KINETIC STUDIES OF THE TWO LIGHT REACTIONS IN PHOTOSYNTHESIS

Abstract— The decay kinetics of the photo-induced absorbance changes in red and green algae are very sensitive to the wavelength of the actinic light. A four to tenfold increase in half-decay time is noted in going from short wavelength (550–650 mμ) to long wavelength (> 700 mμ) excitation. The slow decay rates produced by long wavelength light can be enhanced with a steady background of short wavelength light. A relationship between initial decay rates and O₂ evolution rates is described. This relationship allows a direct correspondence between these spectroscopic studies and the 'red-drop' and 'enhancement' experiments of Emerson.     

PARTICIPATION OF THE TWO PHOTOSYSTEMS IN LIGHT DEPENDENT HYDROGEN EVOLUTION IN Scenedesmus obliquus

Abstract— The H₂-photoproduction in the presence of dithionite measured in wild type and mutant cells of Scenedesmus obliquus demonstrates two sequential phases. In mutants showing only PS I activity phase 1 of H₂-photoproduction is visible with its core activity. When PS II is developed during greening, considerable activity is added to the core of phase I and phase II activity appears. Addition of DCMU reduces H₂-photoproduction by about 90%. The residual activity is completely attributed to the core of phase I. It was concluded that the core of phase I is dependent upon PS I only and can use sources different from water as electron donors. Phase II is dependent upon the capacity of PS II, a functioning photosynthetic apparatus and water as electron donor. The results are supported by studies of wavelength dependent activity of the two separate phases of H₂-photoproduction.     

THERMODYNAMICS OF THE PRIMARY REACTIONS OF PHOTOSYNTHESIS

Abstract. It appears to be widely believed that fundamental thermodynamic limitations prevent the products of the primary electron transfer reactions of photosynthesis from exceeding the reactants in energy by more than about 0.7 hv ₀ where v ₀ is the frequency of the lowest energy absorption band of the reaction center. Specifically, in photosynthetic bacteria, where hv ₀ 1.34 eV, it is often said that the midpoint redox potentials of the primary electron donor and acceptor cannot differ by more than about 0.9 eV. This is incorrect. A simple expression is developed for δμ, the increase in the partial molecular free energy of an absorber under illumination. The magnitude of δμ gives one no information about the nature of the photochemical reactions that can result from excitation of the absorber. It puts no limits on the midpoint redox potentials of the reactants. However, knowing δμ does allow one to calculate the concentrations of the products if the system comes to equilibrium during illumination.     

RELATIONSHIP BETWEEN LIGHT QUALITY DEPENDENCE AND IRRADIANCE ADAPTATION OF PHOTOSYNTHESIS IN Acetabularia mediterranea*

Characteristic differences in the light intensity curves of photosynthesis after growth of cells of Acetabularia mediterranea Lamour. (A. acetabulum (L.) Silva) in weak and strong white light were similar to those for red and blue light-treated cells, respectively. This indicated that responses to white light quantity and those to light quality might be causally related. Small differences in the thylakoid polypeptide composition of cells grown in high and low intensities of white light were not significant and thus did not help to clarify whether the adaptations to blue or red light, respectively, were the same. When the red to blue-light ratio was varied, keeping the total photon fluence rate constant, the photosynthetic capacity (red light saturated O₂-production) was dependent on blue light irradiance in a logarithmic fashion. The specific influence of red light was not detectable, indicating that only blue light was effective for light irradiance adaptation in Acetabularia. The situation was different, at least for a transient period, when adaptation to light irradiance was allowed to proceed from a low photosynthetic activity after preirradiation of the cells with prolonged red light. The effect of low white light irradiances was pronounced, causing a maximum increase of photosynthetic activity within 3 days. The response to blue light was enhanced as well, and a very low photon irradiance added to continuous red light caused a change of the same order as that produced by high irradiances of blue light alone. This elevated action of low intensity white and blue light is most likely due to increased metabolite supply derived from the degradation of starch enhanced by this light quality. Therefore, photosynthetic effectiveness in Acetabularia is regulated by the irradiance of blue light and by feedback via photosynthetic products.     

三向缩聚及共缩聚反应的胶凝化条件

1.本文从合理地修正和发展Flory的概念着手,注意到支化点与其自身相联结的机率在三向缩聚反应胶凝化现象中的意义,建议以中心支化点再现机率等于1作为一般性的胶凝化条件。从此出发,可以不经过推导分子量分布函数而十分方便地推算任何反应类型的胶疑点。 2.本文推导了大批一般化类型的胶凝点,包括了大量的特例,实际应用时很方便。前人所得结果成为特例包括在本文结果之中。     

THE ENERGETICS OF ELECTRON-TRANSFER REACTIONS OF CHLOROPHYLL AND OTHER COMPOUNDS*

Abstract— Quite often the primary photochemical reaction of an excited state molecule is transfer of an electron to or from another molecule in its ground state. Rates of such reactions are closely dependent on differences between ground and excited state redox potentials of the reagents. The solvent also plays an important role in stabilizing ion pairs formed by the electron transfer. This Review discusses experimental data relating rates to electrochemical energy parameters in the context of a scheme which portrays the energy and electron transactions in a unified manner. Three consequences of reaction of a singlet excited state are distinguished: (S1) quenching without detectable products, (S2) exciplex fluorescence, (S3) transient radical ion production, and energetically necessary conditions are derived for each. Similarly, four kinds of reactions involving the triplet state are distinguished, which depend on the relation between the energy of the triplet state and that of the ion pair states: (TI) rapid quenching, (T2) slow quenching, (T3) accelerated intersystem crossing and (T4) generation by reaction between radical ions of like spin. The last may be followed by electrochemiluminescence. Classes of compounds for which data are available include chlorophylls, porphyrins and a few other molecules of biological interest, aromatic hydrocarbons and their derivatives, heterocyclic systems, carbonyl compounds, dyes, and complexes of Ru and U. A Table compiling median or selected values of ground and excited state electrochemical potentials of chlorophylls, some porphyrins, and a few other compounds is presented.     

THE MAXIMUM EFFICIENCY OF PHOTOSYNTHESIS *

Within the ideal assumptions: (1) two Photosystems for photosynthetic fixation of CO₂, (2) all solar photons with Λ≥ 700 nm are absorbed, (3) the photon requirement is 8 for each CO₂ molecule fixed and O₂ molecule evolved and (4) the principal stable product of photosynthesis is d-glucose, the theoretical maximum efficiency of conversion of light to stored chemical energy in green-plant type (oxygen-evolving) photosynthesis in bright sunlight is calculated to be 13.0%. Thermodynamic arguments are presented which indicate that a photosynthetic system with one Photosystem would be highly unlikely to be able to drive each electron from water to evolve O₂ and reduce CO₂. The practical maximum efficiency of photosynthesis under optimum conditions is estimated to be 8–9%.     

LIGHT REACTIONS OF THE CILIATE Stentor coeruleus-A THREE-DIMENSIONAL ANALYSIS

A computer-controlled three-dimensional tracking and motion analysis system was developed to study the responses of Stentor coeruleus to short light pulses and to evaluate its distribution patterns. In addition to photokinesis and phototaxis, the step-up photophobic response was analyzed, which includes a gravity-controlled component at higher fluence rates and a light direction-dependent component at lower fluence rates.     

INTERFERENCE BY NEAR ULTRAVIOLET AND GREEN LIGHT WITH GROWTH OF ANIMAL AND PLANT CELL CULTURES*

Abstract— Near ultraviolet (365 nm) and green-yellow (546-579nm) radiations repress the growth of liquid cell cultures of Ginkgo pollen and of monolayer cultures of HeLa. The deleterious effects of green-yellow wavelengths can be negated by red radiation; near-u.v.-induced growth repressions are insensitive to visible light photorestoration. These wavelengths do not interact synergistically and evoke different kinetics of response.     

ON THE DISTRIBUTION OF EXCITATION ENERGY TO TWO PHOTOREACTIONS OF PHOTOSYNTHESIS

Abstract— From kinetic analysis of the Z-scheme we have derived expressions for fractions of open reaction centers and throughput quantum yield. The fraction of absorbed quanta available to photoreaction II has been treated by the wavelength dependent function, α. However, for purposes of analysis in terms of throughput electron flow it is necessary to introduce a related function, α', for actual processing of excitations by photoreaction II; α' differs from α if the individual quantum yields of the two photoreactions are not equal. 'Included in our findings are the following. As a close approximation quantum yield is maximum at α'= 0.5 and a symmetrical function of α' around the line α' =0.5. As previously noted, lowering of the apparent equilibrium constant between the photoreactions flattens dependence of quantum yield on α' near α' = 0.5 but also lowers the attainable quantum yield. For any given equilibrium constant the effective fractions of open reaction centers are equal at α' = 0.5 and mirror images of each other around the line α' = 0.5. Slow changes in quantum yield following changes in wavelength (the State 1-State 2 phenomenon) are explainable in terms of changes in α' which also have the effect of flattening dependence of quantum yield on wavelength. We have used these changes to estimate the neutral wavelength for α' = 0.5 as 681–682 nm in Chlorella .     

QUANTUM YIELD RATIO OF THE FORWARD and BACK LIGHT REACTIONS OF BACTERIORHODOPSIN T LOW TEMPERATURE and PHOTOSTEADY-STATE CONCENTRATION OF THE BATHOPRODUCT K*

The maximum photosteady state fraction of K, x_K^max, and the ratio of the quantum yields of the forward and back light reactions, trans-bacteriorhodopsin (bR) hArr; K, φ_bR/φ_K, were obtained by measuring the absorption changes produced by illumination of frozen water-glycerol (1:2) suspensions of light-adapted purple membrane at different wavelengths at -165°C. An independent method based on the second derivative of the absorption spectrum in the region of the β-bands was also used. It was found that The quantum yield ratio (0.66 ± 0.06) was found to be independent of excitation wavelength within experimental error in the range510–610 nm. The calculated absorption spectrum of K has its maximum at603–606 nm and an extinction 0.85 ± 0.03 that of bR. At shorter wavelengths there are P-bands at 410, 354 and 336 rim. Using the data of Hurley et al. (Nature 270,540–542, 1977) on relative rates of rhodopsin bleaching and K formation, the quantum yield of K formation was determined to be 0.66 ± 0.04 at low temperature. The quantum efficiency of the back reaction was estimated to be 0.93 ± 0.07. These values of quantum efficiencies of the forward and back light reactions of bR at - 165°C coincide with those recently obtained at room temperature. This indicates that the quantum efficiencies of both forward and back light reactions of bacteriorhodopsin are temperature independent down to -165°C.     

FLASH PHOTOLYSIS OF SEVERAL AROMATICS BY ULTRAVIOLET LIGHT AND VISIBLE LIGHT IN THE PRESENCE OF EOSIN Y*

Abstract— A flash photolysis investigation was made of the photo-oxidation of aqueous aniline, resorcinol, βnaphthol, p-sulfanilic acid, and p-bromophenol induced by ultraviolet and visible light irradiation in the presence of eosin Y. The transient spectra show that u.v. irradiation generates the hydrated electron (except in p-bromophenol) and the radical products of one-electron oxidation. The initial products of the eosin-sensitized oxidations are the dye semi-quinone and aromatic radicals which coincide with the u.v. photolysis products in at least several cases. The investigation of the reaction kinetics by rapid spectrophotometry with analog computer analysis shows that the aromatics quench the triplet state of eosin and also react with it in a slower electron-transfer process, in competition with ‘dye-dye’ quenching and electron-transfer reactions. The u.v. and dye-sensitized oxidations are discussed in terms of their energetics.     

THE FUNCTION OF PHYTOCHROME IN THE NATURAL ENVIRONMENT—IV. LIGHT QUALITY AND PLANT DEVELOPMENT

Abstract— Studies of the effect of light quality on the growth of Cucurbita pepo L . and Chenopodium album L . showed that large changes in morphological development can be induced by altering the proportion of red and far-red in the incident radiation. Although phytochrome photoequilibria established at the beginning of the dark period affected development, similar changes were achieved with long term and continuous irradiation. The change in the growth pattern of C. album (a common weed species in cultivated land) caused by simulated wheat canopy shade, appeared to involve a redirection of growth towards processes which would achieve greater height at the expense of lateral growth. The evidence presented supports the hypothesis that phytochrome may be involved in the detection of mutual shading and the modification of development in an appropriate manner.     

THE EFFECT OF INHIBITORS AND UNCOUPLERS OF PHOTOSYNTHETIC PHOSPHORYLATION ON THE DELAYED LIGHT EMISSION OF CHLOROPLASTS*

Abstract— Measurements were made of the 3.7 msec delayed light emission of chloroplasts treated with a variety of agents which affect the rate of electron transport (Hill reaction) or photosynthetic phosphorylation. The presence of the electron acceptors ferricyanide or pyocyanine increased delayed light emission. Inhibitors of electron transport (3-(3,4-dichlorophenyl)-1, -1-dimethylurea or 1,10(ortho)-penanthroline) inhibited delayed light emission. The addition of a phosphate acceptor system inhibited delayed light emission. This inhibition was reversed by inhibitors of the phosphorylation reaction, e.g. Dio-9 or phlorizin. From these results it was concluded that the 3.7 msec delayed light emission probably occurs as a result of back reactions of intermediates in the coupled electron transport and photosynthetic phosphorylation systems.     

STOCHASTIC MODELING OF LIGHT ENERGY CONVERSION IN PHOTOSYNTHESIS

Abstract— Photosynthetic quantum conversion and early electron transport is modeled as a stochastic process on a digital computer to determine what free-energy losses are a necessary consequence of specific assumptions about the reaction structure, kinetics, and thermodynamics of the participating molecules. Maximal free-energy yield requires that all dark reactions be near equilibrium, so the potentials of all half-cells on each side of the light act are nearly the same. This near equilibrium requires that all forward rate constants be at least 10² times the rate of light absorption, and that all reverse rate constants be at least the rate of light absorption. The behavior of model systems with one primary donor and one primary acceptor is comparatively independent whether there is one or an infinite number of secondary electron donors and acceptors. A system having no metastable (e.g. triplet) state of the light-activated donor can convert light energy with nearly ideal efficiency, provided that the standard electrode potentials of the primary donor and primary acceptor half-cells are precisely located with respect to one another and to the potentials of the ultimate donor and acceptor. While not necessary for near maximal free-energy yield, a metastable intermediate allows a flexibility in the choice of half-cell potentials which is not possible in the absence of such an intermediate.