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.
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.
The Calvin cycle, a crucial part of photosynthesis, is sometimes called the Calvin-Benson cycle, "light independent reactions," or the "dark reactions." ("Dark reactions" is misleadingthe Calvin cycle depends on light.)
In the Calvin cycle, carbon dioxide (CO2) molecules are fixed to sugar with the help of an enzyme called RuBisCO. RuBisCO is short for ribulose-1,5-biphosphate carboxylase/oxygenase. It is the most abundant protein on Earth.
suitable or good enough.
(singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.
having to do with water.
(adenosine triphosphate) chemical found in most living cells and used for energy.
story of a person's life, told by that person.
organism that can produce its own food and nutrients from chemicals in the atmosphere, usually through photosynthesis or chemosynthesis.
person who studies the properties and reactions of chemicals in living or once-living material.
series of reactions that take place during photosynthesis, where carbon dioxide and water from the atmosphere are converted into sugar.
type of sugar that is an important nutrient for most organisms.
chemical element with the symbol C, which forms the basis of all known life.
type of carbon with two extra neutrons, useful in dating geological and archaeological material. Also called radiocarbon.
greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.
method plants use to attach carbon dioxide from the atmosphere to a chemical (RuBP) in order to start the process of photosynthesis.
study of the atoms and molecules that make up different substances.
part of the cell in plants and other autotrophs that carries out the process of photosynthesis.
concentrated solar energy
process of using mirrors to focus a large area of sunlight into a smaller area.
gadget or device.
to change from one thing to another.
material produced by a farmer or farm, usually measured in weight per hectare.
energy associated with the changes between atomic particles (electrons).
set of physical phenomena associated with the presence and flow of electric charge.
capacity to do work.
small task or chore.
nutrient-rich chemical substance (natural or manmade) applied to soil to encourage plant growth.
material, usually of plant or animal origin, that living organisms use to obtain nutrients.
sweet type of sugar found in many fruits and honey.
(glyceraldehyde 3-phosphate) chemical produced during photosynthesis that is used to create sugars.
machine that converts one type of energy to another, such as mechanical energy to electricity.
process of altering the genes of an organism.
increase in the average temperature of the Earth's air and oceans.
"simple sugar" chemical produced by many plants during photosynthesis.
harvested seed of such grasses as wheat, oats, and rice.
gas in the atmosphere, such as carbon dioxide, methane, water vapor, and ozone, that absorbs solar heat reflected by the surface of the Earth, warming the atmosphere.
period in the year when crops and other plants grow rapidly.
organism that eats mainly plants and other producers.
(1911-1997) American biochemist.
smallest physical unit of a substance, consisting of two or more atoms linked together.
(nicotinamide adenine dinucleotide phosphate) chemical found in most living cells and used for energy.
National Medal of Science
honor given by the President of the United States to people "deserving of special recognition by reason of their outstanding contributions to knowledge in the physical, biological, mathematical, engineering, behavioral or social sciences."
one of five awards established by the Swedish businessman Alfred Nobel in 1901. Nobel Prizes are awarded in physics, chemistry, medicine, literature, and peace.
substance an organism needs for energy, growth, and life.
process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars.
able to convert solar radiation to electrical energy.
organism that produces its own food through photosynthesis and whose cells have walls.
introduction of harmful materials into the environment.
having unstable atomic nuclei and emitting subatomic particles and radiation.
second step in the Calvin cycle of photosynthesis, where energy reacts with chemicals to create the simple sugar G3P.
fourth and final step in the Calvin cycle of photosynthesis, where energy and sugar interact to form the molecule RuBP, allowing the cycle to start again.
resource that can replenish itself at a similar rate to its use by people.
all of a plant's roots.
(ribulose biphosphate) molecule that reacts with carbon dioxide in the first phase of the Calvin cycle of photosynthesis.
overflow of fluid from a farm or industrial factory.
material that conducts electricity, but more slowly than a true conductor.
top layer of the Earth's surface where plants can grow.
radiation from the sun.
group of cells that converts sunlight into electricity.
most familiar type of sugar, mostly extracted from sugar cane, sugar beets, and sorghum.
type of chemical compound that is sweet-tasting and in some form essential to life.