1. Photosynthesis:
The Light Reactions

2. Photosynthesis: Overview

3. The Light Reactions

4. 4.3 The Light Reactions General Summary Info
Two events of photosynthesis occur: absorption of solar energy & conversion of it into chemical energy (in the form of ATP).
Chlorophyll in the thylakoid absorbs the light, water is oxidized (loses e-) into H+ and O2, & light energy is converted into chemical energy.

5. Pigments/Photosystems
The light absorbing pigments form clusters in the thylakoid membrane.
2 types of clusters: Photosystem (PS) I and II.
The PS absorbs light energy, which is transferred to different molecules until it ends up at a reaction center
Reaction center = a specific chlorophyll a molecule
The reaction center accumulates so much energy that some of its e- jump to electron carriers

6. 4.3 The Light Reactions

7. Electron Movement & Replacement
These electron carriers form an electron transport system (ETS) between the 2 photosystems.
The e- move from PS II to PS I.
To replace e- lost from PS II, an enzyme near its reaction center splits (oxidizes) water into protons (H+), electrons, & oxygen gas.
2 H2O  4 H+ + 4 e- + O2

8. The Light Reactions

9. Oxidation of Water
When the enzyme oxidizes the water, O2 is released as a gas (waste) & protons/H+ build up in the thylakoid.
The e- replace the lost e- in PS II.

10. Production of NADPH
When e- from H2O reach PS I, they receive an energy boost from the reaction center & this energy is used to reduce NADP+ into NADPH.
Both the H+ (protons) and e- from water are used to convert NADP+ into NADPH (this is a reduction)
The NADPH is then used later to reduce CO2 in the Calvin Cycle (it is sometimes thought of as a H+ carrier).

11. The Light Reactions

12. Electron Transport & ATP
As e- flow through ETS, some of the energy gained by the e- carriers powers the active transport of H+ across the thylakoid membrane (from stroma in to thylakoid space).
A large number of H+ build up inside the thylakoid causing a difference in charge, which creates a difference in potential energy.
This potential energy can do work.

13. 4.3 The Light Reactions
The H+ diffuse out of the thylakoid through an enzyme called ATP synthase & as they pass through, they transfer energy to the ATP synthase.
ATP synthase uses the energy to synthesize ATP from ADP & a phosphate.

15. 4.3 The Light Reactions A Summary
Energy from light forces electrons to flow from water to NADP+ (forming NADPH).
While moving, e- provide energy to carriers to actively transport H+ into thylakoid. The H+ will eventually diffuse out through the enzyme ATP synthase, which creates ATP.

16. 4.3 The Light Reactions Overall End Results O2 produced as a waste.
NADP+ is reduced to NADPH.
ATP is produced by diffusion of H+

18. Light Reactions Animation
Light Reaction Animation