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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Plants need energy to live. They use a process called photosynthesis. There are two parts in photosynthesis. In the first part of photosynthesis, plants use sunlight to make energy. The second part is called the Calvin cycle.

In the Calvin cycle, plants grab carbon dioxide (CO2) from the air. CO2 is a type of gas. Human beings breathe it out, while plants breathe it in. After taking in CO2, the plant transforms it. To do this, it uses the energy from the sun. In the end, the plant creates sugar. It uses the sugar as food.

Plants need the Calvin cycle to survive. Many animals, like deer, need to eat plants. Other animals, like wolves, eat other animals. In a way, life exists thanks to the Calvin cycle.

For hundreds of years, scientists knew about photosynthesis. They did not know exactly how it worked, though. Dr. Melvin Calvin was the scientist who figured it out. Because of his discovery, Calvin won the Nobel Prize for chemistry in 1961. It is the highest prize in science.

Steps in the Calvin Cycle
To create sugar, the plant needs to create chemical reactions. It needs energy to start these reactions. Plants get this energy from two chemicals. The chemicals are called ATP and NADPH. These chemicals store energy from sunlight. The energy is used to power the Calvin cycle.

The Calvin cycle has four main steps. In step one, a plant takes CO2 from the air. It takes a carbon molecule from the CO2.

Molecules are tiny parts of a chemical. In a chemical reaction, molecules join or split to form new molecules. Carbon dioxide is one example. It is made of carbon and oxygen molecules.

In the Calvin cycle, the carbon from CO2 joins another molecule. This other molecule is called ribulose biphosphate (RuBP). The new molecule then splits apart. It splits into two molecules called 3-phosphoglycerate (3-PGA).

In step two, the plant changes the 3-PGA into another chemical. The new chemical is called G3P. It is also called glyceraldehyde-3-phosphate.

In step three, some of the G3P molecules are used to make glucose, or sugar.

In step four, the G3P molecules that are left join together. They form RuBP again. The cycle goes back to step one and starts over.

Understanding the Calvin Cycle
The Calvin cycle has inspired new inventions. It helped people learn how to use the sun to make solar power.

One invention that creates solar power is called a photovoltaic cell. This is a machine that uses sunlight to make electricity. Photovoltaic cells are often grouped together to form large solar panels. Solar panels create electrical energy for buildings.

Understanding photosynthesis can help us in other ways too. Right now, Earth is heating up. Scientists say this is because of the fuel people use. Burning fuel like gasoline creates greenhouse gases. These gases trap the heat in the air, causing global warming. CO2 is a greenhouse gas.

Scientists are studying ways to limit the amount of CO2. For example, plants could grab more CO2 from the air. This could slow 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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