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| Steps Involved in Photosynthesis |
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| Photosynthesis has two main reactions: |
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 Light reaction |
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Dark reaction |
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| It is so called as it takes place only in the presence of sunlight and hence is a light-dependent reaction. Robin Hill, an English biochemist conducted studies on isolated chloroplasts to understand the mechanism of light reactions. Hence, it is also called Hill's reaction. |
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| During this stage, the light energy from the sun is absorbed by the photosynthetic pigments and is used to break down the water molecules. This is called the photolysis ('photo' is light and 'lysis' refers to break down) of water. |
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| An Overview of Photosynthesis |
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| This releases energy, energy is released at the same time setting free hydrogen and oxygen. The energy is temporarily stored in two types of molecules NADP (Nicotinamide Adenine Dinucleotide Phosphate) and ATP (Adenosine triphosphate). |
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| NADPH2 is formed from NADP which accepts the hydrogens formed during the photolysis. |
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| The formation of ATP from ADP and inorganic phosphate (iP) requires energy. |
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| The NADPH2 and the ATP are used in the next stage of photosynthesis. |
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| F.F. Blackman (1905) demonstrated the presence of the dark reactions for the first time. It utilises the chemical energy temporarily stored in the products of light reaction. It is also called the Calvin cycle. It is a series of reactions taking place in a cyclic manner. It can be represented as follows: |
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| The carbon dioxide is accepted by a carbohydrate called the ribulose 1, 5 diphosphate(RuDP). It has 5 carbon atoms. Each RuDP accepts one carbon dioxide molecule. Thus 6 carbon dioxide molecules are accepted by 6 RuDP molecules. |
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| Observe the following chart: |
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| 1 RuDP = 5 carbon atoms |
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| thus, 6 RuDP = 6 x 5 = 30 carbon atoms. |
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| Further, 6 CO2 = 6 carbon atoms. |
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| This means that a there are totally 36 reactant carbon atoms at this stage. |
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| Calvin Cycle |
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| Going back to the Calvin cycle, RuDP on accepting carbon dioxide form 12 molecules of phosphoglyceric acid (PGA). PGA is a 3 carbon compound. |
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| Thus, |
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| 1 PGA = 3 carbon atoms |
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| and 12PGA = 12 x 3 = 36 carbon atoms. |
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| So, all the reactant carbon atoms are accounted for. |
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| Referring to the Calvin cycle, the PGAs are now converted into 12 molecules of phosphoglyceraldehyde. It is at this step that the hydrogens released during the light reaction are used up. |
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| Phosphoglyceraldehyde is also a 3 carbon compound. This step also regenerates the 12 water molecules used in the first step. |
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| Of the 12 phosphoglyceradehyde molecules formed, 2 of them (that is, 6 carbon atoms) go to form other products and the other 10 (which together contain 10 x 3 = 30 carbon atoms) are made to form 6 molecules of RuDP (6 x 5 carbon atoms of each RuDP) again. Thus all the RuDPs are regenerated. |
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| The net product is thus a molecule of hexose sugar like fructose and glucose. These can be used to form other molecules like the glucose (C6H12O6). In such case the equation for photosynthesis becomes as follows: |
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