3.3.8Inquiry Process
Light Dependent Reactions — IB Biology HL Revision Notes & Topical Questions
5 min read 3 parts Lesson 123 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Electron Flow
The light-dependent reactions occur on the thylakoid membranes and need light, water, ADP, phosphate and NADP⁺.
1
Light excites electrons in the reaction centre of photosystem II, and they pass to the first carrier of the electron transport chain.
2
Photolysis of water replaces the lost electrons: 2H₂O → 4H⁺ + 4e⁻ + O₂, which releases oxygen and protons inside the thylakoid.
3
Electrons pass down the chain, losing energy at each step, and the energy is used to pump protons into the thylakoid space.
4
Photosystem I absorbs light and electrons are excited again, and they are passed to the carrier NADP reductase.
5
NADP reductase uses the electrons and a proton to reduce NADP⁺ to NADPH in the stroma.
Part 2 of 4Making ATP
Proton gradient
- Proton gradient Protons from photolysis and from pumping accumulate in the thylakoid space, which makes it more acidic than the stroma.
ATP synthase
- ATP synthase Protons flow down their gradient through ATP synthase channels, back into the stroma.
Chemiosmosis
- Chemiosmosis The energy of the flow drives the joining of ADP and inorganic phosphate to make ATP, a process called photophosphorylation.
Location of ATP
- Location of ATP The ATP is made in the stroma, where it is used in the light-independent reactions.
Oxygen
- Oxygen Oxygen from photolysis is a by-product that diffuses out of the chloroplast, and it is not used in these reactions.
| Input | Output |
|---|---|
| Light energy | Excited electrons |
| Water | Oxygen, protons and electrons |
| ADP + phosphate | ATP |
| NADP⁺ | NADPH |
Part 3 of 4Summary and Comparison
Summary and Comparison
Products used later
- Products used later ATP and NADPH carry energy and hydrogen to the Calvin cycle in the stroma.
Similarity to respiration
- Similarity to respiration Both use an electron transport chain, a proton gradient and ATP synthase,
- but the energy source is light in one and food in the other.
Location
- Location Grana of the thylakoid membranes hold the photosystems, giving a large surface area for the reactions.
Oxygen
- Oxygen Photolysis is the only source of the oxygen released by photosynthesis.
Energy transfer
- Energy transfer Light energy ends up as chemical energy in two forms: the phosphate bond of ATP and the extra electrons carried by NADPH.
Part 4 of 4Exam Checklist & Tips
3.3.8 – Key Points for the Exam
- Photolysis The splitting of water by light energy.
- Photophosphorylation Formation of ATP using light energy.
- NADP A coenzyme that carries hydrogen in photosynthesis.
- Thylakoid space The inside of a thylakoid, where protons build up.
Common Mistakes
- Saying that carbon dioxide is split to release oxygen; the oxygen comes from water.
- Mixing up the pumping directionprotons are pumped into the thylakoid space, not out of it.
- Forgetting that NADP is reduced to NADPH in photosynthesis, whereas NAD is reduced in respiration.
- Placing ATP synthesis in the thylakoid space; ATP is made on the stroma side.
Last updated:
\n