AP Syllabus focus:
‘Protons flow back through ATP synthase by chemiosmosis, driving photophosphorylation and ATP synthesis in chloroplasts.’
Photosynthetic light reactions convert light energy into chemical energy largely by building a proton gradient. Chemiosmosis then harnesses that gradient to power ATP formation via ATP synthase in the thylakoid membrane.
Practice Questions
FAQ
Cyclic photophosphorylation routes excited electrons back to earlier carriers, increasing proton translocation without producing NADPH.
This tends to raise the ATP:NADPH production ratio when extra ATP is needed.
Protons are preferentially accumulated in the thylakoid lumen, reducing stromal $[H^+]$.
Lower $[H^+]$ corresponds to a higher pH, which can influence stromal enzyme activity.
Common approaches include:
pH-sensitive dyes/probes that partition by pH
electrochromic shift measurements indicating membrane potential
monitoring ATP formation while selectively dissipating $\Delta pH$
Uncouplers increase thylakoid membrane permeability to $H^+$, collapsing the gradient.
Electron flow may continue, but ATP synthase lacks the driving force to synthesise ATP efficiently.
ATP synthase has a membrane channel region and a catalytic head. Proton flow drives rotational/conformational changes.
These changes alter binding affinities for ADP, $P_i$, and ATP, promoting ATP formation and release.
