Summary
Photosynthetic electron flow builds a proton gradient across the thylakoid to drive photophosphorylation.
In depth
Protons accumulate in the thylakoid lumen from water splitting and plastoquinone pumping; ATP synthase in thylakoid membranes uses the gradient. Per 3 H+ one ATP is made.
Chloroplast ATP synthase is similar to the mitochondrial one. The thylakoid lumen is acidic (about pH 4).
Remember: chemiosmosis is central in both respiration and photosynthesis.
In the thylakoid membrane, photosystem II absorbs light and splits water (photolysis), releasing O2, protons and electrons. Electrons pass along an electron transport chain (plastoquinone, cytochrome b6f, plastocyanin) to photosystem I, which re-energises them with more light and passes them to NADP+ to form NADPH.
Protons pumped into the thylakoid lumen and released from water create a gradient that drives ATP synthase to make ATP, called photophosphorylation. The non-cyclic pathway produces both ATP and NADPH; the cyclic pathway uses only photosystem I and produces ATP alone. Both products feed the Calvin cycle in the stroma.
Example: oxygen in air came from this reaction.
Remember: the light reactions need light, the Calvin cycle needs their products.
Frequently asked questions
What is Light Reactions and ATP Synthesis?
Photosynthetic electron flow builds a proton gradient across the thylakoid to drive photophosphorylation.
Which area of biology covers Light Reactions and ATP Synthesis, and how advanced is it?
Light Reactions and ATP Synthesis is one of the 53 topics in Metabolism & Bioenergetics. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Mitochondrial Uncoupling, Electron Transport Chain, Chemiosmosis.
Can you give an example of Light Reactions and ATP Synthesis?
Uncouplers collapse the gradient and stop ATP synthesis.
What should I learn before Light Reactions and ATP Synthesis?
Work through the Beginner topics in Metabolism & Bioenergetics first, such as Metabolism, Anabolism and Catabolism, ATP, then return to Light Reactions and ATP Synthesis. Intermediate topics assume the Beginner vocabulary.
What topics are related to Light Reactions and ATP Synthesis?
The closest topics to Light Reactions and ATP Synthesis are Mitochondrial Uncoupling, Electron Transport Chain, Chemiosmosis, Oxidative Phosphorylation. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Light Reactions and ATP Synthesis?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Light Reactions and ATP Synthesis. To practise the whole of Metabolism & Bioenergetics, use the Metabolism & Bioenergetics quiz.
How is Light Reactions and ATP Synthesis different from Mitochondrial Uncoupling?
Light Reactions and ATP Synthesis: Photosynthetic electron flow builds a proton gradient across the thylakoid to drive photophosphorylation. Mitochondrial Uncoupling: Uncoupling proteins dissipate the proton gradient as heat, as in brown adipose tissue. Read both side by side and note what each one explains.
How should I study Light Reactions and ATP Synthesis step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Mitochondrial Uncoupling and explain the difference aloud. Finally, take the Metabolism & Bioenergetics quiz and come back to this page for any question you miss.