The Calvin Cycle Basics

Dr. Melvin Calvin & Others

Dr. Calvin utilized carbon-14 isotopes as radioactive tracers to reveal the chemical processes of photosynthesis. The Calvin Cycle illustrates how plants turn carbon dioxide and water into sugar. The Calvin Cycle is a metabolic pathway found in the stroma of the chloroplast in which inorganic carbon is fixed into an organic form.

Dr. Calvin was also named “Mr. Photosynthesis” in 1961 by Time Magazine. “…Dr. Calvin has long since earned the title: "Mr. Photosynthesis." Shortly after World War II, he began to use radioactive tracers, particularly carbon-14, and other recently developed tools to find out what happens to carbon dioxide when it tangles with chlorophyll in a living green plant cell. Step by painful step, Calvin and his large group of helpers followed CO2, tagged with carbon-14, through the intricate photosynthetic processes that start when green leaves are exposed to sunlight.”

Green algae (chlorella), grown in continuous cultures, were placed in the "lollipop" with the light shining on them. Carbon-14 labeled CO2 was injected into the stream of nonradioactive CO2 for a suitable period, at the end of which the algae were killed. The compounds into which the radioactive carbon had entered were analyzed by paper chromatography.

“But the greatest insight came to Chemist Calvin one day while he was in his car waiting at a traffic light. After that, he and his group were finally able to prove that sugar, the finished product of the process, is built up in six stages, each of which adds a single carbon atom. Now, thanks to Calvin, the chemical action of chlorophyll, on which all life on earth ultimately depends, is fairly well understood…” Time Nov. 10, 1961

Dr. Calvin & Dr. Benson were awarded the Nobel Prize for Chemistry, in 1961, due to their discovery of the cycle.

The Calvin cycle fixes carbon dioxide and the adds energy and hydrogen ions to the resulting molecules to yield a three carbon phosphate sugar called PGAL or G3P. The fixed (organic) carbon of PGAL is used to produce the variety of organic compounds of living organisms. It is used to keep the cycle going, and is used to produce carbohydrates such as glucose.

Here’s What Happens… This is called carboxylation. Carbon is fixed here!

Rubisco

twelve 3-C PGALs

from #3 6) Two 3C PGALs form fructose & then glucose.

PGAL (G3P) and other sugar phosphates from the Calvin cycle are food for plants. PGAL molecules can be used to supply energy and “carbon skeletons” for metabolism in the plant.

Some sugar phosphates are made into lipids, amino acids then protein Some sugar phosphates are made into lipids, amino acids then protein. This can occur in the chloroplast.

Humans and other animals consume plants and thus use the material from photosynthesis for building protein, as well as a source of sugars and energy.

BSCS Biology