Background Knowledge
The Sun
The Sun is 90 million miles from the Earth, yet despite this unfathomable distance, the Earth's surface receives a huge amount of energy from the Sun. This energy, also known as electromagnetic radiation, is formed by nuclear fusion, in which hydrogen atoms fuse producing helium due to the extremely high temperatures in the Sun. The electromagnetic spectrum ranges from long wavelength, low frequency radio waves to short wavelength, high frequency gamma rays. The Earth's atmosphere only allows some of this radiation to reach the surface though. A narrow band of this spectrum that can pass through the atmosphere is known as the visible spectrum, or the photosynthetically active radiation [7]. Depending on your location in the globe, between 105 kW/m 2 (in England) to 1000 kW/m 2 (at the equator) of energy reaches the Earth's surface. One kilowatt can power a 100 Watt lamp for 10 hours, a laptop for 24 hours, or a vacuum cleaner for 45 minutes [8].
Carbon
Carbon is the sixth most abundant element on Earth, but it is one of the most important because it is found in every organic molecule. Carbon can bond with many different elements including itself, and, thus, there are a myriad of molecules that carbon can form. Carbon has four valence electrons, thus each carbon atom will form covalent bonds with up to four other atoms, in order to become stable with eight electrons in the outer shell. Carbon can form various inorganic compounds, such as carbon dioxide, carbon monoxide, and carbonates. Organic molecules are those containing carbon bonded to hydrogen, with other atoms as well. These include fatty acids, alcohols, proteins, carbohydrates, nucleic acids and polymers like plastic.
Related Activities:
It's All About Carbon Episodes 1-5, http://www.npr.org/news/specials/climate/video/
The Carbon Cycle
The carbon cycle explains how carbon moves through ecosystems changing in state and molecular form. The carbon cycle includes two parts. The geological carbon cycle involves long-term weathering, subduction and volcanism. Carbon dioxide dissolves in the oceans to form carbonic acid, which then reacts with calcium and magnesium in the crust to form insoluble carbonates which settle out. The deposition of carbonates and weathering of rocks from the crust provide a source of carbon that is taken into the mantle through subduction. This carbon is released back into the atmosphere from the eruption of volcanoes [9].
The biological carbon cycle involves the movement of carbon through organisms on land and in water through photosynthesis and respiration. Photosynthesis, which is described in the next section, is done by autotrophs by using the energy from light to split water and incorporate hydrogen atoms into carbon dioxide, making a carbohydrate like glucose (C 6H 1 2O 6). The energy stored in carbohydrates is then released by all organisms during respiration. In this process, carbohydrate is burned in the presence of oxygen, and water and carbon dioxide are produced and released back into the atmosphere. The amount of carbon cycled though these biological processes are 1000 times greater than the geological aspect of the carbon cycle [9].
Carbon that is taken in during photosynthesis can be stored for long periods of time in plants, for example in the trunk of a tree. This carbon is released back into the atmosphere as carbon dioxide when the tree dies, or is cut, burned, or decayed. Additionally, carbon is released by plants or trees during respiration. Actively growing trees assimilate more carbon dioxide than is released by respiration, though mature trees reach a point of carbon saturation produce as much carbon dioxide in respiration and decay as is removed from the atmosphere by photosynthesis [10].
Related Activities:
Carbon Neutral Cars? - see Activity #2 in Resources
Photosynthesis
Photosynthesis is the process in which solar energy is converted to chemical energy. In other words, the energy in light is used to convert CO 2 and H 2 O into C 6H 1 2O 6 and O 2 . This process occurs in two steps, the light reactions (which requires light) and carbon fixation (also known as the dark reactions or Calvin cycle). In the first step of photosynthesis, the light energy from the sun is used to split water, which results in the release of oxygen and the production of hydrogen ions and free electrons. The hydrogen ions and electrons continue to the second step of photosynthesis in which carbon dioxide is reduced by hydrogen and electrons to form a carbohydrate like glucose.
In Energy, Plants and Man, David Walker gives numerous anecdotes which illuminate early discoveries about photosynthesis. For example, a scientist, named Van Helmont, planted a willow tree in a tub with soil that he had weighed. He watered the willow tree for five years after which time the willow weighed 164 lbs greater and the soil only two ounces less. Van Helmont concluded that the tree had grown in size because of the water. He also concluded that the difference is soil mass was an error (Walker, 1992). This example is excellent at confronting misconceptions that many students may have. Fertilizers, which contain essential nutrients N (nitrogen), P (phosphorous), and K (potassium) are important cofactors in plant growth, much like vitamins are to a human diet. But fertilizers alone do not result in plant growth. The gas in the atmosphere (carbon dioxide) and the water in the presence of light produce glucose molecules, which combine to form larger molecules like starch and cellulose. Cellulose forms the framework of plant structure, and starch provides storage for glucose, from which the plant can depend on during times of reduced photosynthesis [11].
Almost all of the energy on the Earth originates from the Sun and is made accessible to humans through photosynthesis. Plants are able to use light energy in a way that humans are trying to replicate. Plants absorb light from the Sun using pigments, primarily chlorophyll a, chlorophyll b and caroteniods. These pigments absorb slightly different wavelengths of light; for example, chlorophyll a absorbs red and blue light best, reflecting green light which makes plants appear green. Chlorophylls are found in chloroplasts, cell organelles which are numerous in photosynthesizing plant cells. Chloroplasts contain many compartments, called thylakoids that are surrounded by membranes. The compartments allow multiple chemical reactions to take place simultaneously. Chlorophylls and other pigments are found in embedded in the thylakoid membranes. The matrix that surrounds the thylakoids is called the stroma. These two distinct locations are where the two steps of the photosynthesis reaction take place [11].
The first step of photosynthesis, using light to split water, occurs in two photosystems, which are protein complexes within the thylakoid membrane of a chloroplast. When light strikes a leaf, the light energy is initially harnessed by antennae (which consist of 500-1000 pigment molecules) found within Photosystem 2 (PS2). The antennae pigments funnel energy to a chlorophyll molecule in a reaction center, which is elevated to an excited state. In the excited state, an electron of the chlorophyll molecule is moved to an energy level that is farther away from the nucleus. These elevated electrons are quickly transferred to small electron accepter molecules, which are alternatively reduced and then oxidized as electrons move through the electron transport chain (ETC). This movement of electrons can be thought of as "downhill" movement- electrons moving toward progressively more electron attracting molecules, releasing small amounts of energy at every step. The energy is used to pump (through active transport) hydrogen ions across the thylakoid membrane into the thylakoid (lumen) against the hydrogen ion concentration gradient. The positive charges in the lumen create a proton gradient, an imbalance of charges on either side of the membrane. This proton gradient is a source of potential energy which is used along with the enzyme ATP synthase to produce ATP from ADP. The energy in the form of ATP is used in the carbon fixing reaction in the stroma [11].
The electron moves along the electron transport chain towards Photosystem 1 (PS1), which also captures light using antennae and a reaction center. The energy from the light captured in PS1 is used to kick up the electron to a further excited state. The electron is again carried through an electron transport chain to its final electron acceptor, NADP (nicotinamid adenosine dinucleotide phosphate). NADP + becomes NADPH when it receives a hydrogen ion and two electrons from the ETC of PS1. NADPH along with energy (ATP) that is made during the light-dependent reactions is used in the second part of photosynthesis, in which CO 2 is reduced, forming a carbohydrate like glucose [11,12].
The reduction of carbon dioxide is also known as the dark reactions. This name is a misnomer, because while these reactions do not require light, they occur in the light [11]. It is convenient to separate this set of reactions from the light reactions (water oxidation reactions) because the light reactions produce energy and occur in the thylakoid membrane and lumen, while the dark reactions use up the energy produced in the light reactions and occur outside the thylakoid, in the stroma. The energy in ATP and electrons in NADPH are used in this process which begins with a 5 Carbon sugar phosphate molecule accepting CO 2 in the presence of the enzyme rubisco. Rubisco is extremely abundant in the stroma of a chloroplast, and is, in fact, the most abundant protein on Earth. The 5C sugar and CO 2 combine to form a 6 C sugar which immediately splits into 2 3C molecules. These molecules undergo phosphorylation (by ATP) and reduction (by NADPH) to regenerate the initial 5C sugar and to produce a carbohydrate that will undergo further processing to form glucose. Glucose may be the final product or it could undergo processing to form sucrose (a disaccharide) or starch (a polysaccharide), for example [11].
Related Activities:
Paper Chromatography - http://www.ekcsk12.org/science/lelab/chromatographylab.html, http://www.ekcsk12.org/science/aplabreview/chromatographylab.htm
Starch Pictures - http://www.accessexcellence.org/AE/AEC/AEF/1996/morishita_pictures.html
Biomass: Ethanol vs. Biodiesel
Biomass includes any plant or plant-derived material that was originally the result of photosynthesis [13]. Biomass has been used throughout history, primarily as wood or dung. Recently, scientists have been using plants and plant remains to produce fuel rather than to produce heat or make electricity. What are the benefits of fuel over electricity? Making fuels from plants is precisely what fossil fuels are; though fuels from biomass can be produced in minutes rather than in millions of years.
Biomass does emit carbon dioxide when it is burned. This amount is offset by the carbon dioxide that the plant took in through photosynthesis as it was growing. The plants that formed fossil fuels died millions of years ago and, thus, are no longer photosynthesizing, only releasing carbon dioxide as they are burned. So, biomass is a good alternative to fossil fuels, but, even as a carbon neutral fuel, (photosynthesis balanced by respiration) biomass fuel does not reduce the amount of carbon dioxide in our atmosphere, it merely does not increase the amount. In addition, the production of biomass incurs hidden carbon dioxide costs. For example, the fuel required by tractors, fertilizers, trucks and machinery to grow, transport and produce the biofuel needs to be included in the carbon dioxide equation.
The two major types of biofuels on the market today are ethanol and biodiesel. Ethanol is an alcohol-based fuel that is made by fermenting the glucose in starch from wheat, barley and primarily corn. It can also be made from sucrose found in sugar cane. The Clean Air Act of 1990 mandated the gasoline be oxygenated in order to reduce the carbon monoxide emissions. Ethanol and methyl tertiary-butyl ether (MTBE) were the two additives used for this purpose, though MTBE use has been discontinued because it was found to contaminate ground water. Ethanol is blended with gasoline is various percentages; commonly 10% which reduces emissions and improves the octane rating. Higher blends of ethanol include Ethanol 85 (85% ethanol and 15% gasoline) and Ethanol 95. These blends are considered alternative fuels and can be used in flexible fuel vehicles (FFVs). The use of ethanol is advantageous to the U.S. because it reduces our demand of oil from other countries, and ethanol use supports U.S. farmers, primarily in the Midwest. There are drawbacks to ethanol use, however. Ethanol contains less energy than gasoline, which results in lower mileage per gallon of fuel [14].
The rise of ethanol to replace MTBE as a gasoline oxygenate has been controversial because some researchers concluded that producing ethanol resulted in a limited gain or even a net loss of energy [15]. One reason for the relative uncertainty is that ethanol can be produced by various types of plants, parts of plants and using different processes. For example, ethanol is largely produced from corn in the U.S. and from sugarcane in Brazil. Further, ethanol is the U.S. is made by converting the starch which is found in the corn kernel, while ethanol in Brazil is made using sucrose which is found in sugarcane.
Ethanol is produced from sugar or starch by dry-mill or wet-mill processing [16]. In dry-mill processing, the corn kernel is ground and then mixed with water to form a mash. Enzymes are added to convert the starch to dextrose, and the mixture is heated to kill-off bacteria before fermentation begins. The mash is cooled and yeast is added to begin the fermentation process, which takes 40-50 hours. The ethanol is separated from the stillage, and then concentrated to 200 proof using distillation and a molecular sieve. A denaturant is added to the ethanol to make it undrinkable, and it is then ready from shipment. The remaining stillage is processed further to produce a nutritious livestock feed. Wet-mill processing uses water and sulfuric acid to break down the grain into fiber, starch, corn germ, corn gluten and corn oil. The starch can be further processed into ethanol and the by-products sold or processed for livestock feed. Most ethanol (82%) is made using the dry-mill technique [16].
Biodiesel is a fuel that is primarily made from soybean oil. It is sold primarily in blends of B2 (2% biodiesel, 98% diesel) or B5 (5% biodiesel, 95% diesel). Higher blends of biodiesel (20%) are given credits through the Energy Policy Act of 1992. The benefits of biodiesel include reduced carbon dioxide emissions and displacement of imported petroleum [17].
Biodiesel is made from vegetable oil, which consists of triglyceride molecules, each with a glycerol head and fatty acid tails. Methanol and sodium hydroxide are combined together and added to vegetable oil that has been heated to 100-120 deg F. The mixture is stirred and then allowed to separate. During agitation, the sodium hydroxide reacts with methanol removing a hydrogen, which produces water and methyl oxide. The methyl oxide removes the glycerol head from the triglycerides and replaces glycerol with H 3C groups [18]. The resulting molecule is biodiesel, and glycerin is produced in small amounts as a waste product. After the biodiesel and glycerin separate, the biodiesel is washed with water to remove any remaining methanol which is corrosive. The reaction produces a very high yield, 1000 g of biodiesel for 1030 g of vegetable oil added [19].
Comparisons of the net energy gain of ethanol and biodiesel have been numerous [6,15, 20]. Simply, the amount of energy that it takes to produce ethanol must not be greater than the amount of energy that results. Some studies found a net loss of energy in ethanol production, yet later research criticized these studies for not including the co-products that result from ethanol production, like animal feed, that diverts energy use. Other studies have been faulted for expanding the boundaries of energy inputs to include the manufacture of farm equipment and buildings. Despite varied methodologies, the consensus is that ethanol and biodiesel do provide a net energy gain and a net reduction of greenhouse gases (GHG) over an equal volume of gasoline [6,20]. Factors that serve to reduce this net energy gain includes the production and application of fertilizers, both of which consume petroleum, in addition to the transport of the crop to the production plant and transport of the fuel to the retailer.
Researchers compared ethanol and biodiesel using four standards: net energy gain, environmental effects, economic cost, and capability for growth without negative impact on food production. For net energy gain, biodiesel provides 93% more energy required for its productions, while ethanol provides only 25% more energy than is input. Biodiesel reduced GHGs by 41% compared to diesel and ethanol only provides a 12% decrease. Biodiesel results in a reduction in four of the major emissions including VOCs (volatile organic carbons), CO, Sox (sulfur oxides) and large particulate matter (PM 10, diameter>10 mm). Low-level ethanol-gasoline blends (for ex. 10%) reduce CO, VOCs and particulates during combustion, though ethanol 85 has higher levels of CO, VOC, PM10, SO x, and NO x than an amount of gasoline with the equivalent energy content.
Further, corn requires greater amounts of fertilizer (N and P) and pesticides than soybeans. Neither fuel is economically competitive at this writing with gasoline or diesel without subsidy [6]. Current agricultural practices contribute to GHG emissions (34-44%) and petroleum inputs (45-80%) associated with the ethanol [20]. Improvements in the agriculture industry, could significantly improve the net energy gain and environmental impact of ethanol. For example, conservation tillage can reduce soil erosion, fertilizer and pesticide runoff, as well as petroleum use and GHG emissions.
Further ways to improve the net energy gain and reduce GHG emissions of ethanol involve cellulosic technology. Currently in the U.S., ethanol is made from the starch in the corn kernel, while the husk and stalk remain unusable. Researchers are beginning to work on making biofuel from cellulose, which will allow biofuel to be produced from corn waste (the stalk and husk) and many other plant wastes. This process is currently being developed in pilot programs and in selected areas. The conversion of cellulose into ethanol holds great promise for economic and environmental reasons. Cellulosic technology does not interfere with food production, as corn and soybean conversion does. Plants such as switch grass can be growth in marginally productive areas with little or no fertilizer requirements. Combustion of waste biomass could be used to power the ethanol processing plants. Initial inputs of energy will be required for building these new production plants, and transportation costs may be higher for transporting waste biomass to the production plants. New developments in this technology promise to increase the net energy gain of ethanol and reduce the GHG emissions of the fuel [6].
Related Activities:
Making Biodiesel - http://journeytoforever.org/biodiesel_make.html http://www.biodieselcommunity.org/gettingstarted/
Social and Economic Issues
Beyond the costs of fertilizer and transportation costs of producing biofuels, it appears there are more hidden costs associated with biofuel production. Because of the high demand for ethanol, international corn prices have increased dramatically. The result in Mexico is that prices of tortillas have tripled or quadrupled since the summer of 2006. A cap on tortilla prices has done little to alleviate vendors and consumers who are struggling to survive. Tortillas are a vital part of the Mexican diet, providing 40% of the protein, fiber and essential nutrients, like calcium. Poor Mexicans have been forced to shift their diet to other foods, among them, instant noodles which are far less nutritious. Corn shortages have forced Mexico to import 800,000 tons of corn from the U.S. in 2007, whereas corn was exported the previous year [21].
Soybean production in other countries is also reportedly wreaking havoc on human and ecosystem health. In Paraguay, the number of acres of soy jumped over 200% from 1999 to 2004. The expansion of soy has come at the expense of savanna and rainforest lands. Further, pesticide use, more than 24 million liters every year, has caused increased birth defects and reduced sperm count in exposed males. Paraguayans maintain that wildlife and livestock have died, forcing many to leave their lands. Cheap land in Paraguay brought soy farmers eager to satisfy the demand for biodiesel [22]. Estimates that consider biodiesel production to be a substantial net energy gain and GHG reduction are only valid if the soy is grown on land already in production. Clearing intact forest land to grow soy for biodiesel would likely contribute a net GHG release into the atmosphere [6].
New Technology
Artificial photosynthesis is renewable energy research area that may serve as a means for sequestering carbon dioxide from the atmosphere, thus reducing the amount of heat that is trapped by the Earth's atmosphere. There are many new technologies on the horizon, which could help reduce our oil consumption and carbon emissions. Many of these new technologies, including high altitude wind, tidal power, nanotech solar cells, and designer microbes, though, require time and substantial financial investments [23].
Scientists are actively working on the concept of artificial photosynthesis, and there seems to be two general approaches. One is to use a molecular assembly linking electron donors and electron acceptors. Light stimulates an electron to reach an excited state, and much like in the photosystems in plants, the electron moves to an electron acceptor. The main obstacle for this method is the tendency of the electron to quickly fall back to the electron donor. New carbon nanotube technology may help pull electrons away from donors quickly enough to ensure charge separation [24].
The other approach to artificial photosynthesis uses semiconductors, which transmit electrons, but do not absorb visible light. Yet by attaching a colored molecule, like manganese, to a semiconductor, such as titanium dioxide, sunlight can be used to move electrons, generating electricity. Further, this method can electrolyze or split water, which could provide a renewable source of hydrogen gas. The hydrogen gas can be used directly in fuel cells or converted into a more easily transported fuel, such as methanol [25]. Clearly, there is great potential in these research areas, but a large investment of time and money is necessary before artificial photosynthesis is a large-scale reality.
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