O2 comes from water; sugar's carbon comes from CO2. ATP and NADPH carry light's energy between the two.
Photosynthesis is two linked machines. The light reactions turn light energy into ATP and NADPH, splitting water for electrons. The Calvin cycle spends that ATP and NADPH to build sugar from CO2.
Key terms
- Thylakoid — a flattened membrane sac inside the chloroplast. The light reactions run in its membrane; the space inside is the lumen.
- Stroma — the fluid surrounding the thylakoids. The Calvin cycle runs here, turning CO2 into sugar.
- Photosystems — pigment-and-protein complexes in the thylakoid membrane. Each passes light energy to a reaction center, boosting an electron to high energy.
- Accessory pigments — pigments such as chlorophyll b and carotenoids. They absorb colors chlorophyll a absorbs poorly and pass the energy along.
- ATP synthase — an enzyme spanning the thylakoid membrane. H+ flowing through it, down its gradient, drives ATP production.
- NADPH — an electron carrier. It hauls high-energy electrons from the light reactions to the Calvin cycle.
How it works
- Pigments absorb light and pass the energy to photosystem II's reaction center. An electron there is boosted to high energy and handed to the electron transport chain.
- Photosystem II replaces its lost electron by splitting water. That releases O2, the only O2 photosynthesis makes, and leaves H+ in the lumen.
- Electrons moving down the chain release energy that pumps more H+ from the stroma into the lumen. The small lumen builds a steep gradient.
- Photosystem I re-energizes the electrons with more light and passes them to NADP+, forming NADPH.
- H+ flows back out to the stroma through ATP synthase, which uses the flow to make ATP.
- In the stroma, the Calvin cycle attaches CO2 to a five-carbon acceptor, then spends ATP and NADPH to make sugar and rebuild the acceptor.
Why it matters
Most photosynthesis questions knock out one part and ask what fails. Trace the flow. No water splitting → no O2. A membrane leaky to H+ → no gradient → no ATP, even while electrons keep flowing. No light → no ATP or NADPH → the Calvin cycle stalls within seconds. You'll meet the same trick in respiration: an electron transport chain builds an H+ gradient, and ATP synthase cashes it in.
Exam traps
| Trap | Actually |
|---|
| The O2 plants release comes from CO2. | It comes from water, split at photosystem II. CO2 supplies the carbon that ends up in sugar. |
| The Calvin cycle doesn't use light, so it keeps running at night. | It uses no light directly, but it needs ATP and NADPH from the light reactions. In darkness it stalls within seconds. |
| Protons build up in the stroma. | They build up in the thylakoid lumen. Water splitting releases them there, the chain pumps more in, and they exit through ATP synthase. |
| Electron flow makes ATP directly. | Electron flow builds the H+ gradient, and ATP synthase makes ATP from it. Collapse the gradient and ATP stops while electrons keep flowing. |
Worked example
Leaf disks with the air drawn out of them sink in a cup of liquid. Under bright light, disks in a bicarbonate solution (a CO2 source) float within 12 minutes as O2 collects inside them. Disks in plain water are still sunk after 30 minutes. The O2 comes from water, not CO2. So why does taking away CO2 stop the floating?
- O2 is released only when photosystem II splits water. So ask what keeps photosystem II running.
- Its electrons travel down the chain to NADP+. Electron flow continues only while there's NADP+ to accept them.
- The Calvin cycle turns NADPH back into NADP+ as it reduces CO2 to sugar. No CO2 → no Calvin cycle → NADP+ runs out.
- With no acceptor, electrons back up in the chain. Photosystem II can't hand off electrons, so it splits less water and O2 release falls.
Without CO2, the Calvin cycle stops turning NADPH back into NADP+. Electron flow backs up, water splitting slows, and too little O2 collects to float the disks.
The two stages form one loop: the light reactions make ATP and NADPH, and the Calvin cycle hands back ADP and NADP+. The near-miss is 'the light reactions don't use CO2, so they should run unaffected.' They don't use CO2, but they stall without the Calvin cycle returning NADP+ and ADP.