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Introduction Once the energy that was in sunlight is transformed into chemical energy, the organism has to now convert the chemical energy into a usable form. It may seem a bit odd for they’re still to be more steps. After all, when we eat a candy bar isn't the sugar in the candy bar "burnt" by the body to provide energy? Well the answer is yes and no. First of all when we burn something normally in the air we combine that substance with oxygen releasing energy from the substance. Indeed, an analogous process does happen in our bodies. The process is analogous in the sense that the food we eat is oxidized and energy released that we can use. By oxidation, we mean that electrons are lost by the molecules we take in from our food and given to other molecules. The electrons though aren't given up right away but instead the energy they contain is used to produce an important energy-carrying molecule: adenosine tri-phosphate, more commonly called ATP. Thus, what goes on in living things is not really like burning because the molecules from which we harvest energy give up their energy in a controlled fashion rather than all at once as what happens in a fire. If we think of our car, all the energy in the gas tank when we get in our car is not released all at once but rather in small bursts, which allow us to control the car's movement. In the same way cells take the energy from the "food" and package that energy into manageable bursts that provide just the right amount of energy for the organism's activities be those activities driving a car or flashing a light to attract a mate. We'll examine two main types of respiration: first aerobic respiration, which involves oxygen and then fermentation, and anaerobic respiration, which does not involve oxygen. Why ATP? What's so special about ATP that the cell spends so much time making it? There are several things: 1. When ATP releases energy, the energy release only involves breaking the last of three phosphate bonds in the molecule. This results in the production of a small controlled amount of energy that is just the right amount for most of the energy using processed of the cell. 2. Energy release since it involves breaking just the one phosphate bond means that ATP can easily be put together by taking ADP and adding phosphate to it. Thus the cell to make new ATP can recycle the products of energy release from ATP. If this sort of cycling between ADP+Phosphate and ATP were not possible our bodies would require huge amounts of ATP. Given below is the table for ATP requirements for a "simple" bacteria cell: Biosynthesis in E. coli modified from Ensign (1998) Cell constituent Number of molecules per cell Molecules synthesized per second Molecules of ATP required per second for synthesis DNA 1 0.00083 60,000 RNA 15,000 12.5 75,000 Polysaccharides 39,000 32.5 65,000 Lipids 15,000,000 12,500.0 87,000 Proteins 1,700,000 1,400.0 2,120,000 In the above table the number of DNA molecules per cell excludes plasmids.
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