Photosynthesis: The Calvin Cycle

The Calvin cycle is the second set of reactions in photosynthesis and occurs after the light-dependent reactions. Sometimes the Calvin cycle is called the Light-INDEPENDENT reactions or even the dark reactions because light is not directly needed to make the Calvin cycle happen. Nevertheless, two key products from the light reactions are needed to make the Calvin cycle work: ATP and NADPH.
You may recall that both ATP and NADPH were made during the light reactions at the thylakoids. The Calvin cycle takes place in the fluid surrounding the thylakoids, known as the STROMA.
ATP provides the energy, and NADPH provides the electrons and hydrogens that are needed to reduce carbon dioxide to build sugars in the Calvin cycle.
Here’s how it works:
Phase 1: Carbon Fixation
An enzyme called Rubisco takes CO2 from the air and adds it to a 5-carbon compound called RuBP. This is called carbon fixation. This makes a 6 carbon compound that immediately splits in half to make two molecules of 3-phosphoglycerate. Notice that we did carbon fixation for 3 molecules of CO2. You will see why soon!
 
Phase 2: Reduction
In the reduction phase, ATP from the light reactions provides energy to make an intermediate compound. This reactive intermediate can then be reduced, or gain electrons and hydrogen. The electrons and hydrogen come from NADPH and are used to reduce the intermediate compound to make Glyceraldehyde-3-phosphate, more commonly called G3P (or sometimes triose phosphate). G3P is the key product of the Calvin Cycle that can be used to make sugar! Notice that for every 3 molecules of CO2 brought into the Calvin cycle, there is a net gain of 1 G3P. 3 carbons in, 3 carbons out… makes sense, right?
Phase 3: Regeneration
Notice in our model that only one G3P is removed from the cycle, but 5 more remain. These 5 G3P molecules get converted back to our starting compound, RuBP. Remember, the Calvin cycle is a cycle so we need to make our starting compound again to keep the cycle going. Now, I’m going to give you a moment to think… If we have 5 G3P molecules and each one has 3 carbons, how many carbon atoms do we have? Now that you have the answer, how many 5-carbon RuBP molecules can we make using those carbon atoms? I’m guessing that you’ve figured it out… we can make 3 RuBP molecules. To do this, energy is required. Can you guess where that energy comes from? If you said ATP, you are right. Let’s watch.
Now that we’ve completed the Calvin cycle for 3 carbon atoms, let’s go through the process again so that we can make glucose! Remember, our overall photosynthesis equation has 6 carbon dioxides being used to make 1 glucose molecule with 6 carbons. With this in mind, how many G3Ps are needed to make one glucose? If you said 2, you are correct!
Now you have seen how the Calvin cycle is used to make glucose and how the light-dependent reactions provide the NADPH and ATP that were needed to make the Calvin cycle work!
The glucose and other organic compounds made as a result of these processes in photosynthesis are what allow plants and algae to grow and support practically all life on earth, including you!
To become an expert on the Calvin Cycle by doing it yourself, play Photosynthesis Interactive at BioMan Biology!

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