The light-dependent reactions of photosynthesis produce ATP and NADPH, which are then used in glucose synthesis during the Calvin cycle. As you should know from studying the Krebs cycle, metabolic cycles involve inputs and outputs, and some molecules are recycled to complete the cycle.
In the case of the Calvin Cycle, the input molecules are carbon dioxide, ATP, and NADPH. The output molecules are sugar, ADP, NADP+, and inorganic phosphate (Pi). The recycled molecule is ribulose bisphosphate (RuBP ). Examine the figure below and locate these molecules in the cycle.
Scientific knowledge is gained through observations and controlled experiments, but how does one study a process that can't be seen directly?
About 50 years ago, Melvin Calvin tried to do just that. He was working in a laboratory at the University of California, Berkeley during WWII. The study of radioactive elements had become an important new field in chemistry during the war. Among these newly discovered radioactive elements was carbon 14 . On the day in 1945 that the Japanese surrendered, a friend and colleague told Calvin, "Now is the time to do something useful with radioactive carbon." Calvin turned his focus to the study of photosynthesis, and 16 years later he won a Nobel Prize in chemistry and a metabolic process was named after him.
Calvin knew that photosynthesis could only occur in living organisms. Thus, the study of the chemical process was a difficult one. He devised a method whereby he could raise algae (Chlorella pyrenoidosa ) in a lollipop-shaped disk. He set up a stream of air that could be controlled. He could inject radioactive carbon as carbon dioxide into the air stream for a set period of time. Then he would kill the algae with boiling methanol to stop the process of photosynthesis. He ran the experiment multiple times, each time killing the algae at different lengths of time after injecting carbon 14 .
Calvin analyzed the dead algae to determine which molecules had incorporated the carbon 14 . The technique he used is called chromatography. By comparing the molecules that contained carbon 14 after each time period, he found a sequence of compounds that revealed the path of carbon dioxide as it was turned into glucose (you can see an example of the data in the figure). In the years that followed, other researchers have discovered the enzymes and other compounds that also function in the Calvin cycle.