Calvin Cycle

The Calvin cycle describes what happens to light energy after it has been absorbed into a leaf.

Put on your steampunk magnifying glasses because, today, we are going deep into the molecular level of leaves to learn how they make energy from sunlight.

Also known as the Calvin-Benson cycle, Melvin Calvin won a Nobel Prize in Chemistry for figuring this out. That was in 1961. Until Melvin’s research was complete, everyone thought that photosynthesis consisted of chlorophyll interacting directly with carbon dioxide to create edible organics (sugars) for plants. Instead, his research taught us that light energy causes chlorophyll to trigger plants to produce those organic compounds for themselves!

So, how does photosynthesis work?

Photosynthesis

Photosynthesis occurs in two steps. The first step is the light dependent reaction. This is when the sunlight is absorbed and transformed. To do this, electrons are torn from water molecules, creating oxygen as a waste product. When this happens, hydrogen (H) is released and used to create two compounds: nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP). The second stage of photosynthesis is the light independent reaction, or the Calvin cycle. This is when the ATP is converted into glucose.

Calvin cycle

​Each stage of the Calvin cycle has its own enzyme. Enzymes are chemical catalysts that trigger change. After light energy has entered a plant through the stoma and been converted into ATP and NADPH, the light-independent (or ‘dark’) aspect of photosynthesis can begin. There are four stages to the Calvin cycle:

1. Carbon fixation - a five-carbon enzyme molecule (RuBisCO*) snags a molecule of carbon dioxide (CO2) and turns into a six-carbon molecule
2. Reduction - The six-carbon molecule is split in half by an enzyme, using energy from ATP and NADPH molecules; one half becomes a sugar phosphate molecule, which is later combined to make starch and sucrose; the other half moves on to the next step
3. Regeneration - Again using energy from ATP and NADPH, the three-carbon molecule becomes a five-carbon molecule and the process starts over

​For your chemistry buffs, here's the equation and a graphic:

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Calvin Cycle (Mike Jones) CC BY-SA 3.0

In the illustration above, atoms are represented as black (carbon), white (hydrogen) red (oxygen) and pink (phosphorus).
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For the rest of us, try this story for a more memorable form:

​Rubio marries Connie. Connie has twins. One twin becomes a cook. The other works the family business and marries a girl just like mom. And they have twins. And so it goes….

​Factors that interrupt the Calvin cycle

Any interruption in photosynthesis leads to chlorosis, or yellowing. Chlorosis can be the result of insufficient light (epinasty), disease, a lack of mycorrhizae, or sunburn. Other factors that interrupt the Calvin cycle include:

• excessive heat
• overspray of herbicides and other chemicals
• soil pH - too much acidity
• insufficient magnesium uptake - it may be present in your soil, but other factors can limit uptake

If you see chlorosis, it means your plant is starving. By learning about the Calvin cycle, you may be better equipped to figure out what is wrong with your plants. Get growing!


* RuBisCO - ribulose bisphosphate carboxylase/oxygenase; believed to be the most abundant enzyme on Earth (Wikipedia)