are in the P-700 · P-430^? state or if P-430 is inactivated by heat, then flashing of Photosystem I generates the triplet state chlorophyll with $\text{τ}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{? 0.5}\text{ms}$. The triplet state has been measured by the delayed fluorescence of chlorophyll at 20 °C and 77 °K and by the chlorophyll phosphorescence at 77 °K. The delayed fluorescence at 20 °C arises from the thermal activation of the triplet state up to the excited singlet level of chlorophyll and at 77 °K it is due to triplet-triplet annihilation. The quantum yield of the triplet formation, estimated by a comparison of the light saturation curves of delayed fluorescence at 20 °C and of P-700 photooxidation under the same experimental (optical) conditions, is ≈ 0.9 of the P-700⁺ yield. Only one triplet of chlorophyll can be generated per P-700. Under heat inactivation of P-430 the triplet formation is not observed when P-700 is oxidized.It is assumed that the triplet-triplet annihilation at 77 °K is related with the strong interaction between the chlorophyll molecules in the pigment complex of Photosystem I. The possibility of a triplet participation in the primary processes of photosynthesis is discussed.     

Effects of hydrostatic pressure on the kinetics of electron transfer in an isolated system of chloroplast cytochrome bf complex,plastocyanin and P700

The effects of pressure on the kinetics of redox reactions in and around the chloroplast cytochrome bf complex were studied using a reconstituted system consisting of Photosystem I (PS I) particles, cytochrome bf complex and plastocyanin (PC), all derived from pea chloroplasts. There were no significant permanent effects of pressure in the range 0.1–191 MPa on the reaction kinetics, or on the shape of the absorption spectra of components studied. Discernable effects on rate-coefficients of increasing pressure were observed on the reduction of P700⁺ by PC^I, on the reduction of PC^II by ascorbate, and on the oxidation of decyl plastoquinol by the bf complex. The volumes of activation ΔV^# were determined from the dependence of the rate-coefficient on pressure using: $$(\partial lnk/\partial P)_T = - \Delta V^\# /RT.$$ The volume of activation is the difference in partial molar volume between the activated state and the reactants for the redox reaction. Such data was sought to help define in detail those redox reactions and the corresponding activated states. For the reduction of P700⁺ by PC^I and the oxidation of decyl plastoquinol by the bf complex, the rate coefficient decreased with increase in pressure, whilst for the reduction of PC^II by ascorbate it increased. The corresponding volumes of activation were 9.6±0.6×10^-6 m³ mol^-1, 18±2×10^-6 m³ mol^-1 and -14±1×10^-6 m³ mol^-1, respectively. Much of the pressure-dependence of PC^II reduction by ascorbate was ascribed to an increase in ascorbate ionisation with increase in pressure. There was little effect of pressure on the kinetics of oxidation of ferrocytochrome f by PC^II, or on the equilibrium constant of the redox pair ferrocytochrome f/ferricytochrome f: PC^II/PC^I. Possible physical bases for these activation volumes are discussed, and they are compared with literature values.     

Photoreactions of cytochrome b-559 and cyclic electron flow in Photosystem II of intact chloroplasts

The high potential cytochrome b-559 of intact spinach chloroplasts was photooxidized by red light with a high quantum efficiency and by far-red light with a very low quantum efficiency, when electron flow from water to Photosystem II was inhibited by a carbonyl cyanide phenylhydrazone (FCCP or CCCP). Dithiothreitol, which reacts with FCCP or CCCP, reversed the photooxidation of cytochrome b-559 and restored the capability of the chloroplasts to photoreduce CO₂ showing that the FCCP/CCCP effects were reversible. The quantum efficiency of cytochrome b-559 photooxidation by red or far-red light in the presence of FCCP was increased by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone which blocks oxidation of reduced plastoquinone by Photosystem I. When the inhibition of water oxidation by FCCP or CCCP was decreased by increased light intensities, previously photooxidized cytochrome b-559 was reduced. Red light was much more effective in photoreducing oxidized high potential cytochrome b-559 than far-red light. The red/far-red antagonism in the redox state of cytochrome b-559 is a consequence of the different sensitivity of the cytochrome to red and far-red light and does not indicate that the cytochrome is in the main path of electrons from water to NADP. Rather, cytochrome b-559 acts as a carrier of electrons in a cyclic path around Photosystem II. The redox state of the cytochrome was shifted to the oxidized side when electron transport from water became rate-limiting, while oxidation of water and reduction of plastoquinone resulted in its shifting to the reduced side.     

A demonstration of energy transfer from photosystem II to photosystem I in chloroplasts

Photosystem I activity of Tris-washed chloroplasts was measured at room temperature as the rate of photoreduction of NADP and as the rate of oxygen uptake mediated by methyl viologen in both cases using dichlorophenolindophenol plus ascorbate as the source of electrons for Photosystem I. With both assay systems the rate of electron transport by Photosystem I was stimulated approx. 20 % by the addition of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea which caused the Photosystem II reaction centers to close. Photosystem I activity of chloroplasts was measured at low temperature as the rate of photooxidation of P-700. Chloroplasts suspended in the presence of hydroxylamine and 3-(3,4-dichlorophenyl)-1, 1-dimethylurea were frozen to ?196 °C after adaptation to darkness or after a preillumination at room temperature. The Photosystem II reaction centers of the frozen dark-adapted sample were all open; those of the preilluminated sample were all closed. The rate of photooxidation of P-700 at ?196 °C with the preilluminated sample was approx. 25 % faster than with the dark-adapted sample. We conclude from both the room temperature and the low temperature experiments that there is greater energy transfer from Photosystem II to Photosystem I when the Photosystem II reaction centers are closed and that these results are a direct demonstration of spillover.     

Energy transfer from photosystem II to photosystem I in Porphyridium cruentum

Rates of photooxidation of P-700 by green (560 nm) or blue (438 nm) light were measured in whole cells of Porphyridium cruentum which had been frozen to ?196 °C under conditions in which the Photosystem II reaction centers were either all open (dark adapted cells) or all closed (preilluminated cells). The rate of photooxidation of P-700 at ?196 °C by green actinic light was approx. 80% faster in the preilluminated cells than in the dark-adapted cells. With blue actinic light, the rates of P-700 photooxidation in the dark-adapted and preilluminated cells were not significantly different. These results are in excellent agreement with predictions based on our previous estimates of energy distribution in the photosynthetic apparatus of Porphyridium cruentum including the yield of energy transfer from Photosystem II to Photosystem I determined from low temperature fluorescence measurements.     

Evidence for a new early acceptor in Photosystem I of plants. An ESR investigation of reaction center triplet yield and of the reduced intermediary acceptors

The yield of the triplet state of the primary electron donor of Photosystem I of photosynthesis (P^T-700) and the characteristic parameters (g value, line shape, saturation behavior) of the ESR signal of the photoaccumulated intermediary acceptor A have been measured for two types of Photosystem I subchloroplast particles: Triton particles (TSF 1, about 100 chlorophyll molecules per P-700) that contain the iron-sulfur acceptors F_X, F_B and F_A, and lithium dodecyl sulfate (LDS) particles (about 40 chlorophyll molecules per P-700) that lack these iron-sulfur acceptors. The results are: (i) In Triton particles the yield of P^T-700 upon illumination is independent of the redox state of A and of F_X,B,A and is maximally about 5% of the active reaction centers at 5 K. The molecular sublevel decay rates are k_x = 1100 s^?1 ± 10%, k_y = 1300 s^?1 ± 10% and k_z = 83 s^?1 ± 20%. In LDS particles the triplet yield decreases linearly with concentration of reduced intermediary acceptors, the maximal yield being about 4% at 5 K assuming full P-700 activity. (ii) In Triton particles the acceptor complex A consists of two acceptors A₀ and A₁, with A₀ preceding A₁. In LDS particles at temperatures below ?30°C only A₀ is photoactive. (iii) The spin-polarized ESR signal found in the time-resolved ESR experiments with Triton particles is attributed to a polarized P-700-A^?₁ spectrum. The decay kinetics are complex and are influenced by transient nutation effects, even at low microwave power. It is concluded that the lifetime at 5 K of P-700A₀A^?₁ must exceed 5 ms. We conclude that P^T-700 originates from charge recombination of P-700A^?₀, and that in Triton particles A₀ and A₁ are both photoaccumulated upon cooling at low redox potential in the light. Since the state P-700AF^?_X does not give rise to triplet formation the 5% triplet yield in Triton particles is probably due to centers with damaged electron transport.     

On the functional organization of electron transport from plastoquinone to Photosystem I

Signal I, the EPR signal of P-700, induced by long flashes as well as the rate of linear electron transport are investigated at partial inhibition of electron transport in chloroplasts. Inhibition of plastoquinol oxidation by dibromothymoquinone and bathophenanthroline, inhibition of plastocyanin by KCN and HgCl₂, and inhibition by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide are used to study a possible electron exchange between electron-transport chains after plastoquinone. (1) At partial inhibition of plastocyanin the reduction kinetics of P-700⁺ show a fast component comparable to that in control chloroplasts and a new slow component. The slow component indicates P-700⁺ which is not accessible to residual active plastocyanin under these conditions. We conclude that P-700 is reduced via complexed plastocyanin. (2) The rate of linear electron transport at continuous illumination decreases immediately when increasing amounts of plastocyanin are inhibited by KCN incubation. This is not consistent with an oxidation of cytochrome f by a mobile pool of plastocyanin with respect to the reaction rates of plastocyanin being more than an order of magnitude faster than the rate-limiting step of linear electron transport. It is evidence for a complex between the cytochrome b₆ - f complex and plastocyanin. The number of these complexes with active plastocyanin is concluded to control the rate-limiting plastoquinol oxidation. (3) Partial inhibition of the electron transfer between plastoquinone and cytochrome f by dibromothymoquinone and bathophenanthroline causes decelerated monophasic reduction of total P-700⁺. The P-700 kinetics indicate an electron transfer from the cytochrome b₆ - f complex to more than ten Photosystem I reaction center complexes. This cooperation is concluded to occur by lateral diffusion of both complexes in the membrane. (4) The proposed functional organization of electron transport from plastoquinone to P-700 in situ is supported by further kinetic details and is discussed in terms of the spatial distribution of the electron carriers in the thylakoid membrane.     

The electrophoretic isolation and partial characterization of three chlorophyll-protein complexes from blue-green algae

Three chlorophyll-protein complexes have been resolved from blue-green algae using an improved procedure for membrane solubilization and electrophoretic fractionation. One complex has a red absorbance maximum of 676 nm and a molecular weight equivalency of 255 000 ± 15 000. A second complex has an absorbance maximum of 676 nm, a molecular weight equivalency of 118 000 ± 8000, and resembles the previously described P-700-chlorophylla-protein (CPI) of higher plants and algae. The third chlorophyll-protein has a red absorbance maximum of 671 nm and a molecular weight equivalency of 58 000 ± 5000. Blue-green algal membrane fractions enriched in Photosystem I and heterocyst cells do not contain this third chlorophyll-protein, whereas Photosystem II-enriched membrane fractions and vegetative cells do. A component of the same spectral characteristics and molecular weight equivalency was also observed in chlorophyll b-deficient mutants of barley and maize. It is hypothesized that this third complex is involved in some manner with Photosystem II.     

Plastocyanin redox kinetics in spinach chloroplasts: evidence for disequilibrium in the high potential chain

Reduction kinetics of cytochrome f, plastocyanin (PC) and P₇₀₀ (‘high-potential chain’) in thylakoids from spinach were followed after pre-oxidation by a saturating light pulse. We describe a novel approach to follow PC redox kinetics from deconvolution of 810-860 nm absorption changes. The equilibration between the redox-components was analyzed by plotting the redox state of cytochrome f and PC against that of P₇₀₀. In thylakoids with (1) diminished electron transport rate (adjusted with a cytochrome bf inhibitor) or (2) de-stacked grana, cytochrome f and PC relaxed close to their thermodynamic equilibriums with P₇₀₀. In stacked thylakoids with non-inhibited electron transport, the equilibration plots were complex and non-hyperbolic, suggesting that during fast electron flux, the ‘high-potential chain’ does not homogeneously equilibrate throughout the membrane. Apparent equilibrium constants <5 were calculated, which are below the thermodynamic equilibrium known for the ‘high potential chain’. The disequilibrium found in stacked thylakoids with high electron fluxes is explained by restricted long-range PC diffusion. We develop a model assuming that about 30% of Photosystem I mainly located in grana end-membranes and margins rapidly equilibrate with cytochrome f via short-distance transluminal PC diffusion, while long-range lateral PC migration between grana cores and distant stroma lamellae is restricted. Implications for the electron flux control are discussed.