Calvin cycle: the light independent reactions (carbon fixation)

So far we have seen how light energy has been converted to chemical energy and stored in the high-energy compounds, ATP and NADPH.  But we have not made any food for the plant.  That happens in the next process, the Calvin cycle (named for its discoverer).  The Calvin cycle does not depend upon light and can occur in either light or dark conditions, unlike photophosphorylation.  Here carbon dioxide from the air is used, "fixed", to make organic molecules for the plant, hence the name "carbon fixation".  Remember that carbon dioxide is very low in energy and glucose is very high in stored energy.  Where do you suppose the energy to raise carbon dioxide up comes from?  You guessed it!  The energy stored in ATP and NADPH in the light dependent reactions of photophosphorylation will now be used to power the Calvin cycle.

I have constructed a simplified series of animations of the Calvin cycle to show you how it works.  Only the carbon atoms are shown and I have not identified any of the intermediate reactions involved.   Carbon fixation is much more complicated than what is shown here.   Nevertheless the most important results of the cycle are illustrated.  As you view the animations note the following: 


1.  A carbon atom from carbon dioxide enters the cycle and joins with a five carbon molecule that is present.


2 .The six carbon molecule that results, breaks up into two molecules, each with three carbon atoms.

3.  As the reactions in the cycle continue, ATP is dephosphorylated (loses a phosphate) to ADP.  The energy that was released is used to raise the energy of the molecules reacting in the cycle.  (But remember that not all of the energy that is released is available, some is lost as heat.  Why?  Because of the second law of thermodynamics.)

4.  Further energy is supplied by the oxidation of NADPH to NADP.


5.  A carbon atom (actually connected to other atoms not shown here) breaks off and is available to be used to make G3P, a high energy molecule that has three carbons.  The bonds connecting this carbon atom to hydrogen and carbon are much higher in energy than the original C-O bonds of carbon dioxide.  This carbon is left behind as the cycle continues.  G3P has three carbons, so it will require three turns of the cycle to provide the necessary carbons to make one molecule of G3P.  Since glucose has six carbons, it will take two G3Ps or six turns of the cycle to make one molecule of glucose.

6.  The five carbons that remain, combine and continue in the cycle.  Another ATP is dephosphorylated to ADP to provide the necessary energy.

7.  The cycle repeats continuously, each time making a carbon atom available for G3P.   When three cycles are completed, one G3P can be removed for making glucose and other organic molecules.

Study the animations above until you understand what is happening.  Now let's view the entire Calvin cycle in this abbreviated form. 

This final animation will continue for three cycles, after which a molecule of  G3P will be removed for making glucose and other organic molecules. For each G3P that is produced, 3 carbon dioxide molecules must enter the cycle.  To provide energy, 9 molecules of ATP and 6 molecules of NADPH are required. 

It might help you to understand all of this if we look at what goes in and what comes out of the Calvin cycle in tabular form.     


Three CO2 One G3P
Nine ATP Nine ADP

To make a molecule of glucose requires 6 turns of the cycle.   Eighteen molecules of ATP will be dephosphorylated and 12 molecules of NADPH will be oxidized.  Each molecule of NADPH can be considered to be roughly equivalent to 3 molecules of ATP in terms of energy.  So, it requires approximately the equivalent of 54 ATPs to make a molecule of glucose.  (3 x 12 NADPH = 36 ATP.  36 ATP + 18 ATP = 54ATP)

The final drawing represents a summary of the entire process of photosynthesis.  Notice that light and water go into the noncyclic photophosphorylation wheel.  Light provides the power to turn the wheel.  As the wheel turns it pushes the two smaller wheels.  ATP and NADPH leave the green wheel and enter the Calvin cycle where they provide the power to turn the Calvin cycle wheel.   Carbon dioxide enters and G3P comes out.  The ADP and NADP from the Calvin cycle return to the light dependent reactions.








Now for some sophisticated movies about photosynthesis go to the following website and investigate the plant cell there.  The movies are a bit slow, but you can open up another window of your browser and do other stuff while it is loading.  For example, if you use Netscape, just open it up again and have it running in two places at the same time.

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copyright June B. Steinberg, 2000