Sweet Secret

The Calvin cycle is a process that plants and algae use to turn carbon dioxide from the air into sugar, the food autotrophs need to grow.

Every living thing on Earth depends on the Calvin cycle. Plants depend on the Calvin cycle for energy and food. Other organisms, including herbivores, like deer, depend on it indirectly. Herbivores depend on plants for food. Even organisms that eat other organisms, like tigers or sharks, depend on the Calvin cycle. Without it, they wouldn’t have the food, energy, and nutrients they need to survive.

For centuries, scientists knew that plants could turn carbon dioxide and water into sugar (carbohydrates) using light energy—a process called photosynthesis. However, they didn’t know exactly how this was accomplished.

Fifty years ago, biochemist Dr. Melvin Calvin figured out the photosynthetic process from his lab at the University of California at Berkeley, located in the United States. The Calvin cycle is named after Dr. Calvin.

In a wooden building on the Berkeley campus called the Old Radiation Lab, Calvin grew green algae. Green algae are aquatic organisms that use photosynthesis. Calvin placed the algae into a contraption he called “the lollipop.”

Calvin shone light on the lollipop and used a radioactive form of carbon called carbon-14 to trace the path that carbon took through the algae’s chloroplast, the part of the cell where photosynthesis occurs. By this method, he discovered the steps plants use to make sugar out of carbon dioxide.

Steps in the Calvin Cycle

The Calvin cycle has four main steps. Energy to fuel chemical reactions in this sugar-generating process is provided by ATP and NADPH, chemical compounds that contain the energy plants have captured from sunlight.

In step one, a carbon molecule from carbon dioxide is attached to a 5-carbon molecule called ribulose biphosphate (RuBP). The method of attaching a carbon dioxide molecule to a RuBP molecule is called carbon fixation. The 6-carbon molecule formed by carbon fixation immediately splits into two, 3-carbon molecules called 3-phosphoglycerate (3-PGA).

In step two, 3-PGA is converted into glyceraldehyde-3-phosphate (G3P), a chemical used to make glucose and other sugars. Creating G3P is the ultimate objective of the Calvin cycle.

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, which will continue in the cycle back to step one to capture more carbon from carbon dioxide.

Nobel Prize Winner 

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 the way in which plants turn carbon dioxide into sugar wasn't a straightforward one. Instead, it worked in a circular pattern.

For discovering how plants turn carbon dioxide into sugar, Calvin was awarded the Nobel Prize for chemistry in 1961. Time magazine nicknamed him “Mr. Photosynthesis.”

Calvin received the National Medal of Science from President George H. W. Bush in 1989. He published his autobiography, Following the Trail of Light, in 1992. He died on January 8, 1997, in Berkeley, California. 

Understanding the Calvin Cycle 

Understanding how the Calvin cycle works is important to science in several ways.

“If you know how to make chemical or electrical energy out of solar energy the way plants do it—without going through a heat engine—that is certainly a trick,” Calvin once said. “And I’m sure we can do it. It’s just a question of how long it will take to solve the technical question.”

Melvin Calvin’s research into photosynthesis sparked the U.S. government’s interest in developing solar energy as a renewable resource.
Today, the U.S. Department of Energy researches the uses of photovoltaic cells, concentrated solar energy, and solar water heaters. Photovoltaic cells are made of semiconductors that convert sunlight into electricity. Photovoltaic cells are often grouped together to form large solar panels. Solar panels can help provide electrical energy for homes and businesses.

Concentrated solar power focuses the sun’s heat to run generators that produce electricity. Solar water heaters provide hot water and space heating for homes and businesses.

Scientists are also developing ways to increase carbon fixation, the first step in the Calvin cycle. They are doing so mostly by genetic modification.

Increasing carbon fixation removes excess greenhouse gases—mostly carbon—from the atmosphere. Greenhouse gases contribute to global warming.

Understanding photosynthesis could also increase the crop yields for many plants.

“Our understanding of photosynthesis, and the factors that increase it, such as the length of a growing season and adequate plant access to water in the soil, guides our development of perennial versions of grain crops,” says Jerry Glover of the Land Institute in Salina, Kansas, U.S.

Perennial plants come back year after year, while annual plants last only one growing season. Glover’s research shows that perennial grains are more environmentally friendly than annual grain crops. They use less water and fertilizer, and their deeper root systems mean they hold onto the soil better. This leads to less runoff and, therefore, less pollution into lakes and streams.