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.
This diagram summarizes the light-dependent reactions that create the proton-motive force across the thylakoid membrane. It shows protons accumulating in the thylakoid lumen and then flowing back to the stroma through ATP synthase, which drives . The figure also situates proton pumping within the broader electron transport pathway that links PSII to PSI. Source
Chemiosmosis in chloroplasts
Key terms and locations
Chemiosmosis: The movement of H (protons) down an electrochemical gradient through a membrane protein, using the released potential energy to do cellular work (here, ATP production).
In chloroplasts, chemiosmosis occurs across the thylakoid membrane:
This figure compares chemiosmotic ATP synthesis in mitochondria and chloroplasts, highlighting the shared logic of a proton gradient driving ATP synthase. In the chloroplast panel, the thylakoid lumen is depicted as more acidic (higher ) than the stroma, so protons flow back toward the stroma through ATP synthase. The diagram also makes the enzyme’s orientation explicit, reinforcing where ATP is produced relative to the membrane. Source
High [H] accumulates in the thylakoid lumen
Low [H] is maintained in the stroma
The thylakoid membrane’s selective permeability helps prevent H from freely diffusing back into the stroma
Building the proton gradient (context for coupling)
A proton gradient is created when processes associated with the light reactions move protons to the lumen and remove them from the stroma, resulting in:
a pH gradient (lumen more acidic than stroma)
an associated electrical gradient (lumen relatively more positive)
The stored energy in this gradient is often described as the proton-motive force (PMF), which represents how strongly protons are driven to flow back across the membrane.
ATP synthase: converting gradient energy into ATP
ATP synthase: A membrane-embedded enzyme complex that uses proton flow to catalyse ADP + P → ATP.
ATP synthase provides a controlled path for H to return to the stroma:
H moves through the membrane portion of the complex (a proton channel)
Proton movement drives rotation/conformational changes in the catalytic portion facing the stroma
These conformational changes promote ATP formation from substrates available in the stroma
A key idea is coupling: ATP synthase links an energetically favourable process (H moving down its gradient) to an energetically unfavourable one (ATP synthesis).
= Adenosine triphosphate (energy currency of the cell)
= Adenosine diphosphate (ATP precursor)
= Inorganic phosphate (phosphate group added to ADP)
This ATP production powered by light-driven proton gradients is central to how chloroplasts capture energy in a usable chemical form.
Photophosphorylation
Photophosphorylation: The synthesis of ATP from ADP and P using energy ultimately derived from light, mediated by a proton gradient and ATP synthase.
Photophosphorylation in chloroplasts depends on:
an intact thylakoid membrane to maintain the H gradient
a functional ATP synthase to provide a route for H flow and catalyse ATP production
sufficient ADP and P in the stroma (substrate availability limits ATP output)
Because the gradient is dynamic, the rate of ATP synthesis changes as conditions change:
If H accumulates in the lumen, the driving force for H flow through ATP synthase increases
If the membrane becomes leaky to H, the gradient dissipates and ATP synthesis slows or stops
If ADP or P becomes scarce, ATP synthase cannot sustain high ATP production even with a strong gradient
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.
Practice Questions
State what chemiosmosis is in chloroplasts and name the protein complex that uses it to make ATP. (2 marks)
Describes protons moving down an electrochemical gradient across the thylakoid membrane (1)
Identifies ATP synthase (1)
Explain how a proton gradient across the thylakoid membrane leads to ATP synthesis in photophosphorylation. (5 marks)
Proton concentration is higher in the thylakoid lumen than in the stroma (1)
Protons move back to the stroma through ATP synthase down the gradient (1)
Proton flow provides energy (proton-motive force) to drive ATP formation (1)
ATP synthase catalyses on the stromal side (1)
ATP production requires an intact thylakoid membrane to maintain the gradient (1)
