Summary of Lecture 6 by rrboy


									Summary of Lecture 9
Dr. Thomas Gianfagna One of the earliest known organisms on Earth are the cyanobacteria. They are ancient. There are fossils of cyanobacteria dating back 2.5 billion years ago in mounds known as stromatolites in shallow, clear, tropical waters. At that time lawns of cyanobacteria likely covered every moist sun-lit surface of the planet. Today, modern cyanobacteria still exist in ponds and shallow ocean waters, but animals feed on them limiting their growth. The cyanobacteria are photosynthetic. This means that they produce food in the form of carbohydrate (C6 H12O6) from carbon dioxide (CO2) and water (H2O) in the presence of light energy. Oxygen (O2) is produced as a by-product. This is the equation for photosynthesis:

CO2 + H2O  C6 H12O6 + O2
one of the most important chemical reactions for life on Earth. Cyanobacteria were significant players in the evolution of life as we know it because they produced the Earth’s oxygen. In fact, all of the oxygen that we breathe today is a byproduct of photosynthesis from cyanobacteria, algae and plants. What was the early Earth like? The age of the Earth is about 4.6 billion years. The atmosphere of the young Earth was very different from today. The Earth’s volcanic core was very active, spewing huge amounts of carbon dioxide (CO2), carbon monoxide (CO), hydrogen sulfide (H2S), methane (CH4) and ammonia (NH3) into the air and oceans. Ultraviolet radiation (UV) was intense, and there likely were frequent and severe lightning storms. There was no oxygen. The oldest known fossils date back to 3.5 billion years. If we were to use a time line in which the 4.6 billion year history of the Earth was represented by 1 year, the oldest fossils appear in mid-March, the dinosaur age around Christmas time, and the emergence of early man (various hominid species), at 2 minutes before midnight on December 31. Were the cyanobacteria the earliest forms of life? Probably not. Photosynthesis is a complicated process and early life forms likely derived their energy from the chemical oxidation of hydrogen sulfide (H2S), which was abundantly produced by volcanic activity. How did life originate? In the 1950s Stanley Miller and other scientists constructed an apparatus designed to simulate the early Earth’s atmosphere. They allowed the reactions of carbon dioxide, carbon monoxide, hydrogen sulfide, methane, water and ammonia to proceed with UV light and a spark chamber to simulate a lightning strike. They were amazed to find that most of the chemicals that we associate with living organisms: sugars, amino acids, fatty acids and nitrogenous bases were produced in considerable abundance!


Life, of course, is more than a soup of chemicals. A life form must consist of a separate entity that is able to process compounds to make or extract energy (otherwise known as metabolism), and replicate itself. Fatty acids will spontaneously form spheres like soap bubbles that are stable and resemble membranes. Some processes like the formation of peptides or simple nucleic acids may have occurred within these spheres and a method of producing copies of each molecule may have developed. The conditions of the early Earth, either deep alongside hydrothermal vents, or near the surface of warm tropical waters may have been conducive to the generation of life forms similar to bacteria. Bacteria persist today and are found everywhere on Earth. Some species of bacteria can survive in extremely hostile environments, such as salt ponds, hydrothermal vents beneath the ocean, in hot geyser pools at temperatures over 80C (175F), and in the coldest places on the planet. We call such organisms extremophiles. There are two groups of bacteria, the eubacteria and the archeabacteria. Many of the extremophiles are members of the archeabacteria. Based upon analyses of the biochemistry of bacterial cells, biologists believe that the archeabacteria split away from the eubacteria, and then the eukaryotic organisms diverged from the archeabacteria. We often think of bacteria as harmful organisms to man. Some species do cause human diseases such as cholera, tuberculosis, bubonic plague etc., others spoil our food and some kill our crops. However, most bacteria are either indifferent to man or actually are critically important for life on Earth as we know it. Bacteria are important in decomposing organic matter and improving the soil. They fix atmospheric nitrogen (N2) in a form that can be used by plants (NH3). They produce vitamins in our intestines and inhibit fungal growth. Bacteria are the original source of antibiotics, and some eat oil and can clean up our environment, or breakdown sewage sludge. In fact, human life would be impossible without bacteria! In the 1940s a soil microbiologist at RU named Selman Waksman, who worked in the basement of Martin Hall, was studying a group of bacteria called the actinomycetes. He noticed that when actinomycetes were cultured with other bacteria, the actinomycetes prevented their growth and eventually killed them. Waksman discovered that the actinomycetes produced anti-bacterial compounds that controlled the growth of some types of bacteria. Two of these compounds are streptomycin and actinomycin, common antibiotics that are used today to kill bacterial diseases in humans. These compounds work by preventing the bacteria from synthesizing its cell wall during reproduction. The bacterial cell wall is made out of a substance called peptidoglycan. Most of our known antibiotics work on this principle. Of course, actinomycetes don’t make antibiotics to help us; they make them to prevent other bacteria from growing on their part of the soil. Antibiotics were so effective in controlling bacterial diseases that it was once believed that these organisms would no longer remain a threat to human health. Doctors prescribed


antibiotics for any ailment, even when the likelihood of a bacterial infection was low. Farmers gave their animals antibiotics as part of their feed even when the animals were completely healthy. As a result, the bacteria evolved and the ones that survived were antibiotic resistant. This is a serious problem today, and doctors are much more careful prescribing antibiotics to slow the development resistant strains. In addition to harmful bacteria, there are bacteria that can protect their host and prevent disease. These organisms are known as probiotics. The best known probiotic bacterium is Lactobacillus acidophilus. It is one of the organisms that are used to make yogurt. L. acidophilus converts the lactose sugar that is present in milk to lactic acid. This is what gives plain yogurt its sour taste. Not all yogurts contain live bacterial cultures, so check the back of the container. Most products with live cultures contain a mixture of Lactobacillus species. Our intestinal tract needs lots of probiotic bacteria to function properly. This is why you may be told to eat yogurt when given a course of antibiotics to cure an illness. The antibiotics will kill both the bad and the good bacteria, but the yogurt will add back some of the good bacteria. There are additional claims that probiotic bacteria can regulate immune system function and reduce allergic reactions. More research is needed here to substantiate these claims. We can classify all organisms, but especially bacteria, by the way they obtain energy and food. The cyanobacteria are autotrophs, that is, they make their own food. They use light to produce the biochemical energy needed to drive photosynthesis, so we call them photoautotrophs. Green plants are also photoautotrophs. Some bacteria, including many in the extremophile category, are chemoautotrophs. They use chemical energy, such as the energy produced when hydrogen sulfide is oxidized, to produce the biochemical energy needed to make food. That’s why chemoautotrophs can grow in places such as the hydrothermal vents at the bottom of the ocean where there is no light. All animals are heterotrophs or specifically chemoheterotrophs. They obtain their food and energy by eating other organisms. Some bacteria, like E. coli, which lives in the human intestines, are also chemoheterotrophs. They don’t eat other organisms, but they absorb food that they find in their environment. A small number of bacteria are photoheterotrophs. They obtain their energy from light and use it to produce the biochemical energy needed to digest food that they have taken up from the environment.


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