The light-dependent stage
The light-dependent stage
- In the thylakoids, light energy is used to make ATP.
- This is photophosphorylation, and it comes in two forms.
- One form also splits water and releases the oxygen we breathe.
Cyclic vs non-cyclic
- Cyclic photophosphorylation: uses photosystem I only; chlorophyll is photoactivated; makes only ATP.
- Non-cyclic photophosphorylation: uses photosystems I and II; the oxygen-evolving complex carries out photolysis of water (releasing oxygen); makes ATP and reduced NADP.
Practice
Cyclic photophosphorylation:
Cyclic photophosphorylation uses PSI alone and produces ATP only (no NADP reduced, no water split).
Practice
Non-cyclic photophosphorylation produces:
Using PSI and PSII, it makes ATP and reduced NADP, and photolysis of water releases oxygen.
How the ATP is made
- In both forms:
- energetic electrons pass along an electron transport chain, releasing energy.
- that energy pumps protons across the thylakoid membrane.
- protons flow back into the stroma through ATP synthase, making ATP (chemiosmosis).
Practice
Photolysis is:
Photolysis splits water using light energy; the oxygen released is a by-product of photosynthesis.
Practice
In the thylakoid, ATP is made as protons flow back through:
Protons pumped into the thylakoid space flow back through ATP synthase, driving ATP synthesis.
You've got it
Key idea
- the light-dependent stage = photophosphorylation in the thylakoids
- cyclic: PSI only → ATP only
- non-cyclic: PSI + PSII → photolysis of water (releases O₂) → ATP + reduced NADP
- electrons pump protons → flow back through ATP synthase → ATP (chemiosmosis)