3.2 Anoxygenic Photosynthetic Organisms
While current artificial photosynthesis methods are far less efficient than the natural process, there has been continual progress in the field. One of the reasons that the technology is being pursued is that, compared to current solar panel technology, molecular nanoparticles are cheaper, lighter, and more environmentally sound. Aside from providing a renewable energy source and eliminating our reliance on rapidly diminishing fossil fuels, it has also been suggested that artificial photosynthesis on a large industrial scale could reverse global warming since the process consumes carbon dioxide and releases oxygen. With the potential of such beneficial impacts on the environment and our energy supply, continued research into combining nanotechnology and natural processes should remain a central goal.
We can write photosynthesis like this:
3. CLASSIFICATION OF PHOTOSYNTHETIC ORGANISMS
All life can be divided into three domains, Archaea, Bacteria and Eucarya, which originated from a common ancestor (Woese et al., 1990). Historically, the term photosynthesis has been applied to organisms that depend on chlorophyll (or bacteriochlorophyll) for the conversion of light energy into chemical free energy (Gest , 1993). These include organisms in the domains Bacteria (photosynthetic bacteria) and Eucarya (algae and higher plants). The most primitive domain, Archaea, includes organisms known as halobacteria, that convert light energy into chemical free energy. However, the mechanism by which halobacteria convert light is fundamentally different from that of higher organisms because there is no oxidation/reduction chemistry and halobacteria cannot use CO2 as their carbon source. Consequently some biologists do not consider halobacteria as photosynthetic (Gest 1993). This chapter will follow the historical definition of photosynthesis and omit halobacteria.
The Amazon rainforest, alone, produces 20-30 percent of the total oxygen on the planet.
The fact is that photosynthesis plays a crucial role in making the Earth the only planet with life.
6. Anoxygenic Photosynthesis6.1 Purple Bacteria
The photosynthetic process in plants and algae occurs in small organelles known as chloroplasts that are located inside cells. The more primitive photosynthetic organisms, for example oxygenic cyanobacteria, prochlorophytes and anoxygenic photosynthetic bacteria, lack organelles. The photosynthetic reactions are traditionally divided into two stages - the "light reactions," which consist of electron and proton transfer reactions and the "dark reactions," which consist of the biosynthesis of carbohydrates from CO2. The light reactions occur in a complex membrane system (the photosynthetic membrane) that is made up of protein complexes, electron carriers, and lipid molecules. The photosynthetic membrane is surrounded by water and can be thought of as a two-dimensional surface that defines a closed space, with an inner and outer water phase. A molecule or ion must pass through the photosynthetic membrane to go from the inner space to the outer space. The protein complexes embedded in the photosynthetic membrane have a unique orientation with respect to the inner and outer phase. The asymmetrical arrangement of the protein complexes allows some of the energy released during electron transport to create an electrochemical gradient of protons across the photosynthetic membrane.
Photosynthesis makes food for plants and for animals.
In artificial photosynthesis, scientists are essentially conducting the same fundamental process that occurs in natural photosynthesis but with simpler nanostructures. The fabrication of these nanostructures has only recently been possible due to breakthroughs in nanotechnology in the areas of imaging and manipulation. With the core processes in photosynthesis being light gathering, charge separation, and recombination, the goal of scientists has been to create efficient synthetic nanostructures that can function as antennae and reaction centers. Devens Gust and fellow researchers at Arizona State University created a hexad, or six-part, nanoparticle made of four zinc tetraarylporphyrin molecules, (PZP)3-PZC, a free-base porphyrin, and a fullerene molecule, P-C60.
Amesz, J. (1987) Photosynthesis. Elsevier, Amsterdam.
Photosynthetic electron transport consists of a series of individual electron transfer steps from one electron carrier to another. The electron carriers are metal ion complexes and aromatic groups. The metal ion complexes and most of the aromatic groups are bound within proteins. Most of the proteins involved in photosynthetic electron transport are composed of numerous polypeptide chains that lace through the membrane, providing a scaffolding for metal ions and aromatic groups. An electron enters a protein complex at a specific site, is transferred within the protein from one carrier to another, and exits the protein at a different site. The protein controls the pathway of electrons between the carriers by determining the location and environment of the metal ion complexes and aromatic groups. By setting the distance between electron carriers and controlling the electronic environment surrounding a metal ion complex or aromatic group, the protein controls pairwise electron transfer reactions. Between proteins, electron transfer is controlled by distance and free energy, as for intraprotein transfer, and by the probability that the two proteins are in close contact. Protein association is controlled by a number of factors, including the structure of the two proteins, their surface electrical and chemical properties and the probability that they collide with one another. Not all electron carriers are bound to proteins. The reduced forms of plastoquinone or ubiquinone and nicotinamide adenine dinucleotide phosphate (NADPH) or NADH act as mobile electron carriers operating between protein complexes. For electron transfer to occur, these small molecules must bind to special pockets in the proteins known as binding sites. The binding sites are highly specific and are a critical factor in controlling the rate and pathway of electron transfer.