LIGHT REACTION PHOTOSYNTHESIS PART I

 The Light Reaction occurs within the thylakoid – the inside sac and across the thylakoid membrane.  + It Needs Light energy, H2O, ADP, P, NADP  It creates O2, ATP and NADPH

1. Photosystem II and I (made of Chlorophyll) absorb the Light Energy (called photons).

2. O.E.A breaks down Water by photolysis (splitting water using light energy).  Oxygen Evolving Apparatus (O.E.A.) is an enzyme that breaks 2 water molecules + - into Hydrogen Ions (4H ), Oxygen Gas (O2) and Electrons (4e ) + - 1H2O  2H + O2 + 2e  The electrons pass into the PS II

 The Oxygen Gas (O2) is released into the atmosphere by the stomata of the leaf.  The H+ build up inside the thylakoid.

3. PS II excited electrons flow into PS I  The PS II uses the photons to excite electrons to a higher energy level which then moves through the electron transport chain to the PS I.

4. PS I excited electrons flow into NADPH reductase.  PS I uses the photons to excite electrons to a higher energy level which then moves through the 2nd part of the electron transport chain to the NADPH reductase (where they wait for step 7).

5. H+ flow out of the thylakoid into the stroma, through the ATP synthase.  The H+ from the break down of water build up on the inside of the thylakoid.  This creates a strong concentration gradient across the thylakoid membrane called an “electrochemical gradient”.  The H+ diffuse out of the thylakoid, through an ion protein channel called the ATP synthase, into the stroma.

6. ATP is formed by the ATP synthase (enzyme).  ADP and P are joined by the ATP synthase to make ATP.  Attaching P to ADP to form ATP using light energy is called photophosphorylation.  The energy provided to the ATP synthase comes from the electrochemical gradient of H+ flowing from the inside of the Thylakoid to the Stroma.

7. NADPH reductase (enzyme) forms NADPH from NADP, 2H+ and 2e-  The enzyme NADPH reductase joins NADP, Two hydrogen ions and Two electrons together. NADP+ + 2H+ + 2e-  NADPH + H+

8. Both the NADPH and ATP are formed which will be used in the Light Independent Reaction. Summary: The Light Reaction absorb the energy of sunlight and convert it to the energy that is stored in chemical bonds. The thylakoid membranes of the chloroplast contain clusters of pigments molecules, or photosystems, that are able to absorb the energy in sunlight. In green plants, there are two systems: photosystem I and photosystem II. Each photosystem contains several hundred molecules of chlorophyll. Once the light is absorbed, the electrons of chlorophyll become excited. This energy is passed along until it reaches a particular pair of molecules, which can process this energy. These electrons are transferred to an ETC (Electron Transport Chain) in the thylakoid membranes. A H+ gradient is set up and ATP is produced using an ATP synthase complex. This energy is needed to drive the Light InDependent (L-I-D) cycle. In addition NADP+ is reduced to NADPH, which is also sent to the L-I-D. As a result of the light reaction, water is split and oxygen is given off.

Light Reactions Questions

1. Where does the Light Reactions take place?

2. What are the main input components that are needed for L-R?

3. What is the importance of the Photosystems?

4. What starts the process of light reactions?

5. Describe the pathway of flow of electrons from H2O to NADPH.

6. List each of the enzymes involved in the light reactions.

7. What does each enzyme catalyzes?

8. What is meant by photolysis?

9. Which enzyme produces O2?

10. Describe how an “electrochemical gradient” is established across the thylakoid membrane and how it “helps” make ATP.

11. What is meant by photophosphoryllation? In what enzyme does this take place.

12. What are 3 important “products” of the light reactions?