Sweet Secret

Sweet Secret

What does a plant leaf have to do with the solar energy panels on the White House? Find out how Melvin Calvin’s Nobel Prize–winning photosynthesis research is helping cool the planet, in this article from National Geographic Education.


3 - 12+


Biology, Chemistry, Social Studies, World History

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In order to grow, plants and algae use a special process called the Calvin cycle. They grab carbon dioxide (CO2) from the air and turn it into sugar. Carbon dioxide is a gas that human beings breathe out into the air. It is made of carbon (C) and oxygen (O).

Every living being on Earth depends on the Calvin cycle. Plants depend on the Calvin cycle for energy and food. Other organisms, including herbivores, like deer, eat plants to survive. Carnivores, like tigers (Panthera tigris) or sharks, eat other animals. Without the Calvin cycle, none of these organisms would have the food they need.

For hundreds of years, scientists knew about a process called photosynthesis. In this process, plants use sunlight to create energy and sugar. Even with this information, scientists did not know exactly how the process worked.

Fifty years ago, scientist Dr. Melvin Calvin found the answer at the University of California at Berkeley, located in the United States.

Steps in the Calvin Cycle
Photosynthesis is made of two parts. In the first part, plants use sunlight to create and store energy. In the second part, plants use this energy to power the Calvin cycle, which makes glucose, or sugar.

The Calvin cycle has four main steps. They take place in the chloroplast, which is a tiny structure inside the cells of a plant. Cells are the tiny building blocks that make up a living organism.

For the cycle to happen, the plant first needs energy to fuel its chemical reactions. It gets this energy from ATP and NADPH. These are two chemicals that contain the energy captured from sunlight.

In step one, a carbon molecule from carbon dioxide attaches to a 5-carbon molecule called ribulose biphosphate (RuBP).The new molecule now has 6-carbon molecules. It then splits into two 3-carbon molecules, called 3-phosphoglycerate (3-PGA).

In step two, 3-PGA is changed into glyceraldehyde-3-phosphate (G3P).

In step three, some of the G3P molecules are used to create sugar. Glucose, the type of sugar produced by photosynthesis, is composed of two G3P molecules.

In step four, the G3P molecules that remain combine through a complex series of reactions into the 5-carbon molecule RuBP. This cycle will return back to step one to capture more carbon from carbon dioxide.

Nobel Prize Winner
Melvin Calvin published "The Path of Carbon in Photosynthesis" in 1957. The key to understanding what was going on in the chloroplast came to him one day while "waiting in my car while my wife was on an errand," he said. Calvin realized that the process wasn't simple. Instead, it worked in a cycle.

Calvin was awarded the Nobel Prize for chemistry in 1961. Time magazine nicknamed him "Mr. Photosynthesis."

Understanding the Calvin Cycle
Calvin's discovery led the U.S. government to develop solar energy as a renewable resource. Today, the U.S. government is researching photovoltaic cells, concentrated solar energy and solar water heaters.

Photovoltaic cells are machines that convert sunlight into electricity. These cells are often grouped together to form large solar panels. Solar panels can help provide electrical energy for homes and businesses.

Concentrated solar power takes sunlight and points it into a small area. The sun's heat is then used to make electricity.

Solar water heaters provide hot water and heating for homes and businesses.

Scientists are also finding ways to increase carbon fixation, the first step in the Calvin cycle. Increasing carbon fixation puts a limit on greenhouse gases in the air. These gases, mostly carbon, increase global warming.

Fast Fact

Dark Chemistry
The Calvin cycle, a crucial part of photosynthesis, is sometimes called the Calvin-Benson cycle, "light independent reactions," or the "dark reactions." (However, the term "dark reactions" is misleading because the Calvin cycle depends on light.)

Fast Fact

Ose No! Ose Yes!
Sugars are identified by the ose at the end of their names. Glucose is the most abundant sugar produced in photosynthesis. Other sugars include sucrose and fructose.

Fast Fact

In the Calvin cycle, carbon dioxide molecules are fixed to sugar with the help of an enzyme called RuBisCO. RuBisCO is short for ribulose-1,5-biphosphate carboxylase/ oxygenase.

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Mary Schons
Jeannie Evers, Emdash Editing, Emdash Editing
Kara West
National Geographic Society
Last Updated

October 31, 2023

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