We know that matter has to be recycled because it is in a limited supply on earth.  How do we know where the original matter came from and how is it possible to follow the recycling processes of all of this matter?  We don’t know everything concerning these questions, but we do have evidence to support certain theories.

Photosynthesis

What this says is that 6 molecules of carbon dioxide combine with 12 molecules of water in the presence of sunlight energy to form one molecule of sugar plus 6 molecules of oxygen and 6 molecules of water. Scientists know this because they can actually measure these substances.

Scientists can actually follow the movement of oxygen through the photosynthetic process by using radioactive oxygen (labeled below as red).  Two experiments were conducted.  One experiment used water that contained the radioactive oxygen (shown in red below), and the other experiment used carbon dioxide that contained the radioactive oxygen (also shown in red).  The results of the experiment are shown below:

1:  6CO₂ + 12H₂O + Energy from Sunlight à C₆H₁₂O₆ + 6O₂ + 6H₂O 

2:  6CO₂ + 12H₂O + Energy from Sunlight à C₆H₁₂O₆ + 6O₂ + 6H₂O

"What has this got to do with cycles?" you might ask. Notice in step 1 that the oxygen in water gets released as oxygen gas. In step 2 the carbon in the carbon dioxide in air gets captured in sugar (glucose). Both steps capture the carbon in carbon dioxide into the sugar. In other words, both oxygen and carbon are conserved in sugar. The carbon in the sugar of the last candy bar you ate could have been in the air exhaled by a dinosaur. Can you make a drawing of the cycles for oxygen and carbon as it moves among the environment, plants, and animals?

Another major contribution to understanding photosynthesis came from Melvin Calvin (see Story time) who discovered the process that uses the energy obtained from the sunlight to turn the carbon in carbon dioxide into glucose.  This is known as the Calvin Cycle.  Calvin supplied green algae with radioactively-labeled carbon molecules, and traced the movement of the carbon over different time periods using an identification method known as chromatography.  The carbon gradually moved through different carbon compounds until it finally ended up in glucose.  Using the data obtained in this experiment, Calvin mapped the steps in photosynthesis. 

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