Harnessing the Power of the Sun

While the energy crisis facing America continues to grow, scientists all over the country are continually working on better ways to harness one of earth’s most abundant resources: the sun. According to Crabtree and Lewis, the sun is continually delivering 1.2 x 10⁵ terawatts of power to the earth, naturally making this a very abundant energy source to harness for commercial use. This essay will outline the benefits, as well as the pitfalls, of using solar energy as a renewable source of energy, while also explaining how natural ATP Synthase is considered by some to be an integral to the use of artificial photosynthesis in harnessing the immense power of the sun.

Until recently, solar energy had not been given much attention as an alternative to fossil fuels. Without proper technology, the use of solar panels and other photon gathering devices was both cumbersome and inefficient. As a result of these inefficiencies, the enormous potential for solar energy as an alternative to fossil fuels was overlooked. The sun supplies an estimated 10¹⁷ joules of energy to earth in one second, an estimated 1.7 x 10²² joules of energy in 1.5 days, and an estimated 4.6 x 10²⁰ joules in one hour (Crabtree and Lewis, 37). This staggering amount of energy is extremely versatile and can be harnessed to produce electricity, chemical fuel through the use of artificial photosynthesis, and even heat through the use of solar panels. Despite being such an abundant energy source, harnessing and using solar energy has many problems due to its inefficient conversion.

In their article entitled Solar Energy Conversion, Crabtree and Lewis make the argument that the enormous gap between the potential of solar energy and our use of it is due to cost and conversion capacity (38). Unlike fossil fuels, solar energy is not very concentrated, infinite in quantity, and spread evenly across the entire planet. Fossil fuels on the other hand are extremely concentrated, while limited in quantity, and make a perfect fuel for combustion in motor vehicles. Solar energy’s lack of concentration largely contributes to its inefficient use as fuel up to this point. Crabtree and Lewis state that the best commercial solar cells based on single-crystal silicon are about 18% efficient. Laboratory solar cells based on cheaper dye sensitization of oxide semiconductors are usually less then 10% efficient (Crabtree and Lewis, 38). Less efficient still, green plants convert sunlight into energy with a yearly average of less then .3% efficiency (Crabtree and Lewis, 38). All of these energy harvesting methods severely lack efficiency, however, despite this “dramatic cost-effective increases in the efficiency of solar energy conversion are enabled by our growing ability to understand and control the fundamental nanoscale phenomena that govern the conversion of photons into other forms of energy” (Crabtree and Lewis, 38). With great progression being made everyday in the field of nanotechnology, it shouldn’t be too long before advanced methods of controlling photon conversion become available. One such method of controlling photon conversion that is currently being developed is called artificial photosynthesis

With so much solar energy being absorbed in the photosynthesis process of Earth’s plants, there is naturally a great desire among scientists to artificially reproduce this process for efficiency-optimized use in energy saving applications. As mentioned above, a plant’s use of solar energy is relatively inefficient. This inefficiency, as Crabtree and Lewis noted, is primarily concentrated in the later stages of photosynthesis when carbon dioxide is converted into carbohydrates and sugars. Contrary to these later stages, the earlier stages of photosynthesis where two molecules of water are split to provide four protons and electrons are very efficient. There are several paths to correcting these inefficiencies, but the main one this paper will focus on is using artificial bio-inspired nanoscale assemblies to produce fuel from water and CO₂.

Many years of laboratory research and experimentation has led scientists to discover a technique for improving the efficiency of photosynthesis. The process of artificial photosynthesis relies heavily on the ATP Synthase enzyme to complete the final stages of the energy conversion. Crabtree and Lewis describe this process as being “bio-inspired” and further explain that “light harvesting and charge separation are accomplished by synthetic antennas linked to a porphyrin-based charge donor and a fullerene acceptor” (40). This assembly, with natural ATP Synthase, is then embedded in an artificial membrane to begin converting solar energy into usable chemical fuel. Natural ATP Synthase is used here because its complex function has yet to be replicated by scientists. Carbtree and Lewis even go on to assert that under the right conditions, the components of artificial photosynthesis “self assemble to produce a membrane-based chemical factory that transforms light into the chemical fuel ATP” (40). While this process may sounds simple, the technology and precision utilized in the construction of this artificial assembly are truly remarkable.

The first component that will be analyzed is the artificial proton pump. In an article entitled The Design and Synthesis of Artificial Photosynthetic Antennas, Reaction Centres and Membranes, scientists Moore, Moore, and Gust state that “the availability of artificial reaction centres that self assemble vectorially into bilayer lipid membranes makes it possible to design a proton pump that will generate a transmembrane gradient in proton electrochemical potential” (1490). According to Moore, Moore, and Gust, this is the next step in converting solar energy to a biologically useful form (1490). In order for proton translocation across the membrane to actually occur, lipophilic shuttle quinones are incorporated into the membrane functioning as a redox loop to facilitate transfer of protons across the membrane (Moore, Moore, Gust 1490). With the creation of this artificial pump, methods had to be fabricated to monitor the transmembrane electrical potential (to check the proton pump’s actually effectiveness). Moore, Moore, and Gust described the following method for monitoring proton import: A pH-sensitive fluorescent dye, pyraninetrisulphonate, was trapped inside the liposomes, or 8-aminoacridine was used to measure _pH directly, or 8 anilinonaphthalene-1- sulfonic acid was used to measure the transmembrane electrical potential (1490). With such a complicated artificial device, this method for monitoring proton import is necessary to insure the pump is actively replicating its natural counterpart.

The second component of the artificial photosynthesis process is the ATP Synthase enzyme. The proton pump described above powers the ATP Synthase embedded in the artificial membrane. In naturally occurring biological systems, ATP Synthase is powered by protons entering one side of the enzyme and flowing down their chemical gradient. This causes the rotation of the F₁ subunit the subsequent conversion of ADP into ATP. Artificial photosynthesis utilizes ATP Synthase in a slightly, although nearly identical manner. The enzyme is first extracted from spinach chloroplast thylakoids, and then it is reconstituted into the liposomes containing the components of the proton pump described above. When the ATP Synthase was reconstituted into the membrane, the ATP Synthesizing F₁ subunit was left protruding from the membrane into the external aqueous solution (1491). This protrusion is where the artificial photosynthesis apparatus differs from its natural analogue. Small amounts of Thioredoxin were added to activate the enzyme and start the chemical conversion of energy. In order to make sure the artificial photosynthesis compound was producing ATP from photon emissions, various levels of illumination were applied. The output of ATP based on these illumination levels provided evidence that the artificial apparatus was in fact working. This system, according to research conducted by Moore, More, and Gust, was effective in converting light energy into chemical potential.

Inspired by natural photosynthesis and developed to combat the ever growing consumption of Earth’s finite energy sources, artificial photosynthesis provides a method to effectively capture the sun’s energy. Using natural ATP Synthase, scientists were successful in generating ATP through this method, but due to a lack of efficiency, we are still several years from seeing widespread commercial implementation. Despite inefficiencies, artificial photosynthesis offers enormous promise for the future of clean, renewable energy.

Works Cited

Moore, T A., A L. Moore, and D. Gust. "The Design and Synthesis of Artificial Photosynthetic." The Royal Society (2002): 1490-1491. 4 Apr. 2008 .

Crabtree, George W., and Nathan S. Lewis. "Solar Energy Conversion." Physics Today (2007): 37-42. 4 Apr. 2008 .