Calvin Cycle Definition
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.
Function of the Calvin Cycle
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.
Calvin Cycle Steps
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.
Calvin Cycle Products
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.