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Sec_4 - Carbohydrate Synthesis 4-1 Lec 11 Up to this point...

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4-1 C a r b o h y d r a t e S y n t h e s i s Up 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: (CH 2 O) 6 + 6 O 2 6 CO 2 + 6 H 2 O ' 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 CO 2 into carbohydrate via photosynthesis . 1 . P h o t o s y n t h e s i s - I n t r o d u c t i o n To reverse the above reaction such that CO 2 is reduced or converted into glucose, energy must be supplied and in photosynthetic cells light energy is used. 6 CO 2 + 6 H 2 O (CH 2 O) 6 + 6 O 2 ' 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 acids organic carbon O 2 respiration photosynthesis CO 2 energy released (ATP) energy used (h Q or light energy) Lec # 11 nucleotides
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4-2 Both 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 cases to highlight the commonalities. 1. Green plants and algae 6 CO 2 + 6 H 2 O (CH 2 O) 6 + 6 O 2 is the usual representation but if 6 H 2 O are added to both sides we get: 6 CO 2 + 12 H 2 O (CH 2 O) 6 + 6 O 2 + 6 H 2 O 2. Green sulfur bacteria 6 CO 2 + 12 H 2 S (CH 2 O) 6 + 12 S + 6 H 2 O 3. Purple non-sulfur bacteria 6 CO 2 + 12 CH 3 CH(OH)CH 3 (CH 2 O) 6 + 12 CH 3 COCH 3 + 6 H 2 O (isopropanol) (acetone) Comparison of the three overall reactions produces the generalized overall reaction: 6 CO 2 + 12 H 2 X (CH 2 O) 6 + 12 X + 6 H 2 O electron electron acceptor donor Reflection 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 CO 2 while the molecular oxygen (O 2 ) must have originated from the input electron donor (H 2 O). In short, there may be two stages to the process as follows: S t a g e 1 12 H 2 X 24 H + + 24 e - + 12 X S t a g e 2 24 H + + 24 e - + 6 CO 2 (CH 2 O) 6 + 6 H 2 O Two key experiments addressed and confirmed this idea. 1. The first experiment identified the electron acceptors that became reduced electron carriers ( Hill reagents after the scientist) generated when electrons are removed from H 2 X (clearly electrons don't float around loose in solution), and demonstrated that they were generated independent of CO 2 .
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4-3 Hill found that isolated chlorplasts were capable of generating molecular oxygen (O 2 ) in the absence of CO 2 if they were provided with an electron acceptor. A variety of electron acceptors were found to work in vitro and these became known as "Hill reagents".
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