Photosynthesis

About: Photosynthesis is a research topic. Over the lifetime, 19789 publications have been published within this topic receiving 895197 citations.

The structure of a plant photosystem I supercomplex at 3.4 Å resolution

Abstract: All higher organisms on Earth receive energy directly or indirectly from oxygenic photosynthesis performed by plants, green algae and cyanobacteria. Photosystem I (PSI) is a supercomplex of a reaction centre and light-harvesting complexes. It generates the most negative redox potential in nature, and thus largely determines the global amount of enthalpy in living systems. We report the structure of plant PSI at 3.4 A resolution, revealing 17 protein subunits. PsaN was identified in the luminal side of the supercomplex, and most of the amino acids in the reaction centre were traced. The crystal structure of PSI provides a picture at near atomic detail of 11 out of 12 protein subunits of the reaction centre. At this level, 168 chlorophylls (65 assigned with orientations for Q(x) and Q(y) transition dipole moments), 2 phylloquinones, 3 Fe(4)S(4) clusters and 5 carotenoids are described. This structural information extends the understanding of the most efficient nano-photochemical machine in nature.

Microelectrode studies of the photosynthesis and O2, H2S, and pH profiles of a microbial mat1

Abstract: The profiles of O,, H,S, and pH within a microbial mat of the hypcrsaline pond Solar Lake, Sinai, were measured by 2-208pm-thick microelectrodes during diurnal and artificial light cycles. The oxygen concentration in the photic layer varied from a maximum of 1,400 PM during the day to 0 during the night. The pH in the same layer varied between 9.6 in the early afternoon and 7.7 in the early morning. Sulfide was not present in the photic zone during the day, but built up to about 50 /IM during the night. The diffusion gradients of sulfide and oxygen were very steep and the two compounds coexisted in a layer only 0.25 mm thick during the day. Diffusion flux calculations showed that the average turnover time of sulfide within this layer was 21 s. The rapid turnover indicated that the oxidation of sulfide must be biologically mediated. Oxygenic photosynthesis was measured by a new oxygen microprofile method which accurately determines the vertical distribution of photosynthetic activity. There was no difference in the efficiency of photosynthesis between morning and afternoon. The photosynthetic efficiency of the whole mat was about fourfold higher at low light intensities, ~120 PEinst. m-z.s-1, than at high light intensities, 120-1,600 pEinst*m-“es-‘. Anoxygenic photosynthesis within the mat was not quantitatively important.

Is photosynthesis limited by decreased Rubisco activity and RuBP content under progressive water stress

Abstract: Summary • Whether decreases in Rubisco activity and the availability of ribulose-1,5bisphosphate (RuBP) regeneration are responsible for drought-induced depression of photosynthesis is under debate. • Here, leaf water potential and relative water content, gas exchange, chlorophyll fluorescence, initial and total Rubisco activity and RuBP content were determined during the time course of drought development in five C 3 species: Rhamnus alaternus , Rhamnus ludovici-salvatoris , Nicotiana sylvestris , Phaseolus vulgaris and Vitis vinifera . Water was withheld until photosynthesis approached zero (between 6 and 12 d depending on the species). • Relative water content and water potential progressively dropped with drought in Rhamnus and Vitis , but not in the other two species. While RuBP content and Rubisco activity remained constant, declining eventually only in the more severe drought situations, light-saturated stomatal conductance ( g s ) and photosynthesis ( A N ) decreased progressively during drought in all species. This strongly suggests a dominant role of decreased g s in photosynthesis downregulation during drought in these species, which is supported by increased electron transport to A N ratio. • It is concluded that impairment of Rubisco activity and RuBP content do not limit photosynthesis until drought is very severe. Moreover, the relative water content at which these mechanisms are impaired is strongly species-dependent.

Carbon isotope fractionation in formation of amino acids by photosynthetic organisms.

Abstract: When inorganic carbon is converted to living matter during photosynthesis, there is an isotope effect and the carbon-containing compounds of cells have a slightly lower C13 concentration than the carbonate and carbon dioxide of the environment.' Craig2 has reviewed the existing data on carbon isotope abundances in nature. Park and Epstein3 4 have recently reported on some carbon isotope fractionations in plants grown in the laboratory. The effect of isotopic substitution on the physicochemical properties of molecules has been well studied for inorganic and nonbiological organic systems.5 The relative rates of photosynthesis of the C13and C'2-containing molecules are of a similar order of magnitude as observed for reactions involving formation or rupture of carbon bonds. Although it may be difficult to achieve precise understanding of the carbon isotope effect in living systems, the more qualitative and descriptive features are interesting and have already been the subject of geochemical speculation. Thus, Epstein and Silverman6 have made extensive measurements on petroleum and petroleum fractions. Since marine organisms tend to have a slightly higher concentration of C13 than nonmarine forms, carbon isotope measurements may be important to investigations of the origin and migration of petroleum. In a study of a Finnish Precambrian formation, Rankama7 observed that a problematic fossil, Coryecium enigmaticum (probable age >1.4 X 109 years) has carbon which is depleted in C13 similar to present organic carbon. Rankama's conclusion concerning early life obtained from this evidence has been discussed by Craig.8 Detailed study of this effect might produce information relevant to photosynthesis, biosynthesis, and comparative biochemistry. Accordingly, we have investigated carbon isotope fractionation in a variety of photosynthetic and nonphotosynthetic organisms, including some grown in the laboratory under controlled conditions and some that had grown in a natural environment. A typical experiment consisted of (1) culturing the organisms, (2) extracting the lipides, (3) hydrolyzing the proteins, (4) separating pure amino acids by ion-exchange chromatography, (5) combusting a portion of the amino acids to carbon dioxide, (6) decarboxylating a portion of the amino acids with ninhydrin and purifying the liberated carbon dioxide, and (7) performing an isotopic analysis on the