Light-independent reactions: Do not use light directly but rely on light-dependent products
Both reactions occur in the chloroplast:
Light-dependent reactions: Across the thylakoid membrane
Light-independent reactions: In the stroma
Light-dependent Reactions
Photolysis: Splitting of water molecules using light
Produces:
2 Hydrogen ions (2H+)
2 Electrons (2e-)
1 Oxygen atom (O)
Oxygen is a waste product; Hydrogen ions and electrons are used in reactions
Conversion of light energy into chemical energy in the form of ATP and reduced NADP (NADPH)
NADP: Coenzyme transferring hydrogen; reduces other molecules
Reduced NADP (NADPH): Gains hydrogen & electrons
Reduction: Gain of electrons, gain of hydrogen, loss of oxygen
Oxidation: Loss of electrons, loss of hydrogen, gain of oxygen
ATP and NADPH: Used in light-independent reactions
Production of ATP and NADPH
Occurs via photophosphorylation (light + phosphorylation)
Non-cyclic photophosphorylation: Produces both ATP and NADPH
Cyclic photophosphorylation: Produces ATP only
Involves:
Electron transport chain: Series of proteins; electrons release energy as they move
Chemiosmosis: Production of ATP from energy released in electron transport
Non-cyclic Photophosphorylation
Light hits photosystem II in thylakoid membrane
Excites electrons to higher energy level
Electrons move to an electron transport chain
Electrons replaced by those from photolysis of water
Energy from electrons enables chemiosmosis:
H+ ions pumped into thylakoid space, creating gradient
H+ ions diffuse back via ATP synthase, producing ATP
Electrons pass to photosystem I, excited again
Combine with H+ and NADP to form NADPH
NADPH and ATP used in the light-independent reactions
Cyclic Photophosphorylation
Light hits photosystem I
Electrons excited to higher energy level
Electrons pass through electron transport chain, releasing energy
Energy drives chemiosmosis:
H+ ions pumped into thylakoid space, creating gradient
H+ ions diffuse back via ATP synthase, producing ATP
Electrons cycle back to photosystem I
ATP enters light-independent reactions
Conclusion
The products of the light-dependent reactions are ATP, NADPH, and oxygen. Redundant oxygen is expelled, while ATP and NADPH proceed to support the light-independent reactions.