• Sweet Secret
    The Calvin cycle goes round and round.

    Illustration by Tim Gunther

    RuBisCO
    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.

    Dark Chemistry
    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.)

    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.

    By Mary Schons

    Friday, January 21, 2011

    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 such as deer, depend on it indirectly. Herbivores depend on plants for food. Even organisms that eat other organisms, such as 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. 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 which 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

    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 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, Melvin 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.

    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 less pollution into lakes and streams.

  • Term Part of Speech Definition Encyclopedic Entry
    adequate Adjective

    suitable or good enough.

    algae Plural Noun

    (singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

    annual Adjective

    yearly.

    aquatic Adjective

    having to do with water.

    ATP Noun

    (adenosine triphosphate) chemical found in most living cells and used for energy.

    autobiography Noun

    story of a person's life, told by that person.

    autotroph Noun

    organism that can produce its own food and nutrients from chemicals in the atmosphere, usually through photosynthesis or chemosynthesis.

    Encyclopedic Entry: autotroph
    biochemist Noun

    person who studies the properties and reactions of chemicals in living or once-living material.

    Calvin cycle Noun

    series of reactions that take place during photosynthesis, where carbon dioxide and water from the atmosphere are converted into sugar.

    carbohydrate Noun

    type of sugar that is an important nutrient for most organisms.

    carbon Noun

    chemical element with the symbol C, which forms the basis of all known life.

    carbon-14 Noun

    type of carbon with two extra neutrons, useful in dating geological and archaeological material. Also called radiocarbon.

    carbon dioxide Noun

    greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.

    carbon fixation Noun

    method plants use to attach carbon dioxide from the atmosphere to a chemical (RuBP) in order to start the process of photosynthesis.

    chemistry Noun

    study of the atoms and molecules that make up different substances.

    chloroplast Noun

    part of the cell in plants and other autotrophs that carries out the process of photosynthesis.

    concentrated solar energy Noun

    process of using mirrors to focus a large area of sunlight into a smaller area.

    contraption Noun

    gadget or device.

    convert Verb

    to change from one thing to another.

    crop yield Noun

    material produced by a farmer or farm, usually measured in weight per hectare.

    electrical energy Noun

    energy associated with the changes between atomic particles (electrons).

    electricity Noun

    set of physical phenomena associated with the presence and flow of electric charge.

    energy Noun

    capacity to do work.

    errand Noun

    small task or chore.

    fertilizer Noun

    nutrient-rich chemical substance (natural or manmade) applied to soil to encourage plant growth.

    food Noun

    material, usually of plant or animal origin, that living organisms use to obtain nutrients.

    Encyclopedic Entry: food
    fructose Noun

    sweet type of sugar found in many fruits and honey.

    G3P Noun

    (glyceraldehyde 3-phosphate) chemical produced during photosynthesis that is used to create sugars.

    generator Noun

    machine that converts one type of energy to another, such as mechanical energy to electricity.

    genetic modification Noun

    process of altering the genes of an organism.

    global warming Noun

    increase in the average temperature of the Earth's air and oceans.

    Encyclopedic Entry: global warming
    glucose Noun

    "simple sugar" chemical produced by many plants during photosynthesis.

    grain Noun

    harvested seed of such grasses as wheat, oats, and rice.

    Encyclopedic Entry: grain
    greenhouse gas Noun

    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.

    growing season Noun

    period in the year when crops and other plants grow rapidly.

    herbivore Noun

    organism that eats mainly plants.

    Encyclopedic Entry: herbivore
    Melvin Calvin Noun

    (1911-1997) American biochemist.

    molecule Noun

    smallest physical unit of a substance, consisting of two or more atoms linked together.

    NADPH Noun

    (nicotinamide adenine dinucleotide phosphate) chemical found in most living cells and used for energy.

    National Medal of Science Noun

    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."

    Nobel Prize Noun

    one of five awards established by the Swedish businessman Alfred Nobel in 1901. Nobel Prizes are awarded in physics, chemistry, medicine, literature, and peace.

    nutrient Noun

    substance an organism needs for energy, growth, and life.

    Encyclopedic Entry: nutrient
    perennial Adjective

    happening on a yearly basis.

    photosynthesis Noun

    process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars.

    photovoltaic Adjective

    able to convert solar radiation to electrical energy.

    plant Noun

    organism that produces its own food through photosynthesis and whose cells have walls.

    pollution Noun

    introduction of harmful materials into the environment.

    Encyclopedic Entry: pollution
    radioactive Adjective

    having unstable atomic nuclei and emitting subatomic particles and radiation.

    reduction phase Noun

    second step in the Calvin cycle of photosynthesis, where energy reacts with chemicals to create the simple sugar G3P.

    regeneration phase Noun

    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.

    renewable resource Noun

    resource that can replenish itself at a similar rate to its use by people.

    root system Noun

    all of a plant's roots.

    RuBP Noun

    (ribulose biphosphate) molecule that reacts with carbon dioxide in the first phase of the Calvin cycle of photosynthesis.

    runoff Noun

    overflow of fluid from a farm or industrial factory.

    Encyclopedic Entry: runoff
    semiconductor Noun

    material that conducts electricity, but more slowly than a true conductor.

    soil Noun

    top layer of the Earth's surface where plants can grow.

    solar energy Noun

    radiation from the sun.

    Encyclopedic Entry: solar energy
    solar panel Noun

    group of cells that converts sunlight into electricity.

    sucrose Noun

    most familiar type of sugar, mostly extracted from sugar cane, sugar beets, and sorghum.

    sugar Noun

    type of chemical compound that is sweet-tasting and in some form essential to life.

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