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What happens when cellular respiration doesn't work properly? Disease. Scientists are still studying the connections between glycolysis, cellular respiration and disease, but some interesting links exist. For example, Alzheimer's may be linked to "aerobic glycolysis," which is when cells use glucose that does not go into oxidative phosphorylation.
Tumor cells have an interesting way to acquire energy: instead of oxidative phosphorylation, the electron transport chain, and so on, tumor cells do something called aerobic glycolysis. Aerobic glycolysis makes lactic acid, so it is like fermentation…except it is done with oxygen. This is a pretty inefficient process, which means cancer cells have to use way more glucose than healthy cells to survive and reproduce.
When a cancerous cell changes its metabolism from normal to aerobic glycolysis, it is called the Warburg effect, named after the scientist and Nobel laureate who discovered it. Dr. Warburg thought that this type of metabolism might be what drives cancer. But it's a hot topic of research. Scientists are currently trying to figure out if aerobic glycolysis is a cause or effect of cancer (Taubes 2012). In other words, it's a chicken-or-the-egg situation.
Alzheimer's disease might also be linked to aerobic glycolysis. Research has linked high levels of aerobic glycolysis in the brain to deposition of plaques later in life (Vlassenko et al. 2010). We all know plaque on our teeth is bad news. After all, dentists insist that we remove our plaque daily with toothbrushes. Plaques in the brain are to blame for Alzheimer's. Since they don't make brain brushes yet, researchers will have to keep looking for a cure.
Leigh's syndrome is a rare neurological disease in children in which the central nervous system degenerates. As the disease progresses, motor skills and muscle development weaken. Although not totally understood, Leigh's syndrome is related to problems with cellular respiration. Breakdowns in both the electron transport chain and the conversion of pyruvate to acetyl CoA have been implicated as causes of Leigh's. Cells in Leigh's syndrome patients have slower metabolisms and take up less oxygen and glucose than healthy cells. In some cases, Leigh's syndrome has been treated successfully with certain vitamins that are necessary for cellular respiration, such as thiamin; coenzyme Q10; and vitamins C, K3, and E. (Vo et al. 2007).
Fermentation is not only a biological process, but a critical component of making alcohol and bread. Cultures around the world and throughout history have used fermentation to produce beer, wine and bread.
We already learned that alcohol fermentation, done by bacteria and yeast, produces ethanol, which is alcohol, and carbon dioxide. The carbon dioxide makes bubbles, which makes beer frothy, and causes bread to rise. Some would even say beer is the most important invention in the world—watch How Beer Saved the World for more details.
Human use of fermentation goes way back—ancient civilizations made alcohol, as early as nine thousand years ago in Neolithic China (Dietler 2006). The ancient Sumerians made beer (Michel and Patrick 1992), and some researchers even think that beer is why we have civilization at all. Back in the day, humans lived nomadic lives, roaming around to hunt. About 10,000 years ago, they started settling down and grew crops, domesticated animals (really cut back on the need for orange hunting vests), and lived in cities and villages. Some researchers believe humans started growing crops so that they could make fermented beverages from them. One of the first crops grown during the agricultural revolution was barley. Since beer is made from barley, anyone who wanted to brew (and drink) beer would have to grow the barley first. Then they had to stick around to brew the beer. Ancient people said goodbye to their previously nomadic lives, planted barley, and became the first farmers and brewers. The first brewers probably stumbled upon beer by accident, but they quickly figured out how to keep making it, and today brewing is down to a science.
Archaeologists can tell that certain pots used to hold beer because of a residue they find on artifacts. In ancient Mesopotamia, evidence suggests that the wealthy elite drank wine and lower classes drank beer. This is not so surprising, since water was not purified and beer was probably safer to drink. In both ancient and modern societies, wine and other alcoholic beverages are integral parts of ceremonies and rituals (Dietler 2006). Before water purification and refrigeration, it was probably useful to have a beverage around that would keep for awhile.
Alcohol was vital to ancient Greek and Roman societies, not just as an intoxicant, but also as an important economic commodity. The Romans shipped huge quantities of wine to their colonies. In the 1700s, alcohol was an important trade item to the colonial powers participating in the West African slave trade. Rum, made from sugar cane, was traded for slaves, who were then forced to work on those very same sugar plantations (Dietler 2006).
Humans are not the only ones who enjoy fermented products. Lots of other animals ingest ethanol when they eat ripe fruit. Fruit growing in the tropics, with the warm and wet climates there, is especially prone to hosting fermenting yeast. Many tropical fruits contain low levels of ethanol, so animals that eat large quantities of fruit are also steadily taking in alcohol (Dudley 2002). The alcohol content of fruit adds more calories, and therefore more nutrition, to the fruit (to animals living in the wild that have to search for food all day, the more calories, the better). In fact, the fruit-eating patterns of our primate ancestors and the high-calorie rewards from ethanol-laden fruit may provide the evolutionary basis for human alcoholism (Dudley 2000, 2002).
That is enough about alcohol…on to a more important thing: bread. Bread is another major product from fermentation. A world without bread would be like a day without sunshine. Bread has had a pretty big impact on world history. After all, didn't the French Revolution start because people had to wait in long lines for their baguettes? Hmm…maybe it is time to go back and read A Tale of Two Cities.
The ancient Sumerians started making bread around 6000 BC (Belderok 2000). Although grains had been cooked before, it was around then that people actually made leavened bread—bread that has yeast in it, causing it to rise. Compare a tortilla to a loaf of bread and you can easily see the difference.
Three thousand years after the invention of bread, the Egyptians really made heated things up by inventing ovens to bake bread (Belderok 2000). In Europe, wheat was not originally the preferred grain—barley was most popular, probably because of its use to beer making. In the Middle Ages, rye became very popular, and finally in the 1700s wheat was the basis of most breads.
The old school way of harvesting wheat:
Bread, like alcohol, became a staple to human diets and economies. Wheat and rye were traded in large quantities in Europe as the population grew quickly in the 1200s. So much of these grains went through Amsterdam that it became known as the "Granary of Europe" (Belderok 2000). In the late 1700s, the center of grain production for Europe shifted away from Amsterdam to Russia and the up-and-coming American economy. Wheat production grew very rapidly in the U.S. and Canada, supplying grain for an exponentially growing world population. Of course, farm and transportation technology were influenced by the need to grow and transport large quantities of wheat. So it might not be such a stretch to say our modern railroads and interstates owe their existence to alcohol fermentation.
Many people are interested in another application of fermentation: biofuels. Biofuels are a hot topic of research these days, as people become more and more aware of rising prices and shorter supplies of oil. Trying to get cars and trucks to run on plant material instead of oil is an interesting and challenging problem.
The product people talk about when it comes to biofuels is ethanol. Ethanol is currently used as an additive to gasoline, mostly in the U.S. and in Brazil. Gasoline bought in the US is usually 10% ethanol, but some cars and trucks are equipped to deal with higher ratios of ethanol to gasoline. Read more about the economics and politics of global ethanol production here.
What is ethanol? As mentioned earlier in this module, it comes from alcohol fermentation. Usually plant material such as corn, sugar cane, or potatoes are the sugar source, and bacteria do the fermentation work. They produce ethanol through alcohol fermentation, and that ethanol is used as fuel. Pretty cool, huh?
Of course nothing is as simple as it seems. A lot of controversy exists about biofuels, because it seems wasteful to use food crops for fuel when they could be feeding people. Since the cost of food is currently increasing, the economic benefit of replacing gasoline with ethanol can be hard to see. Also, food crops, particularly corn, take a lot of input, such as fertilizer, pesticides, and the cost of transporting them. It is unclear if the economic and environmental cost of the inputs is worth the output.
Using other plant products that are not as energy-intensive or potential food sources to produce ethanol might help this problem. This is where cellulosic ethanol comes in. Cellulose is a sugar found in the cell walls of plants, but it is harder to break down than other plant parts. Technological advances have made it possible to break down cellulose and use more plant products, such as wood chips and fast growing grasses. However, the cellulosic industry has struggled to get off the ground because of funding, the need for more research, and the lack of infrastructure to support it.
Another biofuel being researched is algae biodiesel. Biodiesel is not based on ethanol, but on actual oil harvested from algae. Since that is not as clearly related to cellular respiration, we won't spend time talking about it here.
Although researchers are looking for alternatives to oil, there is still a lot of oil being transported around the globe. With oil transportation comes the occasional oil spill. When examining the effects of oil spills in the ocean on marine life, scientists have found something really awesome: there are actually bacteria that eat oil!
Well "eat" may be a bit of an exaggeration, since bacteria do not have mouths or digestive systems, per se. These bacteria metabolize the compounds in oil because they can use other molecules as electron donors and acceptors. Crude oil is made up of many compounds, and different types of bacteria make the enzymes that can break down the different compounds in oil. Scientists are also studying bacteria that live in freshwater areas contaminated by oil. Identifying and growing bacteria from these sites may help clean up water polluted by oil.
There are lots of kinds of marine bacteria that naturally break down oil (Head et al. 2006). Currently, researchers are studying how these bacteria are related, how they interact with each other and their environment, and how we can use them for cleaning up pollution.
Many types of pollution, not just oil spills, have the potential to be cleaned up by bacteria. This is called bioremediation—basically, remedying a problem with biological solution.
Interestingly enough, not all the bacteria that might help clean up pollutants eat oils, plastics and old iPods naturally. They treat these objects as special foreign substances that might be toxic, and break down or transform them to be less harmful. Bacteria have special metabolic pathways that allow them to transform oil and other substances into different compounds.