Light Independent Reactions
The Calvin Benson cycle is (of-course) named after Melvin Calvin, the one who figured out the process. Calvin is a botanist at the University of California at Berkeley, he received a Nobel Prize for his part in determining these reactions. The Calvin Benson cycle goes in (you guessed it) cycles. For every one complete turn of the cycle fixes one CO2 molecule and regenerates an RuBP molecule. Also, every three turns of the cycle produce a PGAL molecule, and since we need two of PGAL molecules to make one simple sugar, such as glucose, the cycle has to run six times straight. Glucose is the real source of energy for most living organisms because it is the monomer (basic unit) for any types of carbohydrate.
Keep in mind that some of the products produced in light reaction go to the dark reaction, and some of the products of dark reaction go to the light reaction. (What a nice recycle and reuse system.) It shows a summarized inputs and outputs of the pathways in Fig. 8 (http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.html, 2001). Sugar created during dark reaction is used for energy storage.
Calvin Benson cycle consists of three subsequencing stages: CO2 fixation, CO2 reduction, and regeneration of ribulose bisphosphate (RuBP). Shown in Fig. 9 (http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.html, 2001), the upper chain of reactions is the dark reaction that occurs in the stroma, and the bottom is the light reaction at the thylakoid. The Calvin Benson cycle starts when RuBP, the five-carbon molecule combines with carbon dioxide. This creates a (recall: 5 + 1 = 6) six-carbon molecule, which then immediately breaks down to from two molecules of phosphoglycerate (PGA). The reason why the Calvin Benson cycle is also known as the C3 pathway is because PGA is a (recall: 6 ÷ 2 = 3) three-carbon molecule.
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Fig. 9 (click for a better view) |
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The cycle starts with an enzyme (a specific catalyst: used as to speed up a specific reaction) called RuBP carboxylase (or rubisco). Figure 10 (http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.html, 2001) shows how rubisco transforms RuBP on the structural side. Rubisco is responsible for the attachment of carbon dioxide to RuBP (or any organic compound), and this process is called the carbon dioxide fixation. Mader (1990) noted that rubisco makes up about 20% to 50% of the protein content in chloroplasts, and the reasoning for its abundance is that it works unusually slow and there has to be a lot of it in order to keep the Calvin Benson cycle moving. (Fascinating!)
The next step is the reduction of carbon dioxide. Each molecule of PGA can now reduces to phosphoglyceraldehyde (unless you have too much time on your hands, PGAL). This reduction happens in two steps: The reactions that reduce PGA to PGAL use up the NADPH (from the light reaction) and at the same time some of the ATP (wow, energy) are created in the thylakoid membrane. The reduction of carbon dioxide caused PGAL contains more hydrogen atoms than does PGA. Each PGA molecule is phosphorylated by ATP to PGAP. Then, each PGAP is reduced to PGAL by the conenzyme NADPH2, and inorganic phosphate is also released. It is calculated that there is a net gain of one PGAL molecule for every six turns of the Calvin Benson cycle. If you know something about respiration, you would be familiar of knowing that it takes two PGAL to from one sugar, glucose. (Which is the end product of this cycle.) Plants can now use the glucose made as the basic building blocks for sucrose, cellulose, starch, or to provide energy for ATP formation in mitochondria.
Finally, we need to get to where we started to complete this cycle by a process called regeneration of RuBP. "The Calvin Benson cycle goes round and round, round and round, round and round; the Calvin Benson cycle goes round and round, and we all live happily after..." (Base on the famous children folk music "the Wheels on the Bus", re-mastered by, me.) How can we get back to the starting point then? Well, for every six turns of the Calvin Benson cycle, five molecules of PGAL are used to reform three molecules of RuBP. This conversion requires a number of intermediate steps which (for the good purpose of not killing anymore of our brain cells) are not shown, but what we should know is that some of the ATP produced along the pathway of photosynthesis are utilized in this step. This cycle goes on and on (as you've guessed).
Now we have covered almost everything we have to know for photosynthesis, and what we will be looking next, is the discovery of photosynthesis.
