The Light-Independent Reaction
Building the sugar, in the dark — sort of.
Called the dark reaction because it needs no light — but it does not wait for night. It runs at the same time as the light reaction, less than a thousandth of a second behind it.
Not in the grana. The dark reaction takes place in the fluid stroma surrounding them, so the two phases are separated in space but not in time.
Free play — step through the process at your own pace, and move the slider to see what changes. Switch to when you're ready to test yourself.
What this lesson covers
Challenges to clear
- Advance to the step where carbon dioxide is captured.
- Reach the step where the first sugar is built.
- Step through the whole dark phase.
The task
Called the dark reaction because it needs no light — but it does not wait for night. It runs at the same time as the light reaction, less than a thousandth of a second behind it.
What this lesson covers
- In the stroma — Not in the grana. The dark reaction takes place in the fluid stroma surrounding them, so the two phases are separated in space but not in time.
- Carbon dioxide arrives — CO₂ diffuses in through the stomata and reaches the stroma.
- Hydrogen is transferred — NADPH, made in the light phase, hands over its hydrogen to combine with the carbon dioxide.
- ATP supplies the energy — The ATP made in the light phase is spent here. This is why the dark reaction cannot run for long without light — it runs out of both ATP and NADPH.
- Glucose is formed — The result is C₆H₁₂O₆. The syllabus notes that the detailed equations are not required — what matters is that carbon dioxide is reduced to sugar using hydrogen and energy from the light phase.
- Polymerised into starch — Many glucose molecules are joined together into starch for storage — polymerisation. That stored starch is exactly what the iodine test goes looking for.
- 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂↑
Hint
The name is misleading and examiners exploit it. Light-independent does not mean it happens in darkness — it means light is not directly required for these particular steps.