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The final stage of photosynthesis where carbon dioxide is fixed into organic molecules.
Every single carbon atom in your body was once a gas floating in the atmosphere. How do plants perform the 'magic' of turning invisible air into the solid sugar that fuels all life on Earth?
The Calvin Cycle occurs in the stroma of the chloroplast. Unlike the light reactions, it doesn't need photons directly, but it requires the energy 'currency' (ATP and NADPH) produced earlier. The process begins with Carbon Fixation. An enzyme called RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) captures an inorganic molecule from the air and attaches it to a 5-carbon sugar called RuBP. This creates an unstable 6-carbon intermediate that immediately splits into two molecules of 3-PGA (3-phosphoglycerate). RuBisCO is often called the most important enzyme on Earth because it is the bridge between the non-living atmosphere and living organic matter.
Quick Check
What is the specific role of RuBisCO in the first stage of the cycle?
Answer
RuBisCO catalyzes the reaction that attaches inorganic carbon dioxide (CO2) to the organic 5-carbon molecule RuBP.
Now that the carbon is 'fixed,' it needs to be energized. In the Reduction phase, the 3-PGA molecules are transformed into a high-energy 3-carbon sugar called G3P (Glyceraldehyde-3-phosphate). This transformation requires a massive investment of energy: ATP provides a phosphate group, and NADPH donates high-energy electrons (reduction). For every three molecules that enter the cycle, six G3P molecules are produced. However, only one of these G3P molecules is considered a 'profit' that can leave the cycle to build glucose () or other carbohydrates.
Let's track the carbons for 3 turns of the cycle: 1. Start with 3 molecules of (3 carbons total). 2. Combine with 3 molecules of 5-carbon RuBP (15 carbons total). 3. Total carbons in the system = carbons. 4. These form 6 molecules of 3-carbon G3P ( carbons). 5. 1 G3P exits (3 carbons), leaving 15 carbons behind to restart the cycle.
If the plant kept using all the G3P to make glucose, it would quickly run out of RuBP and the cycle would grind to a halt. In the Regeneration phase, the remaining five G3P molecules (15 carbons total) are rearranged through a series of complex reactions to reform three molecules of the 5-carbon RuBP. This step requires even more ATP. By regenerating the starting material, the chloroplast ensures the factory is ready to process the next batch of . This circularity is why we call it a 'cycle' rather than a linear pathway.
Quick Check
Why must the plant 'spend' 5 out of every 6 G3P molecules it creates?
Answer
To regenerate RuBP, the starting 5-carbon molecule, so the cycle can continue to fix more carbon dioxide.
The Calvin Cycle is the ultimate 'builder.' It takes the products of the light reactions (ATP and NADPH) and uses them to create G3P. This G3P is the precursor to glucose, which serves as the primary fuel for Cellular Respiration. Notice the beautiful symmetry: the and glucose produced in the chloroplast are the exact reactants needed by the mitochondria to produce ATP. In turn, the and released by mitochondria are the reactants for photosynthesis. They are two halves of a global energy loop.
To produce just one molecule of glucose (), the Calvin Cycle must turn 6 times. Calculate the total energy cost: 1. Each turn uses 3 ATP and 2 NADPH. 2. For 6 turns: ATP. 3. For 6 turns: NADPH. 4. Total: 6 + 18 ATP + 12 NADPH 1 Glucose.
In which part of the chloroplast does the Calvin Cycle take place?
Which molecule is reduced by NADPH during the cycle?
RuBisCO is used during the Regeneration phase of the Calvin Cycle.
Review Tomorrow
In 24 hours, try to sketch the Calvin Cycle from memory, specifically labeling where ATP and NADPH are used and where RuBisCO acts.
Practice Activity
Compare the Calvin Cycle to a kitchen: identify the 'chef' (enzyme), the 'ingredients' (reactants), and the 'finished meal' (product).