AP® Biology · Unit 3 · Topic 3.4
The Light-Dependent Reactions
The light-dependent reactions use sunlight to strip electrons from water and push protons across the thylakoid membrane, building a gradient that ATP synthase spins into ATP, while the electrons end up loaded onto NADPH.
Photosynthesis needs electrons to run, and it needs to get rid of the ones it uses up. Water is the source. At photosystem II, light energy knocks an electron loose from chlorophyll, and that missing electron gets replaced by splitting a water molecule, which releases oxygen as a byproduct. Without splitting water first, photosystem II would run out of electrons after one round and the whole chain would stall.
The excited electron leaves photosystem II and travels down an electron transport chain embedded in the thylakoid membrane, losing energy at each step. That energy is used to pump protons (H+) from the stroma into the thylakoid space, building up a steep concentration gradient. The electron eventually reaches photosystem I, where a second burst of light re-energizes it, and it gets passed to NADP+ to form NADPH. The proton gradient built along the way is the point: protons flow back out through ATP synthase, spinning part of the enzyme and driving the attachment of a phosphate to ADP, making ATP. This whole process is called chemiosmosis.
The ATP and NADPH made here don't get used for anything inside the light reactions themselves. They're shipped off to the Calvin cycle, where ATP's energy and NADPH's electrons power the fixation of CO2 into sugar. That's also where the carbon and oxygen from CO2 actually end up, not in the oxygen gas released earlier at photosystem II. The two halves of photosynthesis are separate reactions that sit back to back, trading energy carriers for raw material.
Key terms
- Photosystem II
- The protein complex where light splits water, releasing oxygen and sending an energized electron into the transport chain.
- Photosystem I
- The protein complex that re-energizes the traveling electron with light before it's passed to NADP+.
- Chemiosmosis
- The process of using a proton gradient across a membrane to drive ATP synthesis through ATP synthase.
- NADPH
- An electron carrier molecule that stores energized electrons from the light reactions for use in the Calvin cycle.
- ATP synthase
- A membrane enzyme that lets protons flow down their gradient and uses that flow to attach phosphate to ADP, forming ATP.
Watch it explained
- How it worksWalk me through an electron leaving photosystem II and arriving at photosystem I.
- How it worksHow does the electron transport chain pump protons into the thylakoid space?
- How it worksHow does ATP synthase turn a proton flowing back out into a molecule of ATP?
- Common mistakeWhy isn't the oxygen you breathe coming from the carbon dioxide a plant takes in?
- Going furtherWhat happens to ATP and NADPH production when a herbicide blocks photosystem II?
Check yourself
End of topic.