Lesson 3 ·Class 10 · Photosynthesis · Flow Lab
The Light-Dependent Reaction
Splitting water, and where the oxygen comes from.
Also called the photochemical phase. It happens in the thylakoids of the grana, and it cannot proceed without light.
→ → → → →
💡 Step 1 of 6 · Step I — Activation of chlorophyll
Chlorophyll absorbs photons of light and becomes activated, releasing energised electrons.
2H₂O --energy of 4 photons--> 4H⁺ + 4e⁻ + O₂
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.
Note the overall equation Selina gives: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂. Twelve water molecules go in and six come out — because the water released is re-formed, not the same water that went in.
What this lesson covers
Challenges to clear
- Advance to the step where water is split.
- Reach the step that says where the oxygen came from.
- Step through the whole light phase.
The task
Also called the photochemical phase. It happens in the thylakoids of the grana, and it cannot proceed without light.
What this lesson covers
- Step I — Activation of chlorophyll — Chlorophyll absorbs photons of light and becomes activated, releasing energised electrons.
- Step II — Photolysis of water — The energy of four photons splits water: 2H₂O → 4H⁺ + 4e⁻ + O₂. Splitting by light is what photolysis means.
- Oxygen is released — The oxygen given off comes from the WATER, not from the carbon dioxide. This is the single most misunderstood point in the whole chapter.
- Hydrogen is captured — The H⁺ ions are picked up by the carrier NADP, forming NADPH — a store of reducing power for the next phase.
- ATP is made — The energised electrons are used to convert ADP + inorganic phosphate into ATP. Because light drives it, this is called photophosphorylation.
- Handed to the dark reaction — The light phase produces nothing edible. Its output is ATP and NADPH — energy and hydrogen — which the next phase uses to build sugar.
- 2H₂O --energy of 4 photons--> 4H⁺ + 4e⁻ + O₂
Hint
Note the overall equation Selina gives: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂. Twelve water molecules go in and six come out — because the water released is re-formed, not the same water that went in.