Along with the development of the most important elements of the photosynthetic apparatus, a functional apparatus could have been assembled through a multi-staged recruitment of reaction center proteins and antenna proteins, which could conceivably have had separate evolutionary histories and performed different functions before the recruitment. The recruitment process may have undergone several intermediate stages, producing products with various degrees of complexity.
In essence, the precise picture of early evolution of photosynthesis still remains to be understood. To reveal the true color of the origin of photosynthesis will require years of painstaking biogeochemistry and molecular phylogenetic studies.
National Center for Biotechnology Information , U. Journal List Genome Biol v. Genome Biol. Published online Jan 3. Jin Xiong 1. Author information Copyright and License information Disclaimer. Corresponding author. Jin Xiong: ude. This article has been cited by other articles in PMC.
Abstract Recent studies using geological and molecular phylogenetic evidence suggest several alternative evolutionary scenarios for the origin of photosynthesis. Geological evidence on the origin of photosynthesis The advent of photosynthesis is known from geological studies to be a very ancient event.
Molecular phylogenetic evidence on the evolution of photosynthesis Although the geological records provide the timing information for the evolutionary events, finding the sequence with which the five main groups of photosynthetic micro-organisms evolved from a common ancestor requires molecular phylogenetic analysis of the genetic components of extant photosynthetic organisms.
Studies of whole organisms and genomes In the early days of molecular phylogenetics, bacterial relationships were usually resolved using 16S ribosomal RNA rRNA [ 18 , 19 ], which allowed classification and identification of the major bacterial groups. Open in a separate window. Figure 1. Studies using chlorophyll biosynthesis markers Because bacterio chlorophylls are integral components of the photosynthetic apparatus, enzymes involved in the biosynthesis of this pigment encoded by the bch genes could be used as specific indicators for the evolution of photosynthesis.
Studies using reaction centers The reaction center is the core of the bacterio chlorophyll-containing protein complex where the primary electron transfer event takes place during photosynthesis. The selective loss model The selective loss model [ 24 , 25 , 30 ] postulates an ancestral photosynthetic organism, similar to oxygenic cyanobacteria, containing both types of reaction center. The fusion model The fusion model [ 4 , 34 ] postulates that the type I and type II reaction centers could have been established independently in two different ancestral lineages one in proteobacteria or Chloroflexi and the other in heliobacteria or Chlorobi before being brought together into one lineage to produce the cyanobacterial dual photosystem.
Acknowledgements I thank the Welch Foundation for support grant no. Hydrogen-based carbon fixation in the earliest known photosynthetic organisms. The cyanobacterial genome core and the origin of photosynthesis. Conservation of distantly related membrane proteins: photosynthetic reaction centers share a common structural core.
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New York: Academic Press; Genetic analysis of chlorophyll biosynthesis. Ann Rev Genet. Evolution of reaction centers in photosynthetic prokaryotes. The antenna system of the green sulfur bacteria is composed of bacteriochlorophyll and carotenoids and is contained in complexes known as a chlorosomes that are attached to the surface of the photosynthetic membrane.
This antenna arrangement is similar to the phycobilisomes of cyanobacteria. Green sulfur bacteria can fix CO2 without Rubisco. It has been proposed that they accomplish this by using the respiratory chain that normally oxidizes carbon known as the Krebs cycle , resulting in the release of CO2. With the input of energy this process can be run in the reverse direction, resulting the uptake and reduction of CO2.
Like the green sulfur bacteria, green gliding bacteria harvest light using chlorosomes. The green gliding bacteria appear to have reaction centers similar to those of the purple bacteria Fig. For example, instead of two monomer bacteriochlorophyll molecules, C. The three-dimensional structure of the reaction center of Rhodopseudomonas viridis and Rhodobacter sphaeroides reveals the distances between the electron donors and acceptors Deisenhofer et al.
There is currently a controversy concerning the importance of specific amino acid composition of the protein on the rate of intraprotein electron transfer. In part, the disagreement centers on whether the protein between the donor and acceptor molecules can be treated as a uniform material, or whether the specific amino acid composition of the protein significantly alters the rate.
For example, it has been proposed that aromatic amino acids may provide a particular pathway that facilitates electron transfer between a donor and acceptor pair.
However, in other cases, replacement of an aromatic by another non-aromatic residue has resulted in relatively minor changes in the rate of electron transfer. Dutton and coworkers Moser et al.
Marcus and others DeVault, Dutton and coworkers argue that protein provides a uniform electronic barrier to electron tunneling and a uniform nuclear characteristic frequency. They suggest that the specific amino acid residues between an electron transfer pair is generally of less importance than the distance in determining the rate of pairwise electron transfer.
In their view, protein controls the rate of electron transfer mainly through the distance between the donor and acceptor molecules, the free energy, and the reorganization energy of the reaction.
The importance of distance is demonstrated by electron transfer data from biological and synthetic systems showing that the dependence of the electron transport rate on the edge to edge distance is exponential over orders of magnitude when the free energy is optimized Moser et al. Increasing the distance between two carriers by 1. The extent to which this view is generally applicable for intraprotein transfer remains to be established Williams, One of the challenges in understanding pairwise electron transfer rates from first principles is illustrated by the reaction centers of Rhodopsuedobacter sphaeroides in which the redox components are arranged along two-fold axis of symmetry that extends from the primary donor P to the non heme Fe.
Despite the fact that the reaction center presents two spatially similar pathways for electron transfer from P to quinone, nearly all electrons are transferred down the right-arm of the reaction center as shown in Fig. The same is true for the reaction center of Rhodopseudomonas viridis, in which it is estimated that electron transfer down the left-arm is less than Kellogg et al.
The challenge to theorists is to explain the surprisingly high probability that electron flow goes down the right-arm. Since the distances are similar, it has been suggested that electron transfer down the left-arm is less probable due to an endothermic free energy change Parson et al. The amount of CO2 removed from the atmosphere each year by oxygenic photosynthetic organisms is massive.
This is equivalent to 4 x kJ of free energy stored in reduced carbon, which is roughly 0. Each year the photosynthetically reduced carbon is oxidized, either by living organisms for their survival, or by combustion.
The result is that more CO2 is released into the atmosphere from the biota than is taken up by photosynthesis. The oceans mitigate this increase by acting as a sink for atmospheric CO2. This carbon is eventually stored on the ocean floor. Although these estimates of sources and sinks are uncertain, the net global CO2 concentration is increasing.
Direct measurements show that each year the atmospheric carbon content is currently increasing by about 3 x grams. Over the past two hundred years, CO2 in the atmosphere has increased from about parts per million ppm to its current level of ppm.
Based on predicted fossil fuel use and land management, it is estimated that the amount of CO2 in the atmosphere will reach ppm within the next century. The consequences of this rapid change in our atmosphere are unknown. Such a large temperature increase would lead to significant changes in rainfall patterns. Little is known about the impact of such drastic atmospheric and climatic changes on plant communities and crops. Current research is directed at understanding the interaction between global climate change and photosynthetic organisms.
This text is a revised and modified version of "Photosynthesis" by J. It is published here with full permission from the Managing Editor Dr. Abrahams, J. Leslie, R. Lutter and J. Walker Structure at 2. Nature Amesz, J. In: D. Bryant ed. The Molecular Biology of Cyanobacteria, pp. Kluwer Academic, Netherlands. Hoff eds. Kluwer Academic, the Netherlands. Baker, N. Barber, J. Barry, B.
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The flow of electrons in this way is called the Z-scheme. Only PSI is used during cyclic photophosphorylation; the high-energy electron of the PSI reaction center is passed to an ETS carrier and then ultimately returns to the oxidized PSI reaction center pigment, thereby reducing it.
Figure 6. Click to see a larger image. The electron flow described here is referred to as the Z-scheme shown in yellow in [a]. After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules having lifespans of millionths of a second , photoautotrophs have the fuel needed to build multicarbon carbohydrate molecules, which can survive for hundreds of millions of years, for long-term energy storage.
The carbon comes from CO 2 , the gas that is a waste product of cellular respiration. The Calvin-Benson cycle named for Melvin Calvin [—] and Andrew Benson [—] , the biochemical pathway used for fixation of CO 2 , is located within the cytoplasm of photosynthetic bacteria and in the stroma of eukaryotic chloroplasts.
The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration see Metabolic Pathways for a detailed illustration of the Calvin cycle. Additionally, other bacteria and archaea use alternative systems for CO 2 fixation. Although most bacteria using Calvin cycle alternatives are chemoautotrophic, certain green sulfur photoautotrophic bacteria have been also shown to use an alternative CO 2 fixation pathway.
In prokaryotes, in which direction are hydrogen ions pumped by the electron transport system of photosynthetic membranes? Skip to main content. Microbial Metabolism. Search for:. Photosynthesis Learning Objectives Describe the function and locations of photosynthetic pigments in eukaryotes and prokaryotes Describe the major products of the light-dependent and light-independent reactions Describe the reactions that produce glucose in a photosynthetic cell Compare and contrast cyclic and noncyclic photophosphorylation.
Think about It In a phototrophic eukaryote, where does photosynthesis take place? Think about It Why would a photosynthetic bacterium have different pigments? Think about It Describe the three stages of the Calvin cycle. Key Concepts and Summary Heterotrophs depend on the carbohydrates produced by autotrophs, many of which are photosynthetic, converting solar energy into chemical energy.
Different photosynthetic organisms use different mixtures of photosynthetic pigments , which increase the range of the wavelengths of light an organism can absorb. Photosystems PSI and PSII each contain a light-harvesting complex , composed of multiple proteins and associated pigments that absorb light energy.
In oxygenic photosynthesis , H 2 O serves as the electron donor to replace the reaction center electron, and oxygen is formed as a byproduct. In anoxygenic photosynthesis , other reduced molecules like H 2 S or thiosulfate may be used as the electron donor; as such, oxygen is not formed as a byproduct. Multiple Choice During the light-dependent reactions, which molecule loses an electron?
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