Lesson 2 of 5 / Photosynthesis and energy capture
Light-driven electron transfer
Where do the replacement electrons and released oxygen come from?
I can explain non-cyclic light-dependent reactions and energy conversion.
I can explain light-dependent energy conversion in chloroplasts.Syllabus
Syllabus 9477, Core 3(c). Explain light-dependent energy conversion in chloroplasts.
About 8 min
Make a guess. You are not marked.
Photosynthesis takes in CO2 and gives out O2. Where does the released oxygen come from?
Show the answer
Water, split in the light-dependent reactions.
Water oxidation at photosystem II replaces lost electrons and releases oxygen.
The key idea
Light excites electrons; water supplies replacements, electron transfer supports ATP formation, and NADP accepts reducing power.
Explore the idea
Trace water electrons to reduced NADP
Water oxidation supplies electrons to replace those lost by photosystem II. Protons enter the thylakoid lumen and oxygen is released; CO2 is not the electron source in this route.
This is the order of electron transfer, not a spatial cross-section or a graph of electron energy. The first box lists products qualitatively and is not a balanced chemical equation. ATP synthesis uses the proton gradient explained in the next lesson.
Explanation
Pigment molecules absorb photons and transfer excitation energy to a photosystem reaction centre. In non-cyclic electron flow, light excitation at photosystem II raises an electron to a higher-energy state and it is transferred to an acceptor. Water is split, supplying replacement electrons, protons and oxygen as a by-product.
Electrons pass through carriers in the thylakoid membrane. Energy released during transfer is coupled to movement of protons into the thylakoid lumen, contributing to a proton electrochemical gradient. The energy is not destroyed; it is transferred into a form that can support ATP synthesis.
At photosystem I, further light excitation raises electron energy again. Electrons ultimately reduce NADP, together with protons, to reduced NADP (NADPH) on the stromal side. Reduced NADP supplies reducing power for carbon reduction in the Calvin cycle.
The oxygen released comes from water, not directly from carbon dioxide. ATP and reduced NADP link light capture to carbohydrate synthesis, but the light-dependent reactions do not themselves fix carbon into glucose. Names of individual electron-carrier complexes and detailed ATP synthase mechanics are outside the specified requirement.
Step by step
- 1
Track the electron source
Begin with water rather than carbon dioxide.
- 2
Track energy twice
Excitation occurs at both photosystems in non-cyclic flow.
- 3
Track the products
Separate oxygen, ATP and reduced NADP by role.
Worked example
Work through the evidence
If water is labelled with a heavy oxygen isotope, where would the label be expected in the released gas?
One way to explain it
In oxygen gas produced by photolysis of water. This supports water as the source of photosynthetic oxygen.
Why this answer works
- Trace atoms as well as energy.
- Carbon dioxide supplies carbon for fixation but is not the direct source of the evolved oxygen in this mechanism.
Watch out for this
A student says: "The oxygen released in photosynthesis is split directly from CO2 during the Calvin cycle." What is wrong with this?
Show the answer
It is produced by water oxidation in the light-dependent reactions.
Check your understanding
What is the main role of reduced NADP in the Calvin cycle?
Show the answer
It provides reducing power for carbon reduction.
Its electrons support reduction of carbon intermediates.
Quick recall
Where do the replacement electrons and released oxygen come from?