Sec_4 - 4-1CarbohydrateSynthesisUp to this point in the...

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Unformatted text preview: 4-1CarbohydrateSynthesisUp to this point in the course, the main focus has been the breakdown of metabolites, including carbohydrates, lipids and amino acids. The primary purpose of these pathways is to extract energy in useable form with the common end product being ATP, the "energy currency" of the cell.In the case of glucose, the break down can be expressed by:(CH2O)6+ 6 O26 CO2+ 6 H2O'G'o= -2868 kJ/mol (energy released)Obviously, the material to be degraded must have originated somewhere and the starting point for all organic carbon is the fixation of CO2into carbohydrate via photosynthesis.1.Photosynthesis-IntroductionTo reverse the above reaction such that CO2is reduced or converted into glucose, energy must besupplied and in photosynthetic cells light energy is used. 6 CO2+ 6 H2O (CH2O)6+ 6 O2'G'o= +2868 kJ/mol (energy used)In this one process, not only is reduced carbon (ie. carbohydrate) produced but molecular oxygen, required for respiration, is produced. When the two processes of photosynthesis and respiration are combined, the carbon cycle is generated.carbohydrates lipids amino acidsorganic carbonO2respiration photosynthesisCO2energy released(ATP)energy used(hQor light energy)Lec # 11nucleotides4-2Both procaryotes and eucaryotes are capable of carrying out photosynthesis. While the overall reactions are similar, there are differences apparent and the following section compares three casesto highlight the commonalities.1. Green plants and algae6 CO2+ 6 H2O (CH2O)6+ 6 O2is the usual representation but if 6 H2O are added to both sides we get:6 CO2+ 12 H2O (CH2O)6+ 6 O2+ 6 H2O2. Green sulfur bacteria6 CO2+ 12 H2S (CH2O)6+ 12 S + 6 H2O3. Purple non-sulfur bacteria6 CO2+ 12 CH3CH(OH)CH3(CH2O)6+ 12 CH3COCH3+ 6 H2O(isopropanol) (acetone)Comparison of the three overall reactions produces the generalized overall reaction:6 CO2+ 12 H2X (CH2O)6+ 12 X + 6 H2Oelectron electronacceptordonorReflection on this generalized reaction in relation to the most common reaction from plants and algae leads to the realization that the oxygen atoms in the product waters must have originated in the input CO2while the molecular oxygen (O2) must have originated from the input electron donor (H2O). In short, there may be two stages to the process as follows:Stage112 H2X 24 H++ 24 e-+ 12 X Stage224 H++ 24 e-+ 6 CO2(CH2O)6+ 6 H2O Two key experiments addressed and confirmed this idea.1. The first experiment identified the electron acceptorsthat became reduced electron carriers (Hill reagentsafter the scientist) generated when electrons are removed from H2X (clearly electrons don't float around loose in solution), and demonstrated that they were generated independent of CO2.4-3Hill found that isolated chlorplasts were capable of generating molecular oxygen (O2) in the absenceof CO2if they were provided with an electron acceptor. A variety of electron acceptors were found to work in vitroand these became known as "Hill reagents"....
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This note was uploaded on 02/27/2011 for the course MBIO 2370 taught by Professor Spearman during the Winter '11 term at Manitoba.

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Sec_4 - 4-1CarbohydrateSynthesisUp to this point in the...

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