Dark Reactions of Photosynthesis: The Calvin-Benson Cycle

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  • 0:06 Photosynthesis
  • 1:18 Calvin Cycle
  • 3:10 Lesson Summary
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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

Photosynthesis starts out using the energy from sunlight to get things started, but it ends with the dark reactions, which don't need sunshine to complete sugar production. In the Calvin cycle, ATP and NADPH from the light reactions are used to produce sugars.

Photosynthesis

Photosynthesis is the process that converts energy from sunlight into food. Photosynthesis feeds the world, taking rays of sunlight and turning them into food on your plate with just a few steps in between. Remember that in photosynthesis, light energy, carbon dioxide and water are used to produce glucose (or sugar) and oxygen.

Illustration of the interior of a chloroplast
Chloroplast Illustration

After shedding some sunlight on how photosynthesis begins with the light reactions, we're now ready to learn the second part of photosynthesis, called the dark reactions, otherwise known as the Calvin cycle. This step is also sometimes referred to as the 'Calvin-Benson Cycle,' as it gets its name from the scientists who discovered it. While sunlight is used during the light reactions to produce ATP and the electron carrier NADPH, the dark reactions use these products to complete photosynthesis without the use of light energy.

The light reactions produce ATP and NADPH in the thylakoids of chloroplasts. These products are released into the stroma of the chloroplast. Here, the Calvin cycle can take off, using carbon dioxide from the air to create sugars.

This is the 'synthesis' part of 'photosynthesis,' where the plants will finish cooking up some food with the help of a little carbon dioxide.

Calvin Cycle

The key to creating sugars comes from carbon fixation, where inorganic carbon is fixed into organic carbon, such as sugars. In the Calvin cycle, carbon dioxide is fixed into an organic molecule through the help of an enzyme called RuBisCO. Here, the carbon dioxide that we exhale as waste is used by plants performing photosynthesis. Let's highlight some of the steps that are in this Calvin cycle.

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