Mixing up what the stroma and the thylakoids do
- The idea
- Predict
- See it
- Why
- Check
The idea
“Sugar gets built inside the thylakoids, and H+ piles up in the stroma to make ATP.”
The chlorophyll is in the thylakoids, so it's easy to think all of photosynthesis happens there. Three spaces with similar names don't help.
The short version
The mistake: Sugar gets built inside the thylakoids, and H+ piles up in the stroma to make ATP.
What's actually true: The thylakoids use light to make ATP and NADPH, piling H+ inside. The Calvin cycle in the stroma uses both to make sugar.
- The light reactions run on the thylakoid membrane. Splitting water and the electron transport chain both put H+ into the thylakoid space.
- That H+ flows back out through ATP synthase, and the flow makes ATP. NADPH is made on the stroma side too, so both end up in the stroma.
- Rubisco fixes CO2 in the stroma, and the Calvin cycle uses ATP and NADPH to build a sugar called G3P. The leftover ADP and NADP+ go back to be recharged.
On the exam: Light hits the membrane, H+ piles up inside the thylakoid, and sugar gets built outside it in the stroma.