What do plants do with the high-energy sugar molecules they produce during the calvin cycle?

The Calvin cycle is the cycle of chemical reactions performed by plants to “fix” carbon from CO2 into three-carbon sugars.

Later, plants and animals can turn these three-carbon compounds into amino acids, nucleotides, and more complex sugars such as starches.

This process of “carbon fixation” is how most new organic matter is created. The sugars created in the Calvin cycle are also used by plants for long-term energy storage, unlike ATP which is used up quickly after it is created.

These plant sugars can also become a source of energy for animals who eat the plants, and predators who eat those herbivores.

The Calvin cycle is also sometimes referred to as the “light independent” reactions of photosynthesis, since it is not powered directly by photons from the Sun. Instead, the Calvin cycle is powered by ATP and NADPH, which are created by harnessing the energy from photons in the light-dependent reactions.

The function of the Calvin cycle is to create three-carbon sugars, which can then be used to build other sugars such as glucose, starch, and cellulose that is used by plants as a structural building material. The Calvin cycle takes molecules of carbon straight out of the air and turns them into plant matter.

This makes the Calvin cycle vital for the existence of most ecosystems, where plants form the base of the energy pyramid. Without the Calvin cycle, plants would be unable to store energy in a form that herbivores could digest. Carnivores would subsequently not have access to energy stored in the bodies of herbivores!

The carbon backbones created in the Calvin cycle are also used by plants and animals to make proteins, nucleic acids, lipids, and all the other building blocks of life.

The Calvin cycle also regulates the levels of carbon dioxide, a greenhouse gas, in the Earth’s atmosphere. Scientists have raised concerns because, in addition to putting huge amounts of CO2 back into the air by burning coal, oil, and gasoline, humans have also cut down about half of all Earth’s forests, which play an important role in removing CO2 from the air.

We will discuss how the Calvin cycle creates simple sugars from CO2 below.

In carbon fixation, a CO2 molecule from the atmosphere combines with a five-carbon acceptor molecule called ribulose-1,5-bisphosphate (RuBP).

The resulting six-carbon compound is then split into two molecules of the three-carbon compound, 3-phosphoglyceric acid (3-PGA).

This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, also known as RuBisCO. Due to the key role it plays in photosynthesis, RuBisCo is probably the most abundant enzyme on Earth.

In the second stage of the Calvin cycle, the 3-PGA molecules created through carbon fixation are converted into molecules of a simple sugar – glyceraldehyde-3 phosphate (G3P).

This stage uses energy from ATP and NADPH created in the light-dependent reactions of photosynthesis. In this way, the Calvin cycle becomes the way in which plants convert energy from sunlight into long-term storage molecules, such as sugars. The energy from the ATP and NADPH is transferred to the sugars.

This step is called “reduction” because NADPH donates electrons to the 3-phosphoglyceric acid molecules to create glyceraldehyde-3 phosphate. In chemistry, the process of donating electrons is called “reduction,” while the process of taking electrons is called “oxidation.”

Some glyceraldehyde-3 phosphate molecules go to make glucose, while others must be recycled to regenerate the five-carbon RuBP compound that is used to accept new carbon molecules.

The regeneration process requires ATP. It is a complex process involving many steps.

Because it takes six carbon molecules to make a glucose, this cycle must be repeated six times to make a single molecule of glucose.

To accomplish this equation, five out of six glyceraldehyde-3 phosphate molecules that are created through the Calvin cycle are regenerated to form RuBP molecules. The sixth exits the cycle to become one half of a glucose molecule.

Each turn of the Calvin cycle “fixes” one molecule of carbon that can be used to make sugar.

It takes three turns of the Calvin cycle to create one molecule of glyceraldehyde-3 phosphate.

After six turns of the Calvin cycle, two molecules of glyceraldehyde-3 phosphate can be combined to make a glucose molecule.

Each turn of the Calvin cycle also uses up 3 ATP and 2 NADPH in the processes of reducing (adding electrons to) 3-phosphoglyceric acid to produce glyceraldehyde-3 phosphate, and regenerating RuBP so that they can accept a new atom of carbon from CO2 from the air.

This means that to produce a single molecule of glucose, 18 ATP and 12 NADPH are consumed.

• Chloroplast – The organelle in plant cells where energy from sunlight is turned into ATP and sugar.
• Energy Pyramid – A diagram that illustrates the flow of energy through an ecosystem.
• Photosynthesis – The process by which living things capture energy from sunlight and use it to make fuel and organic materials to build their cells.

1. Why is the Calvin cycle important to most ecosystems?
A. It turns carbon dioxide from the air into carbon that living things can use to make sugars, proteins, nucleotides, and lipids.
B. It stores energy from sunlight into the long-term storage form of sugar, which can be used by plants, or eaten by animals to form the basis for the food chain.
C. It removes carbon dioxide, which is a greenhouse gas, from the air.
D. All of the above.

D is correct. All of the above are reasons why the Calvin Cycle is important!

2. Why is the second phase of the Calvin cycle called “reduction”?
A. Because it reduces the number of carbon atoms in 3-phosphoglyceric acid.
B. Because it reduces the amount of energy in the overall system.
C. Because NADPH gives electrons to 3-phosphoglyceric acid, which is a chemical process called “reduction.”
D. None of the above.

C is correct. In chemistry, to “reduce” something means to give it electrons. NADPH gives electrons to 3-phosphoglyceric action in the “reduction” stage of the Calvin cycle.

3. What is the source of the ATP and NADPH used in the Calvin cycle?
A. Aerobic respiration taking place in the mitochondria.
B. Energy harnessed from sunlight in the chloroplasts.
C. Energy harvested from volatile chemicals, such as iron, hydrogen, or ammonia.
D. None of the above.

B is correct. The Calvin cycle is powered by energy harnessed from sunlight in the chloroplasts. The cycle is performed by photosynthetic plants: aerobic respiration and chemosynthesis, described in the other answers, are used by other types of life forms.

The process of photosynthesis is often described as turning sunlight into sugars, and while that's broadly true, there are two distinct biochemical reactions taking place. The first uses the sunlight to create energy inside the cell and the second takes carbon dioxide and uses it to make sugars. The second is the Calvin cycle although the name is a little unfashionable nowadays. It's politer to refer to it as the Calvin–Benson-Bassham cycle or the reductive pentose phosphate cycle, but with all due apologies to Misters Benson and Bassham, the Calvin Cycle is quicker to write.

Turning carbon dioxide into sugar may sound fairly magical, but it becomes a more conceivable when you consider that both carbon dioxide (CO2) and glucose (C6H12O6) contain roughly the same sort of elements. The Calvin cycle just adds on all the extra elements required. Having said that, the 'just' is still a fairly major task, requiring different enzymes all working in the correct order.

The carbon dioxide molecules diffuse into the cells through small holes in the underside of the leaf. The first enzyme that picks them up is called Rubisco. Despite sounding like a small corporate venture, Rubisco is actually one of the most important enzymes in the world. Without Rubisco, plants would not be able to make sugars, which means that animals would not be able to survive on plants.

Rubisco catalysis the connection of the small molecule ribulose-1.5-bisphosphate phosphate (RuBP) to carbon dioxide - therefore fixing the inorganic CO2 as an organic molecule. RuBP contains 5 carbons as well as oxygen, hydrogen and phosphate and it bonds to the CO2 to create a 6 carbon molecule. This promptly splits into two small 3 carbon molecules as shown in the reaction scheme below:

These two 3 carbon molecules then go through a series of reduction stages, during which they react with the ATP (energy molecule) and NADPH (reducing molecule) that were produced during the light reactions of photosynthesis. Even though the Calvin cycle doesn't require any light itself, it is completely reliant on molecules created by the light-reactions. This stage creates two molecules of the 3-carbon "glyceraldehyde 3-phosphate" - which can be turned into useful plant sugars by further reactions.

In order to continue running the Calvin cycle, and the reason that it is a cycle rather than just a process, the Rubisco must be recycled in order to go and pick up new carbon dioxide molecules. To do this also requires molecules of glyceraldehyde-3-phosphate - which are modified and then joined together to re-form the RuBP. The final result of all this is that for every 3 rounds of the cycle three molecules of RuBP go in, 3 RuBP come out, and one new glyceraldehyde 3-phosphate is made.

When put like that it might seem like a lot of effort for very little, in reality it's a very stable and important cycle. As the components of the cycle are all recycled, the Rubisco can just keep picking up carbon dioxide and shooting out sugars, turning an inorganic gas into an energy molecule useful for life.

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Featured Image by Jon Sullivan. More of his awesome photos can be found here.

The first draft of this was incorrectly posted too early and contained some of my place-holder notation and Unchecked Mathematics. Apologies to all who followed a broken link, and thanks to the commentators for alerting me to the issue (it slipped my mind that I'd set it to publish).