Why is the Calvin Cycle Considered a Dark Reaction?

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Humans are not the only ones who love carbohydrates. Plants also need them to survive, and carbs are an important energy source. During photosynthesis, plants combine water with carbon dioxide and sunlight to make carbohydrates. There are two parts to photosynthesis: the light-dependent reactions and the light-independent reactions or dark reactions.

TL;DR (Too Long; Didn't Read)

Scientists consider the Calvin cycle a dark reaction because it does not require light to work. It is a stage in the photosynthesis process that plants use.

Why the Calvin Cycle Is a Dark Reaction

The Calvin cycle is a dark reaction because it does not need sunlight. Although it can happen during the day, this process does not require energy from the sun to work. Other names for the Calvin cycle include the Calvin-Benson cycle, light-independent reaction, carbon fixation and C3 pathway.

During the Calvin cycle, the plant captures carbon dioxide, which reacts with the sugar, ribulose bisphosphate -- RuBP -- to make a six-carbon sugar. Next, this six-carbon sugar breaks down with the help of the enzyme RuBisCO to make two molecules of 3-phosphoglyceric acid, or 3PGA. Then, adenosine triphosphate, ATP and nicotinamide adenine dinucleotide phosphate hydrogen, called NADPH, convert the 3PGA to glyceraldehyde-3-phosphate, abbreviated as G3P. A portion of the G3P becomes RuBP, so the cycle can start again. Another portion of G3P helps create fructose diphosphate, which can become carbohydrates like glucose or sucrose.

Final Product of the Calvin Cycle

The final product of the Calvin cycle is a simple sugar. This sugar can become a carbohydrate such as starch, which is a vital energy source for plants. For example, the plants can transport glucose to do important processes such as aiding respiration to release energy. They can also convert glucose for storage purposes or use it as a building block to grow bigger.

Factors Affecting the Calvin Cycle

The amount of carbon dioxide that the plant can access affects the Calvin cycle. A higher concentration of carbon dioxide means that the rate of the photosynthesis process can increase. In addition, the temperature affects the cycle. Since it requires enzymes, a temperature that is either too high or too low will affect it.

History of the Calvin Cycle

Melvin Calvin, an American chemist, discovered the Calvin cycle. He later won the 1961 Nobel Prize in Chemistry. While working at the University of California, Berkeley, he used a carbon-14 isotope to understand the photosynthesis process in plants. This radioactive isotope helped him determine how the light-independent reaction works in single-celled algae.


About the Author

Lana Bandoim is a freelance writer and editor. She has a Bachelor of Science degree in biology and chemistry from Butler University. Her work has appeared on Forbes, Yahoo! News, Business Insider, Lifescript, Healthline and many other publications. She has been a judge for the Scholastic Writing Awards from the Alliance for Young Artists & Writers. She has also been nominated for a Best Shortform Science Writing award by the Best Shortform Science Writing Project.