Part 2: Light-Independent Reactions (Dark Reactions)
🔷 What Are Light-Independent Reactions?
These reactions do not require light directly. They are called light-independent or dark reactions, but don’t let the word “dark” confuse you—they can happen during the day too, as long as the products of the light reactions (ATP and NADPH) are available.
These reactions happen in the stroma—the fluid part inside the chloroplast but outside the thylakoids.
Their goal is to make sugar (glucose) using:
- Carbon dioxide (CO₂) from the air,
- ATP for energy, and
- NADPH (which provides hydrogen and electrons)
🔷 Discovery of the Calvin Cycle
The Calvin Cycle is the name given to the series of reactions in light-independent photosynthesis.
- Discovered by Melvin Calvin and his team.
- He received the Nobel Prize in 1961 for his work.
🔷 Phases of the Calvin Cycle (Dark Reactions)
Let’s break it into 3 main phases, and explain each step simply.
✅ Phase I: Carbon Fixation
🧪 This is the process of taking carbon from the air and turning it into a solid organic compound.
- The enzyme Rubisco (full name: ribulose bisphosphate carboxylase) helps this process.
- Rubisco combines:
- 3 molecules of CO₂ with
- 3 molecules of RuBP (a 5-carbon sugar)
This forms:
- 6 molecules of 3-PGA (3-phosphoglyceric acid) — each has 3 carbon atoms.
Think of this step like collecting raw ingredients (CO₂) to start cooking.
✅ Phase II: Reduction
🧪 Here, the plant adds energy and hydrogen to the carbon molecules to make them useful.
- ATP is used to add phosphate groups to 3-PGA, converting it to a higher-energy compound: 1,3-bisphosphoglyceric acid
- Then, NADPH donates electrons and hydrogen to reduce it into G3P (Glyceraldehyde 3-phosphate)
Now, we have 6 molecules of G3P, but only one G3P leaves the cycle.
- This one G3P goes on to make glucose (by combining with another G3P later).
- The other five G3P molecules stay in the cycle.
✅ Phase III: Regeneration of RuBP
🧪 This step is about resetting the cycle.
- The 5 G3P molecules are rearranged (through multiple chemical steps) to form 3 molecules of RuBP.
- This requires 3 ATP molecules.
Now, the cycle is ready to start again by fixing more CO₂.
🌟 End Result of the Calvin Cycle
After two turns of the Calvin cycle, the plant has:
- Made two G3P molecules, which can combine to form one molecule of glucose (C₆H₁₂O₆)
- Used 6 CO₂, 18 ATP, and 12 NADPH
✅ Final Summary
| Process | What Happens | Where | Purpose |
| Light-dependent reactions | Capture light, make ATP & NADPH | Thylakoid membranes | Power the next step |
| Chemiosmosis | Uses H⁺ gradient to make ATP | Thylakoid membrane | Energy production |
| Calvin Cycle (Dark Reactions) | Uses CO₂ + ATP + NADPH to make sugar | Stroma | Make glucose (food) |