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To lift an object x meters it is necessary to expend, at a minimum, an amount of energy corresponding to the increase in that objects potential energy associated with the increase in height. When putting the object down, returning it to the position it was moved from, its potential energy with respect to gravity returns to the original amount. However, this potential energy is not returned to the person who did the lifting. On the contrary, putting an object down gently requires additional energy to be expended to slow what would otherwise be a free fall.
Can any organism convert gravitational potential energy to chemical energy? Is it at all biologically feasible?
Biological Engineering is an interdisciplinary area focusing on the application of engineering principles to analyze biological systems and to solve problems in the interfacing of such systems -- plant, animal or microbial--with human-designed machines, structures, processes and instrumentation. The biological revolution continues to mature and impact all of us. Human-based gene manipulation affects nearly all North American food supplies. Plants and animals are already being defined on a molecular basis. Living organisms can now be analyzed, measured and "engineered" as never before. Designer "bugs" are being produced to enhance biological processes. These changes continue to redefine our research and graduate programs that continue to emphasize biological, environmental and food and fiber engineering. Our connections to agriculture and food systems remain, but modern agriculture is greatly influenced by biotechnology, and our connections to agriculture reflect this fact. A basic goal is to design technology that operates in harmony with the biology of living systems. In many cases, currently available knowledge is inadequate to support engineering design of food and biological processes. Hence, greater fundamental knowledge of biology and its potential applications are also of concern to the biological engineer.
What Is a Chemical Reaction?
A chemical reaction is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a reactant, and a substance that forms as a result of a chemical reaction is called a product. During the reaction, the reactants are used up to create the products.
Another example of a chemical reaction is the burning of methane gas, shown in Figure (PageIndex<2>). In this chemical reaction, the reactants are methane (CH4) and oxygen (O2), and the products are carbon dioxide (CO2) and water (H2O). As this example shows, a chemical reaction involves the breaking and forming of chemical bonds. Chemical bonds are forces that hold together the atoms of a molecule. Bonds occur when atoms share electrons. When methane burns, for example, bonds break within the methane and oxygen molecules, and new bonds form in the molecules of carbon dioxide and water.
Figure (PageIndex<2>): Flames from methane burning
The short answer is NO - a biological fission reactor is also a flawed design and it's going to cook your biological organism so it's just not going to happen. The energy cost alone to reach break even point is far beyond the biological framework we live in to store and release in such concentrated amounts.
That said, it is perhaps important to note that biological organisms are already molecularly driven even if that doesn't provide the same power as we would get if we were atomically driven.
Terrestrial biology (at least) stores and releases energy at the molecular level. That is to say, plants take molecular compounds like CO2 and H2O and via an endothermic reaction called photosynthesis, change the molecules to different ones like O2 and carbohydrates, which take more energy to form, hence storing the energy taken from sunlight in a molecular form. Plants use some of this for their own metabolism, and animals like us then eat the plants, taking their stored carbohydrates and oxygen from the atmosphere and generate an exothermic reaction that converts them back to CO2 and water, releasing that energy inside us for our own needs.
The energy stored and released during these processes is pocket change compared to fission let alone fusion, but as it turns out we don't really need all that much energy to survive even the human nervous system seems to operate at around 0.07 volts, meaning that you could run around 21 people in parallel on a standard AA battery if you had to (but for how long I'd need to do more research to say).
Bottom line is that we don't need atomic power to drive our biology and the energy storage capacity of our current biological design would render us incompatible with it in any event. It turns out that being molecularly powered rather than atomically powered gives us all the energy we need and is much safer and sustainable for us into the bargain.
Can an organism evolve to use a concentrated mass of radioactive material to superheat a pressurised fluid and use that fluid to turn an organic turbine which rotates a permanent magnet inside a coil of conducting metal to generate electricity? Almost certainly not. Does that mean an organism can't make use of nuclear fission to energise itself? Not at all.
On an atomic scale, fissile elements are basically tiny bombs: they're happily sitting there as one element in whatever molecule, then bam, one or more high-energy fragments ricochet off in a random direction potentially screwing up another nearby molecule, and the original atom is now two totally different elements with different chemistry. We make fission reactions more energetic by concentrating fissile material so that the fragments are often captured by other fissile atoms which prompt them to react in turn, creating a chain reaction but even in its simplest form a pile of fissile material releases energy by capturing the high-energy fragments in something and thereby getting slightly hot, no feedback loop required. The challenges for an organism safely utilising this energy source, then, are:
- Extract fissile materials from its food and concentrate them
- Deal with the heavy element byproducts of the fission reaction
- Deal with the highly-energetic (usually neutron) fragments and convert their kinetic energy into heat, then convert that heat into a more useful form of energy for the organism
Biological organisms have evolved to manipulate chemistry on a molecular level they can catalyse pretty much any viable chemical reaction, concentrate particular atoms on one side or other of a boundary, and so forth. It if were evolutionarily favourable for the creature to extract, say, uranium or plutonium from its food, it could do so with no more difficulty than mammals extract magnesium, calcium or potassium ions on Earth. Filtering atoms by isotope is much more challenging, but it's not impossible that an exciting protein could evolve that would bind fractionally more strongly to one isotope than another. Problem 1 solved.
Problem 2 is equally plausible: while the exact byproducts of a fission reaction are random, they'll cover a small range of elements, and if the organism is 'expecting' them then there can be mechanisms for controlling and excreting them if they're not usable. Poisons are only poisonous if the organism isn't adapted to deal with its presence: arsenic is highly toxic to humans, but plenty of terrestrial organisms are evolved to tolerate high concentrations of it.
The main challenge an organism would face is actually doing something useful on a molecular level with the energy released by the fission reaction. By dissolving the fission products in water or a more powerful neutron capture liquid. Boron is an excellent neutron absorber, so boric acid might be a good choice here. The net effect of the nuclear reaction, then, is to turn cold boric acid into hot boric acid. Then there are already terrestrial organisms which survive on reactions which exploit temperature gradients like those found at hydrothermal vents: the organism could either exploit those reactions themselves, or live in symbiosis with a bacteria that did.
In short, while you're unlikely to see an organism evolve to set up the highly unstable, low-entropy, complicated arrangement of materials that humans would consider to be "a fission reactor", fission is an energy-releasing process, and an organism evolved to harness that process in a more relaxed, stream-like, organic way, is perfectly possible.
Prospects for conversion of solar energy into chemical fuels: the concept of a solar fuels industry
There is, at present, no solar fuels industry anywhere in the world despite the well-publicized needs to replace our depleting stock of fossil fuels with renewable energy sources. Many obstacles have to be overcome in order to store sunlight in the form of chemical potential, and there are severe barriers to surmount in order to produce energy on a massive scale, at a modest price and in a convenient form. It is also essential to allow for the intermittent nature of sunlight, its diffusiveness and variability and to cope with the obvious need to use large surface areas for light collection. Nonetheless, we have no alternative but to devise viable strategies for storage of sunlight as biomass or chemical feedstock. Simple alternatives, such as solar heating, are attractive in terms of quick demonstrations but are not the answer. Photo-electrochemical devices might serve as the necessary machinery by which to generate electronic charge but the main problem is to couple these charges to the multi-electron catalysis needed to drive energy-storing chemical reactions. Several potential fuels (CO, H2, HCOOH, NH3, O2, speciality organics, etc.) are possible, but the photochemical reduction of CO2 deserves particular mention because of ever-growing concerns about overproduction of greenhouse gases. The prospects for achieving these reactions under ambient conditions are considered herein.
As a rough approximation, our current energy consumption on a worldwide scale amounts to something like 15 trillion watts of power, of which about 85 per cent comes from combustion of fossil fuels such as oil, coal and natural gas. This massive fossil fuel consumption leads to the inevitable production of unfortunate side-products that themselves present major problems, including climate change, acidified oceans and oil spills. There can be little doubt that these problems will grow in importance over the coming years, not least because of population growth and an escalation in our energy usage the latter is expected to double before 2050 (as discussed by Lewis & Nocera ). Additional concerns can be raised about national security in terms of providing sustainable energy supplies. Indeed, Europe (and most of the individual countries) depends heavily on external supplies of energy and this situation is far from healthy. As a consequence, environmentally friendly fuel production has become a goal of major strategic importance across the globe at both national and international levels. The urgent, if not immediate, introduction of renewable energy sources presents serious challenges to contemporary science  but key steps have been taken already. To be successful and to maintain the need to cover critical scientific challenges, it is necessary to pursue a targeted, multi-disciplinary approach that covers chemistry, physics, engineering and biology. We are at the beginning of this adventure and it is unclear whether sufficient funding will be made available to meet the scientific demands. It can only be hoped that lessons from the past, where the price of gasoline determined the level of support for artificial photosynthesis, will be put to good use.
Renewable power sources, especially in the guise of solar cells [3,4], tidal  and wind  turbines, are set to help replace fossil fuels, and there are streams of reports offering improved efficiencies and lower costs in terms of electricity production. This is important and needs to be further encouraged. The main problem with electricity, however, is that it is hard to store on a massive scale and, by itself, does little to offset escalation of CO2 levels. As such, it cannot be relied upon for most heavy industry and transportation applications, such as ships, planes or agricultural vehicles. Many citizens worry about building new nuclear power stations. The answer in many researchers' minds is to capture sunlight in the form of chemical potential  and to devise strategies for the production of a range of fuels that could fulfil the role as feedstocks. There are indications that this route is viable but there is no solar fuels industry at this moment. The choice of putative solar fuels is large, perhaps this is a detrimental point that loses focus, and there can be little doubt that we must explore all of these possibilities. Even so, there is an overwhelming necessity to demonstrate the scope of this (relatively) new field and to avoid overconfidence.
Of the potential renewable energy sources, solar energy is the most abundant and, if harvested efficiently, is capable of meeting global energy needs for the foreseeable future . Most ongoing solar energy research is focused on direct conversion to electricity, by way of photovoltaic devices [8,9,10], or to high-temperature heat, via solar thermal procedures . Cost-effectiveness and conversion efficiencies are of major importance but so too is storage. The latter is primarily in the form of low-cost batteries . Additional research is directed towards the provision of small power units for application as personal energy supplies or for remote installations where cost is less important. We now direct attention to the main problem: the provision of massive supplies of energy by capture and storage of sunlight, allowing for the known difficulties associated with solar energy conversion in a non-intense (if not gloomy) environment as found in (say!) northern Europe.
3. Solar fuel
A fuel is any material which stores energy that can later be extracted to perform mechanical, electrical or chemical work in a controlled manner. Most fuels used by humans undergo a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air. Other processes used to convert fuel into energy include various exothermic chemical reactions and nuclear processes. Fuels are also used in cellular respiration, where organic molecules are oxidized to release usable energy. Hydrocarbons are by far the most common source of fuel. We can recognize the so-called biofuels, which can be broadly defined as solid, liquid or gas fuel consisting of, or derived from, biomass. Biomass can also be used directly for heating or power. A major advantage for biofuel relates to the fact that it can be produced from any carbon source that itself can be replenished rapidly. We are familiar with fossil fuels, primarily coal and petroleum (both liquid and gas), formed from the fossilized remains of ancient plants and animals over some hundreds of millions of years. Finally, we are aware of nuclear fuel, this being any material that is consumed to derive nuclear energy. There is, at present, no consensus as to what is meant by a solar fuel!
A solar fuel can be defined as ‘an energy-rich material generated from an abundant, cheap substance, such as water or CO2, using sunlight as the only energy input’. In essence, the goal is to produce a CO2 lean fuel from water, or readily available reagents produced as by-products from existing industrial processes, under illumination by sunlight. The product, to be classified as a fuel, must be storable. Some important compromise must be built into the approach from the beginning that deals with the lack of sunshine at certain times, stretching over extended periods in some densely populated locations. Above all, future funding for scientific research into this field demands that large-scale demonstration models are introduced sooner rather than later.
4. Identifying needs
The worldwide production of electricity in 2008 was 20 261 TWh (Statistical Review of World Energy 2009, BP), which represents something in the region of 11 per cent of the solar energy the Earth receives in 1 h (174 000 TWh). Sources of electricity were fossil fuels 67 per cent, renewable energy 18 per cent and nuclear power 13 per cent: the direct combustion of oil was only 5.5 per cent. Ninety-two per cent of renewable energy was hydroelectric, followed by wind at 6 per cent and geothermal at 1.8 per cent. Most scientists agree that fossil-fuel-based electricity generation accounts for a significant portion of world greenhouse gas emissions in the USA, electricity generation accounts for nearly 40 per cent of emissions, whereas fossil fuel combustion for electric power generation is responsible for 65 per cent of all emissions of SO2, a major contributor to acid rain, and other important pollutants such as NOx, CO and particulate matter. It is also important to recall that most large-scale thermoelectric power stations consume considerable amounts of water for cooling purposes and that this fact, by itself, is a cause for concern. A significant fraction of our total electricity production is used for industrial and domestic heating and/or cooling. It is clear that solar energy conversion could be invaluable in this area.
Again in reference to 2008, solar photovoltaic cells accounted for only 0.06 per cent of total electricity production, whereas solar thermal devices amounted for a mere 0.004 per cent. Unlike solar heat concentrators, photovoltaic panels convert sunlight directly to electricity. Although sunlight is free and abundant, solar electricity is still usually more expensive to produce than large-scale mechanically generated power owing to the cost of the panels. Low-efficiency silicon solar cells have been decreasing in cost and multi-junction cells with close to 30 per cent conversion efficiency are now commercially available. Over 40 per cent efficiency has been demonstrated in certain experimental systems. Until recently, photovoltaics were most commonly used in remote sites where there is no access to a commercial power grid, or as a supplemental electricity source for individual homes and businesses. Recent advances in manufacturing efficiency and photovoltaic technology, combined with subsidies driven by environmental concerns, have dramatically accelerated the deployment of solar panels. Installed capacity is growing by 40 per cent per year, led by increases in Germany, Japan, California and New Jersey.
Within the UK, we can make a crude breakdown of our energy needs (DECC report: Energy consumption in the UK) according to table 1. This shows that the two main areas for energy consumption are transport and domestic needs. This might be surprising in view of the widespread notion that the UK is a region of heavy industry. Nonetheless, it is clear that we could make an important contribution to our global energy picture by replacing fossil fuels used in domestic uptake with solar fuels. Looking at the breakdown of UK domestic energy consumption, we can identify heating as being the biggest single factor. This is critical information for defining our priorities for the type of solar fuels we might consider.
Table 1. Breakdown of the main energy consumption modes for the UK.
a Includes what might be termed loosely as ‘commercial’.
5. Photochemical heat production
Alternative routes for using sunlight as a source of domestic heating have been considered. One interesting observation in this respect relates to the so-called South Andros Black Holes. Here, a dense 1 m thick layer of phototrophic purple-sulfur bacteria is present at a depth of approximately 18 m and gives rise to the characteristic colour. These bacteria function as a highly effective heat engine and maintain the temperature in the vicinity of the bacteria at a steady 36 ° C. This is a remarkable achievement but one that we might conceivably duplicate at an artificial level . In fact, there is no difficulty in devising photochemical cycles capable of converting photonic energy into (low-temperature) heat by way of non-radiative channels. The problem is to generate high-temperature heat and to store the energy in such a manner that the desired heat can be released as and when needed. This is quite different to simply depositing the heat into the local environment immediately after excitation.
This situation was considered first some 30 years ago, and several putative cycles were identified. One of the best photochemical storage systems is the interconversion between norbornadiene and quadricyclane . This type of photoisomerization is particularly clean and essentially independent of temperature. Although near-UV light is required to drive the isomerization of norbornadiene, it is possible to promote the reaction using triplet state sensitizers  that absorb in the visible region. The isomerization step can store appreciable amounts of energy that can be converted to heat by passing the quadricyclane product over a transition metal catalyst (figure 1). The main drawbacks are poor light-harvesting capabilities, the need to use organic solvents and poor catalytic properties of the recycling agent. Nonetheless, this is a valuable practical application of solar energy conversion.
Figure 1. Cycle used for the storage and release of heat generated by photolysis of norbornadiene in solution. The exothermic conversion of quadricyclane to norbornadiene occurs only in the presence of a catalyst.
It is worth considering if new forms of photoisomer-based cycles could be found that operate in aqueous solution. Ideally, this would involve a photochemical adenosine triphosphate (ATP) synthesis driven by visible light. In itself, this is a major challenge but it is not unrealistic. As a simpler alternative, it is conceivable to design photochemical cycles around the interconversion of nicotinamide adenine dinucleotide (NAD + ) and its reduced form (NADH), although it is necessary to avoid dimerization of the intermediary NAD⋅ radical. This cycle can be driven enzymatically using photonic energy as the input but this process is usually achieved  at the expense of consuming an added sacrificial organic substrate, such as triethanolamine. It is more difficult to design tandem photosystems that combine oxidation of NADH (or reduction of NAD + ) with a non-sacrificial reaction. In part, this problem is acerbated by the imbalance of protons: for example, reduction of NAD + can be considered as a hydride transfer reaction, whereas O2 evolution from water leads to the accumulation of four protons. Nonetheless, these cycles are important as a means by which to overcome the problems associated with collection and storage of the fuel. It is highly likely that further attention will be given to such systems, especially artificial ATP synthesis, in the near future.
6. Photochemical synthesis
Photochemistry can be used as a tool to synthesize valuable materials, including novel pharmaceuticals. It is often proposed that sunlight could be used profitably to generate industrial-scale quantities of key starting compounds, such as epoxides. Within reason, this can be achieved by sensitized formation of singlet molecular oxygen and its subsequent reaction with an alkene. There are, however, problems of selectivity and specificity with regards to singlet molecular oxygen attack on organic compounds. Could the oxygen be derived from water? Well, this possibility has been considered but the outcome is not too promising.
An alternative approach is to use a waste organic substrate as a sacrificial agent by which to generate a valuable fuel using photochemical methods . Such a strategy is often used to isolate one half of a redox cycle so as to optimize a particular catalytic cycle (e.g. H2 generation) but leads to serious problems at a later stage. We can raise the question here as to whether there exists a suitable dump of waste product that could be upgraded by way of a photochemical reaction that generates a viable fuel. On a slightly different tack, a similar reaction could be used to remove an important pollutant that accompanies an industrial process. One substrate that springs to mind is glycerol, a by-product of biodiesel production . Although glycerol is a useful ingredient in cosmetic products, there is a large surfeit to such an extent that it is now burned at high temperature in order to keep stocks at a reasonable level. Because it contains easily removable hydrogen atoms, glycerol can be used as a substrate for H2 generation using a simple benzophenone derivative as sensitizer  under near-UV illumination (figure 2). Indeed, illumination of the water-soluble benzophenone derivative in aqueous solution containing glycerol at concentrations above a few per cent by volume and a suitable reduction catalyst (e.g. colloidal Pt) leads to H2 generation. Most of the hydrogen atoms can be extracted from glycerol in this manner and converted into collectable H2. Such benzophenone-based sensitizers are non-toxic, being used as UV initiators for dental fillings and activators for emulsion paint, and cheap. Of course, they are destroyed during the process, but the overall reaction is autocatalytic because other carbonyl compounds are generated during the free-radical cycle. Furthermore, the final carbon-based residue can be suitably downgraded as a non-toxic reagent or converted to a crude fertilizer by adding ammonia to the initial reaction mixture, which becomes fixed into the carbon backbone. Other waste substrates, such as aqueous ethanol or biomass, can be considered along these lines and used as substrates for H2 formation.
Figure 2. Cycle proposed for the photochemical dehydrogenation of glycerol as produced as a side-product during biodiesel manufacture. The benzophenone derivative (BP) is shown at the bottom left, where R is a water-solubilizing residue.
Looking back over the myriad of artificial photosynthetic systems published during the late 1970s and early 1980s, one particular system stands out as being especially relevant to the present discussion . This photosystem uses a complex mixture of bioinspired ideas to decompose ethanol into acetaldehyde and H2 in water under ambient conditions (figure 3). The key component is a water-soluble tin(IV) porphyrin (SnP), which acts as a replacement for chlorophyll, that is easily reduced to the corresponding π-radical anion. Unlike most metalloporphyrins , the π-radical anion formed from SnP is stable in aqueous solution and resistant towards disproportionation this is a critical point in terms of overall stability of the system. Under illumination, SnP is reduced by added NADH but the resultant π-radical anion reduces methyl viologen (MV 2+ ) to give the stable mono-cation radical that itself reduces water to H2 on the surface of a colloidal Pt catalyst. This strategy leaves behind the oxidized form of NADH, namely NAD, but this latter species reacts with ethanol in the presence of alcohol dehydrogenase (ADH) to give the protons necessary for H2 evolution. There is no change in pH and the system operates over prolonged illumination periods, in part because breakdown products from SnP (i.e. phlorin- and chlorin-type compounds) also function as sensitizers for the photochemical cycle. The overall quantum efficiency for H2 production at pH 7 is 55 per cent, and a considerable fraction of the solar spectrum is harvested. Furthermore, the system uses only 1 per cent by volume ethanol in water and, as such, could be applied to extract the residual ethanol from biofuel production after the distillation step.
Figure 3. Photochemical cycle used to dehydrogenate ethanol using a variety of bioinspired ideas. (Online version in colour.)
7. Photochemical water splitting
The ideal methodology for solar fuel production is based on the photochemical dissociation of water under ambient conditions. This is a non-trivial operation, made particularly difficult by the need to transfer multiple electrons. There is also a problem of inconsistency in as much as H2 formation is easiest in acidic solution but O2 generation occurs better at alkaline pH. The energy requirement for water splitting is high because of the need to overcome substantial over-potentials associated with the individual electrochemical reactions. Indeed, as a crude approximation, the dissociation of one gallon of water requires an input of about 25 kW h. A further problem related to photochemical water splitting is that there are several important intermediates that can enter into side-reactions and thereby serve to lower overall efficiencies. This latter problem requires that electrons and/or positive holes are injected into the catalytic sites at a high rate in order to push the cycle through to the final product. The main problem here is associated with O2 evolution.
At this point, it is important to stress that the application of sacrificial redox reagents, such as persulfate ions or triethanolamine, must be avoided. Traditionally , these reagents have been used to isolate either H2 or O2 evolution. They work extremely well but hide the fact that the intermediary radicals play critical roles in the subsequent chemistry—this point is often overlooked in the enthusiasm to report H2 or O2 yields. Replacing the sacrificial reagent with a reversible redox couple causes a massive fall in reaction efficiency, or stops the reaction entirely. There are, indeed, few cases of O2 evolution from water where a reversible redox shuttle is involved. The most successful of these processes uses Fe 2+ /Fe 3+ as the redox couple  and allows O2 evolution to proceed in acidic solution with modest quantum yield (figure 4). Although it has long been argued that sacrificial reagents help to optimize the catalytic performance, this is not really the case. Of the many thousands of examples where sacrificial redox reagents have been used to isolate the required catalytic step, only a small handful have been successfully carried through to the next stage.
Figure 4. Photochemical cycle used for the liberation of oxygen from water using a reversible redox couple.
Regarding the photonic input, there seems no alternative but to rely on a semiconductor as the actual light collection unit, perhaps used in consolidation with an organic-based solar concentrator. The latter ensures an adequate supply of photons/electrons to the catalytic site. It seems appropriate to go one stage further and insist on coupling together two such units, rather than rely on a single unit with or without the ancillary solar concentrator. The main advantage of tandem units is that their individual properties can be tailored for particular purposes. Continuing along this line, it can be argued that water splitting can only be realized by way of two coupled photo-electrochemical steps, each performed at a specific electrode. Such a system is an upgrade of the original Fujishima–Honda photo-assisted electrolysis of water—this remains one of the most important observations in the field . A prototype of this design was described recently by Mallouk and co-workers [23,24] and serves as a useful starting point for future discussion. It might also be mentioned that a molecular-based photo-electrochemical system was introduced  recently whereby prior reduction of an appended electron acceptor was used as a means by which to amplify the reduction potential of a nearby excited state. This system, being the first of its kind, functions as a crude harmonic generator to increase redox power (figure 5) and could have important applications for light-driven energy storage.
Figure 5. Scheme used for the photo-assisted electrolysis of a substrate (S) by a zinc porphyrin (ZnP) covalently attached to a polyoxometallate (POM). The approach requires prior reduction of the POM at an electrode surface. Illumination at ZnP causes a charge-shift reaction (csr) to occur and so generate a strongly reducing form of ZnP.
Regardless of the exact nature of the solar fuel, it is essential that any large-scale operation evolves O2 from water during the photochemical cycle . This reaction involves the transfer of four electrons and liberates four protons consequently, the pH will decrease during illumination, thereby changing the chemical potential, unless there is a complementary light-driven process that uses these protons at the same rate. Several metal oxides are known to function as good anodes for O2 evolution and a critical comparison of such materials has been made under photochemical conditions . The main results of this latter study are compiled in table 2. Unfortunately, most of the effective materials are based on very expensive and rare elements, although Co3O4 stands out as being a useful alternative. Despite its involvement in natural oxygen evolution, MnO2 is a relatively poor catalyst for water oxidation under these conditions. This study does not address the mechanism of water oxidation, which might differ according to the type of oxide used as catalyst, but makes clear recommendations as to which materials are likely to work under illumination in any prototypic photo-electrochemical system. As such, we can identify IrO2, RuO2, Co3O4 and (to a lesser extent!) Rh2O3 and MnO2 as being suitable materials for the anode. It is important to note that these experiments were made at pH 5, rather than in alkaline solution where O2 evolution is more favoured thermodynamically. Other work with similar RuO2- and IrO2-based colloidal catalysts has stressed the importance of pH in terms of reaction kinetics and, in particular, the over-potential needed for water oxidation [28,29,30,31]. The need for a wide pH profile stems from the requirement to couple O2 evolution with fuel formation.
Table 2. Comparison of the observed rates and quantum efficiencies of O2 evolution observed for various metal oxide catalysts under photochemical conditions.
Of course, it is important to consider the physical state of the catalyst before reaching critical conclusions about which materials should be used in a large-scale practical system. Similar studies were carried out to assess the ability of hydrous metal oxides to function as good O2 evolving catalysts under photochemical conditions. Again, it was found  that IrO2⋅nH2O forms the most effective catalytic centres at pH 5, with RuO2⋅nH2O and MnO2⋅nH2O being less effective but still viable candidates for the anode (table 3). A point of interest with colloidal IrO2⋅nH2O is that the material possesses a clear blue colour, presumably due to intervalence charge-transfer transitions, that should favour detailed mechanistic investigations. Indeed, some preliminary work has been performed [30,31] under pulse radiolytic conditions to monitor the dynamics of hole injection into the colloid. It was found that the rate of the initial hole injection was sensitive to pH but always remained slow. Clearly, this step is going to be one of the rate-limiting issues in water oxidation, even allowing for the slow provision of holes at the catalytic site.
Table 3. Rates of O2 evolution using various metal oxide hydrates (x H2O) as catalysts and the effect of heating the hydrate for 2 hours in air at a particular temperature.
The natural water oxidation catalyst found  in green plants is based on a tetrameric manganese cubane structure that shuttles between the +2 and +3 oxidation states. At present, it is beyond our capabilities to prepare an artificial analogue based on manganese, although considerable research has been expended on designing manganese porphyrins able to sample multiple redox states . The situation with cobalt catalysts is much improved, and recent results indicate that an effective electrochemical catalyst for water oxidation can be engineered  from cobalt(II) phosphate. Thus, the cathode developed therein exhibits many elements of natural photosynthesis, including (i) its in situ formation from Earth-abundant metal ions in aqueous solution, (ii) a potential route for self-repair, (iii) a carrier for protons in neutral water, and (iv) the generation of O2 at low over-potential at neutral pH, atmospheric pressure and room temperature. Furthermore, the new material has been interfaced  with light-harvesting semiconductors so as to develop devices capable of the solar-to-fuels conversion at an overall efficiency of 4.7 per cent for a wired configuration and 2.5 per cent for a wireless configuration when illuminated with 1 sun (100 mW cm −2 ) of air mass 1.5 simulated sunlight. These cells consist of a triple junction, amorphous silicon photovoltaic interfaced to a NiMoZn H2-evolving catalyst and the above-mentioned oxygen-evolving catalyst (figure 6). The devices described here are able to realize the desired solar-driven water-splitting reaction at reasonable efficiency and hold considerable promise for the future, provided the systems can be increased in scale from laboratory demonstration models to practical entities.
Figure 6. Illustration of the system developed by Nocera et al. for the photochemical splitting of water. The cathode is fabricated from a Ni mesh coated with a NiMoZn H2-evolving catalyst. The photoanode is prepared from a stainless steel band coated with Si in contact with an indium-tin-oxide (ITO) layer. Oxygen evolution occurs at a selective catalyst prepared from cobalt phosphate. (Online version in colour.)
Fujishima and Honda reported  the first example of the UV photo-assisted electrolysis of water using a platinized TiO2 electrode in 1972 and more efficient multi-junction photoelectrodes have been developed subsequently [36,37,38]. More recently, Mallouk and co-workers reported a system (figure 7) that uses visible light to split water into hydrogen and oxygen assisted by a small applied voltage . Their system uses the above IrO2⋅nH2O cluster as the water oxidation catalyst, together with a Pt cathode. The photochemical reaction centre comprises a TiO2 film impregnated with a ruthenium(II) poly(pyridine) complex as sensitizer. The system is a hybrid between the Fujishima–Honda cell and the well-known Grätzel-type dye-injection solar cell. The low quantum efficiency of approximately 1 per cent is due to slow hole injection into the IrO2⋅nH2O nanoparticles from the oxidized dye. This reaction does not compete effectively with back electron transfer from TiO2 to the dye. However, further tuning is possible  and a viable prototype looks likely to emerge in the near future.
Figure 7. Illustration of the set-up used by Mallouk et al. for the photodissociation of water using IrO2 nanoparticles as the O2-evolving catalyst. The photoanode is a ruthenium(II) poly(pyridine), RuC, sensitized TiO2 electrode. (Online version in colour.)
Catalysts for the reduction of water to H2 have been available for many decades , although most are based on expensive and rare metals such as Pt, Ru or Ir. These materials require small over-potentials but are not selective and tend to catalyse hydrogenation of any unsaturated organic matter present in the system. Suitable photocathodes for water reduction can be fabricated from stabilized Cu2O, and this p-type semiconductor is a promising material for the photo-assisted electrolysis of water . A critical operating problem in this domain relates to the simultaneous presence of H2 and O2. This situation makes separation of the gases a difficult task and, because a catalyst is usually in the close vicinity, results in recombination reactions (i.e. a Groves type fuel cell). Molecular oxygen competes with protons for the solar-generated reducing equivalents and leads to corrosion of the materials. The search for highly effective catalysts that display excellent specificity for H2 evolution has included certain enzymes  that can be adhered to semiconductor electrodes. The enzymes can demonstrate high turnover numbers and good selectivity but most are poisoned by the presence of molecular oxygen. Recent research, however, has identified  certain hydrogenase strains able to tolerate a low pressure of O2.
The competing chemistry introduced by the presence of O2, together with the difficulty associated with the collection of H2 over a widespread area, has helped direct research towards other targets. Most notable among these is the photochemical reduction of O2 to hydrogen peroxide. The latter is a useful product provided it is generated at concentrations in excess of approximately 10 per cent by volume. Again, there are problems to identify selective catalysts, although Au nanoparticles look promising in this field. It is necessary to avoid the simultaneous presence of H2 and H2O2 because of catalysed chemical reactions, but rapid progress seems highly likely in the very near future. As with H2 liberation, enzyme catalysts are available to promote the key reduction step.
8. Carbon dioxide reduction
Solar-based production of organic chemicals by the reduction of CO2 is an increasingly important area that addresses global warming and fossil fuel shortages. A major advantage of this route is that CO2 can be converted to other energy-storing chemicals such as syngas, formic acid, methane, ethylene, methanol and dimethyl ether: an excellent review of the available directions for CO2 reduction has appeared recently . In developing a viable strategy along this direction, it must be accepted that water is the sole reagent to be considered as the source of both electrons and protons. There are, in fact, many metal complexes able to reduce CO2 with low quantum efficiency under illumination in the presence of sacrificial reagents. Usually, these systems operate in anhydrous organic media, typically N,N-dimethylformamide, saturated with high concentrations of CO2. These conditions are not conducive to setting up a solar fuels industry! On the other hand, natural photosynthesis does a fine job of the photochemical fixation of CO2 to carbohydrates under ambient conditions. This realization has led to the installation of large-scale facilities using microalgae to extract CO2 from effluent gases produced by essential industrial processes. Most notable in this area is the Secil process operating in Portugal that uses CO2 from cement manufacture as a feedstock for biomass production (see http://www.secil.pt/default_en.asp).
Major problems associated with the photochemical reduction of CO2, which is clearly the best way forward in terms of environmental protection, stem from the high over-potentials associated with most electrodes, poor selectivity because of competing H2 formation or O2 reduction, and low concentration of CO2 in the atmosphere. Nonetheless, certain semiconductors are known to display photocatalytic activity towards CO2 reduction in water, albeit with low quantum efficiencies. A recent breakthrough  in this field has seen the introduction of a tandem arrangement that couples an InP photocathode for CO2 reduction in water with a TiO2 photoanode for H2O oxidation (figure 8). Selectivity for the reduction of CO2 to formate is provided by coating the zinc-doped indium phosphide p-type photocathode with a ruthenium-based polymer, [Ru<4,40-di(1H-pyrrolyl-3-propyl carbonate)-2,20-bipyridine>(CO)2], as the electrocatalyst. The efficiency for converting sunlight into chemical energy is only around 0.04 per cent but this is a promising start.
Figure 8. Tandem (or Z-scheme) photo-electrochemical approach used to reduce CO2 to formate in aqueous solution. The photoanode comprises band gap excitation of platinized TiO2, whereas the photocathode is a Zn-doped InP electrode coated with a polymeric ruthenium(II) complex. (Online version in colour.)
While many attempts at the two-electron photoreduction of CO2 to CO or formic acid have been described in the literature, little success has been achieved in regards to the six-electron reduction of CO2 to methanol. The complete reduction of CO2 to CH4 using water vapour as the source of hydrogen has been observed with wide-band-gap semiconductors such as TiO2  or zinc orthogermantate (Zn2GeO4) nanostructures . The latter material, which has a band gap of 4.5 eV, is a poor sensitizer for CH4 production but, in line with earlier reports concerning water cleavage on TiO2, performs better with co-deposits of Pt and RuO2. Even so, the amount of CH4 produced under UV illumination remains very small. Somewhat surprisingly, Bocarsly and co-workers  have found that the pyridinium cation and its substituted derivatives are effective homogeneous electrocatalysts for the aqueous multiple-electron, multiple-proton reduction of carbon dioxide to products such as formic acid, formaldehyde and methanol. Importantly, high faradaic yields for methanol have been observed at low over-potentials such that detailed mechanistic studies become possible . In this aqueous system, the pyridinium cation functions as redox mediator and might pre-adsorb CO2 in the form of a carbamate. Its involvement in the reductive chemistry is critical. Furthermore, it was demonstrated that similar chemistry could be transferred to a photo-electrochemical set-up using a p-GaP semiconductor as the supplier of electrons. In this case, CO2 was reduced to methanol in water at pH 5.2 with near 100 per cent faradaic efficiency at under-potentials greater than 300 mV below the standard potential of −0.52 V versus saturated calomel electrode. Early work on optimization of the cell has produced sustained cathodic currents as high as 0.20 mA cm −2 with no applied bias. This intriguing system opens the way to sustainable photoreduction of CO2 and is one of the most significant breakthroughs to emerge in recent years.
9. Nitrogen fixation
Ammonia is essential for life, because it provides the nitrogen needed for building proteins and DNA, and because of its critical role as a fertilizer. It is considered that in excess of almost one billion tonnes of ammonia are formed each year via reduction of atmospheric nitrogen by nitrogenase enzymes. About the same amount is produced abiotically through the Haber–Bosch reaction, which is arguably the single most significant industrial process ever discovered. This artificial route to NH3 is energy intensive but necessary to sustain agriculture and human development. It is natural to consider developing photochemical routes to N2 fixation that might circumvent the Haber–Bosch process, possibly by incorporating some aspects of nitrogenases in the chemistry. Discovering an effective catalyst for the mild generation of NH3, either by direct reduction of N2 or its combination with H2, would be a tremendous advance over current technology and, by itself, could ensure stable future funding for artificial photosynthesis. Of course, this chemistry is exceedingly difficult.
It is generally considered that the abiotic fixation of N2 requires a Mo-based catalyst. Indeed, few catalytic cycles are known that will reduce N2 to NH3 under ambient conditions . In one case, a single Mo(III) centre serves as catalyst , whereas a second system yields hydrazine as the primary product, which disproportionates into nitrogen and ammonia . Quite separately, the radiolytic and UV-photochemical reduction of N2 to NH3 has been realized in aqueous solution using colloidal metal particles as catalyst . In this case, the catalysts were far from selective for N2 reduction, with H2 evolution occurring in acidic solution, whereas the overall efficacy of the process was found to depend on the type of reducing radical. Nonetheless, this pioneering work provides encouragement for further exploratory research in this important area.
10. Conclusions and prospects
The supply of secure, clean, sustainable energy is a major scientific and technological challenge that must be solved within the next few decades in order to avoid catastrophic changes in society. There are related issues for national and economic security and environmental control that are likely to raise our urgency for seeking genuine solutions to the renewable energy problem. To overcome the technical problems associated with the development of a national solar fuels industry demands the identification of new protocols for the photochemical production of carbon-based fuels and for the recovery of CO2 from the atmosphere. The conversion of sunlight into chemical potential is based on the capture and conversion of solar energy, the liberation of molecular oxygen from water and the subsequent collection and storage of the fuel. In addition to concerns about cost-effectiveness of this basic strategy, it is necessary to accept that, relative to fossil fuel-derived energy, solar energy is diffuse and seasonal, at least in northern Europe. As such, materials costs must be kept very low and long-term storage is essential if we are to succeed in establishing a viable solar fuels industry on these shores.
Molecular-based artificial photosynthetic models have played important roles in the historical development of the field. There have been some hugely imaginative attempts to construct molecular analogues of both the natural light-harvesting antennae and the bacterial reaction centre complex. Although numerous photosystems have been set up that evolve H2 or O2 under illumination, there seems little likelihood that such approaches can be made practical. The same situation holds for reduction of atmospheric CO2 in the presence of molecular oxygen. By contrast, the photochemical dehydrogenation of waste materials is both viable and economical, provided adequate stocks of the waste compound are available and that the entire process is carried out locally. Equally promising is the development of practical outlets for light-induced heat storage. There can be little doubt that commercial outlets could be found along this route and that domestic heating could be delivered by such bioinspired strategies. The only remaining requisite here is to develop water-based systems using small molecular fragments such as ATP analogues.
Large-scale solar capture and conversion can be accomplished by making use of photovoltaic devices. The primary challenge here is to increase the cost-effectiveness for delivered solar electricity. Do we have sufficient sunlight in the UK to drive such technology? If not, is it possible to harvest solar energy from the desert and transport the generated electricity several thousands of miles to the UK? These are major issues but, in terms of solar fuel production, change nothing: there is still the critical need to develop effective and selective catalysts capable of water oxidation and fuel generation under ambient conditions. Other reports will focus attention on the cost of solar electricity but we have to recognize that storage is best achieved by way of fuel generation. The latter requires water oxidation to O2 at minimum over-potential and on a huge scale. What about the fuel?
The storage of sustainable energy in the form of chemical fuels (e.g. hydrocarbons and alcohols) by means of artificial photosynthesis using CO2 and H2O should enable a CO2 neutral power generation infrastructure close to our present commitment to fossil fuels. This would require minimum disruption of our current machinery but demands the identification of selective and ultra-efficient catalysts. The on-site collection of gases over a wide area is problematical, as is the prior removal of O2 from the cell housing the cathode. In this respect, the conversion of O2 to HOOH looks to be the easiest option. The corresponding conversion of CO2 to HCOOH is more attractive, in many respects, but an order of magnitude more difficult to achieve. In terms of setting up a large-scale demonstration of solar fuel production, we could do worse that develop a robust protocol for local production of hydrogen peroxide. On a longer-term basis, the route to methanol production from CO2, no doubt this is a hazardous journey, has been opened by the concept of involving a pyridinium-based relay and this could be our most realistic target. Mimicking the role of the FeMo nitrogenases in N2 fixation might be a step too far at the moment but fundamental research in this area is needed desperately. A high fraction of worldwide energy supplies is diverted to the fixation of N2, a process that by itself is responsible for massive population growth, using established chemistry at high temperatures and pressures. The selective reduction of N2, probably the most stable diatomic molecule, to NH3 by protons and electrons at room temperature and atmospheric pressure remains a challenge for future generations.
Biological Transformation of Solar Energy
This chapter presents a description of a hypothetical solar energy conversion plant in which an algal culture pond, algal digester, and a thermal power generator are combined to transform solar energy into electrical power. Description, specifications, and cost estimates are provided for designing, maintaining, and operating each of the units. The cost of line power as a function of latitude and photosynthetic efficiency is estimated in the chapter. According to the data and information presented, the cost of line power in the lower latitudes of the earth would be from 15 –20 mills/kw.-hr. This compares with the estimated 16.7 mills/kw.-hr. For a fission plant of equivalent capacity, but is about three times the cost of present day thermal power in the U. S. By increasing the efficiency of the solar energy collecting ponds and improvements in digester design, power costs could be decreased to the extent that solar energy would compare favorably in cost with other sources of energy in special areas of the world.
Ask An Engineer
“Imagine a straight-line, one-hill roller coaster. It’s boring to ride, but it’s a useful example,” says Aaron Johnson, a PhD student in aeronautics and astronautics. If you start your ride at ground level, the first challenge is getting the coaster to the top of the hill. This takes energy, so you expend what you need against gravity and friction to get the coaster to the top. When the coaster crests the hill, all of the energy you have just expended against gravity is now stored as potential energy — though exactly how much depends on the height of the hill and the weight of the cars and passengers.
In an ideal world, Johnson says, a perfectly efficient energy collector on a frictionless coaster in a vacuum should be able to harness that potential energy and convert it to enough kinetic energy on the way down to drive you up a hill of exactly the same height. But in the real world, energy collection is more complicated. Much of the potential energy you have just gathered is going to scatter. Friction generates heat energy in the track and wheels, and drag buffets the cars and passengers, heating them and the air around them and dissipating more of your energy. This dispersion means realizing all of your potential is nearly impossible.
Interestingly, not all of this energy needs to be completely lost — whether it’s from a roller coaster or any other moving vehicle. One way to collect some of the energy that dissipates from moving vehicles is through something called regenerative braking, as used in hybrid cars such as the Toyota Prius. Regenerative brakes use energy normally lost to heat and friction during braking to charge batteries that power the car. “It works, but you’re never going to get enough energy to bring you back up the hill again,” says Johnson. “You’ve lost some of it to heat, plus the regenerative braking process isn’t 100 percent efficient.” (In Pittsburgh, engineers recently created just such a demonstration coaster and used the collected energy to power a display of amusement park lights.)
Another approach is to add a turbine to the coaster to collect wind energy. Airplanes do this. In an emergency, such as a power failure, a plane drops a so-called ram air turbine from a hatch. The turbine collects wind energy to power hydraulics and critical instruments. “It’s very inefficient, so it’s usually only for emergency use,” says Johnson.
While no solution is perfect, regenerative brakes and turbines are constantly being reworked and redesigned to become more efficient. For instance, in turbines, the shape and twist of the blade matters. “There’s a lot of people trying to design the most efficient blade possible,” says Johnson.
Is it possible to biologically convert potential energy to chemical energy? - Biology
Anabolic pathways require an input of energy to synthesize complex molecules from simpler ones. Synthesizing sugar from CO2 is one example. Other examples are the synthesis of large proteins from amino acid building blocks, and the synthesis of new DNA strands from nucleic acid building blocks. These biosynthetic processes are critical to the life of the cell, take place constantly, and demand energy provided by ATP and other high-energy molecules like NADH (nicotinamide adenine dinucleotide) and NADPH (Figure 1).
ATP is an important molecule for cells to have in sufficient supply at all times. The breakdown of sugars illustrates how a single molecule of glucose can store enough energy to make a great deal of ATP, 36 to 38 molecules. This is a catabolic pathway. Catabolic pathways involve the degradation (or breakdown) of complex molecules into simpler ones. Molecular energy stored in the bonds of complex molecules is released in catabolic pathways and harvested in such a way that it can be used to produce ATP. Other energy-storing molecules, such as fats, are also broken down through similar catabolic reactions to release energy and make ATP (Figure 1).
Figure 1. Anabolic pathways are those that require energy to synthesize larger molecules. Catabolic pathways are those that generate energy by breaking down larger molecules. Both types of pathways are required for maintaining the cell’s energy balance.
It is important to know that the chemical reactions of metabolic pathways don’t take place spontaneously. Each reaction step is facilitated, or catalyzed, by a protein called an enzyme. Enzymes are important for catalyzing all types of biological reactions—those that require energy as well as those that release energy.
Chemical Energy to Thermal Energy Examples
1. Coal is burned at a power plant. The chemical energy released as the coal is burned heats water and turns it into steam. The chemical energy causes the liquid water molecules to move faster increasing their thermal energy.
2. Gasoline is burned in a car engine. As the gas burns, small explosions release heat or thermal energy which makes the pistons move so the car go.
3. A furnace burns natural gas in a chemical reaction. As the natural gas burns, heat is released causing air molecules to move faster. The temperature of the air increases because they have more thermal energy which is used to heat a house.
4. Hand warmers used for hunting operate by chemical reaction. When the contents of the bag are mixed together, a chemical reaction causes chemical bonds to break releasing energy. This energy causes the thermal energy of the solution inside the bag to increase and we feel heat to warm our hands.
5. A butane torch burns butane gas. As the chemical bonds are broken during the reaction, heat is released. The heat is used to increase the thermal energy of a metal which melts and is used to weld pipes together.
Is it possible to biologically convert potential energy to chemical energy? - Biology
UCB chemistry professors James McCusker and Charles Shank, who is also the director of Berkeley Lab, along with graduate student Alvin Yeh, co-authored a paper in the August 11 issue of the journal Science in which they described a time-resolved spectroscopic study that followed the evolution of a photo-induced charge-transfer state in an inorganic chromophore. Their findings show, for the first time, two distinct factors that contribute to this evolution one which is strongly influenced by the chromophore's immediate surroundings, and the other which is influenced only by the molecule's internal electronic structure.
A TYPICAL SOLAR CELL CONSISTS OF A COVER GLASS WITH AN ANTI-REFLECTIVE LAYER, A FRONT CONTACT TO ALLOW ELECTRONS INTO A CIRCUIT AND A BACK CONTACT TO ALLOW THEM TO COMPLETE THE CIRCUIT, AND THE SEMICONDUCTOR LAYERS WHERE PHOTO-INDUCED CHARGE TRANSFER PROCESSES TAKE PLACE.
Image courtesy Dept. of Energy
"Although this work is very fundamental in nature, it suggests that medium-induced charge localization could be an important component of photo-induced charge transfer in a variety of settings that employ inorganic compounds as chromophores," says McCusker. "Since this is the necessary first step in almost any scheme one can come up with to convert light into usable energy, we believe that our results will help shape the way people think about this aspect of the problem."
In the process of photo-induced charge transfer, incident light upon a molecule redistributes electron density to create the chemical potential necessary for energy conversion. This process is central to a wide range of physical and chemical phenomena including photosynthesis in plants, and also forms the basis of the photovoltaic effect in semiconductors.
"Prior to our work, very little was known concerning the dynamics of photo-induced charge-transfer in inorganic chromophores," McCusker says. "But its only been within the last 10 years or so that the study of processes on such short time-scales has been experimentally accessible."
Understanding the dynamics behind photo-induced charge-transfer in inorganic chromophores is especially important for certain classes of solar cells where light absorption by dye-sensitized wide-bandgap semiconductors such as titanium oxide forms the basis for photoelectric conversion. Recently there has been considerable interest "both theoretically and experimentally" McCusker says, in the "dynamics of solvation." That is, scientists want to know what happens when the effects of incident light upon the chromophore and the dynamics of its nearby molecular neighbors take place on a comparable time-scale.
In their Science paper, McCusker, Shank, and Yeh report their observations of the factors that contributed to the formation of a charge-transfer state following the absorption of light by an inorganic chromophore in solution on a time-scale of less than one trillionth of a second. A chromophore known as [Ru(bpy) 3 ] 2+ , which is the prototype for the most widely used inorganic chromophores in sensitized solar cells, was photoexcited with flashes of light that were a mere 25 femtoseconds in duration. (A femtosecond is to one second what one second is to roughly 30 million years.) The chromophore's absorption of this light was then monitored as a function of time.
"Analysis of the data revealed that the excited electron is initially delocalized over all three bpy ligands, but eventually becomes trapped on a single ligand due to the rapid motion of the molecules in the surrounding solvent which occurs in response to the charge-transfer event," says McCusker. "In contrast, electronic relaxation from the initial excited state of the compound to lower energy states appears to occur independent of this charge localization process."
Whether or not the intramolecular effects of photoexcitation are totally independent of the environmental effects when it comes to localizing the charge-transfer is still not clear, the scientists report, but the identification of solvation dynamics as having a role to play answers some long-standing questions about the dynamics of charge-transfer states in inorganic chromophores. The researchers say it is possible that the surrounding medium might also play a similar role in localizing or directing charge-transfer states in the organic chromophores of biological systems.
"In this circumstance, the nearby residues within the proteins would act in much the same way as the solvent does in our experiment," says McCusker. "We have no evidence that this is the case, but it's interesting to think about."
Fact or Fiction?: Energy Can Neither Be Created Nor Destroyed
The conservation of energy is an absolute law, and yet it seems to fly in the face of things we observe every day. Sparks create a fire, which generates heat&mdashmanifest energy that wasn&rsquot there before. A battery produces power. A nuclear bomb creates an explosion. Each of these situations, however, is simply a case of energy changing form. Even the seemingly paradoxical dark energy causing the universe&rsquos expansion to accelerate, we will see, obeys this rule.
The law of conservation of energy, also known as the first law of thermodynamics, states that the energy of a closed system must remain constant&mdashit can neither increase nor decrease without interference from outside. The universe itself is a closed system, so the total amount of energy in existence has always been the same. The forms that energy takes, however, are constantly changing.
Potential and kinetic energy are two of the most basic forms, familiar from high school physics class: Gravitational potential is the stored energy of a boulder pushed up a hill, poised to roll down. Kinetic energy is the energy of its motion when it starts rolling. The sum of these is called mechanical energy. The heat in a hot object is the mechanical energy of its atoms and molecules in motion. In the 19th century physicists realized that the heat produced by a moving machine was the machine&rsquos gross mechanical energy converted into the microscopic mechanical energy of atoms. Chemical energy is another form of potential energy stored in molecular chemical bonds. It is this energy, stockpiled in your bodily cells, that allows you to run and jump. Other forms of energy include electromagnetic energy, or light, and nuclear energy&mdashthe potential energy of the nuclear forces in atoms. There are many more. Even mass is a form of energy, as Albert Einstein&rsquos famous E = mc 2 showed.
Fire is a conversion of chemical energy into thermal and electromagnetic energy via a chemical reaction that combines the molecules in fuel (wood, say) with oxygen from the air to create water and carbon dioxide. It releases energy in the form of heat and light. A battery converts chemical energy into electrical energy. A nuclear bomb converts nuclear energy into thermal, electromagnetic and kinetic energy.
As scientists have better understood the forms of energy, they have revealed new ways for energy to convert from one form to another. When physicists first formulated quantum theory they realized that an electron in an atom can jump from one energy level to another, giving off or absorbing light. In 1924 Niels Bohr, Hans Kramers, and John Slater proposed that these quantum jumps temporarily violated energy conservation. According to the physicists, each quantum jump would liberate or absorb energy, and only on average would energy be conserved.
Einstein objected fervently to the idea that quantum mechanics defied energy conservation. And it turns out he was right. After physicists refined quantum mechanics a few years later, scientists understood that although the energy of each electron might fluctuate in a probabilistic haze, the total energy of the electron and its radiation remained constant at every moment of the process. Energy was conserved.
Modern cosmology has offered up new riddles in energy conservation. We now know that the universe is expanding at a faster and faster rate&mdashpropelled by something scientists call dark energy. This is thought to be the intrinsic energy per cubic centimeter of empty space. But if the universe is a closed system with a finite amount of energy, how can it spawn more empty space, which must contain more intrinsic energy, without creating additional energy?
It turns out that in Einstein&rsquos theory of general relativity, regions of space with positive energy actually push space outward. As space expands, it releases stored up gravitational potential energy, which converts into the intrinsic energy that fills the newly created volume. So even the expansion of the universe is controlled by the law of energy conservation.
ABOUT THE AUTHOR(S)
Clara Moskowitzis Scientific American's senior editor covering space and physics. She has a bachelor's degree in astronomy and physics from Wesleyan University and a graduate degree in science journalism from the University of California, Santa Cruz.