1. Photosynthesis: The Light Reactions & The Calvin Cycle

2. Photosynthesis: Overview

3. 4.3 The Light Reactions General Summary Info  Conversion of visible light into chemical energy needed to produce sugars in the Calvin cycle  Chlorophyll in the thylakoid absorb the light, water is split into Hydrogen and Oxygen, & light energy is converted into chemical energy.

4. 4.3 The Light Reactions Light absorbing pigments form 1 of 2 kinds of clusters: Photosystem (PS) I and II. Light absorbing pigments form 1 of 2 kinds of clusters: Photosystem (PS) I and II. The PS absorbs light energy & this energy gets transferred to different molecules until it ends up at a reaction center The PS absorbs light energy & this energy gets transferred to different molecules until it ends up at a reaction center Reaction center – a specific chlorophyll a molecule Reaction center – a specific chlorophyll a molecule The reaction center accumulates so much energy that some of its e - jump to electron carriers The reaction center accumulates so much energy that some of its e - jump to electron carriers

5. 4.3 The Light Reactions

6. These electron carriers form an electron transport system between the 2 photosystems. These electron carriers form an electron transport system between the 2 photosystems. Electrons from PS II replace electrons lost from PS I. Electrons from PS II replace electrons lost from PS I. PS II receives replacements from an enzyme near its reaction center that splits water into protons, electrons, & oxygen. PS II receives replacements from an enzyme near its reaction center that splits water into protons, electrons, & oxygen. 2 H 2 O  4 H + + 4 e - + O 2

7. 4.3 The Light Reactions When the enzyme oxidizes the water, oxygen is released as a gas & protons build up in the thylakoid. When the enzyme oxidizes the water, oxygen is released as a gas & protons build up in the thylakoid. The e - replace the lost e - in PS II. The e - replace the lost e - in PS II.

8. 4.3 The Light Reactions When e - from H 2 O reach PS I, they receive an energy boost from the reaction center & this energy is used to reduce NADP + into NADPH. When e - from H 2 O reach PS I, they receive an energy boost from the reaction center & this energy is used to reduce NADP + into NADPH. Both the protons and electrons from water are used to convert NADP + Both the protons and electrons from water are used to convert NADP + The protons & electrons of NADPH is then used later to reduce CO 2 in the Calvin Cycle. The protons & electrons of NADPH is then used later to reduce CO 2 in the Calvin Cycle.

9. 4.3 The Light Reactions

10. As electrons are flowing, some of the solar energy powers the active transport of H+ across the thylakoid membrane. As electrons are flowing, some of the solar energy powers the active transport of H+ across the thylakoid membrane. So a large number of H+ build up inside the thylakoid causing a difference in charge, which creates a difference in potential energy (think of a battery). So a large number of H+ build up inside the thylakoid causing a difference in charge, which creates a difference in potential energy (think of a battery). Just like a battery, this potential energy can do work. Just like a battery, this potential energy can do work.

11. 4.3 The Light Reactions The protons diffuse out of the thylakoid through an enzyme complex (ATP synthetase) & as they pass through, they transfer energy to the ATP synthetase. The protons diffuse out of the thylakoid through an enzyme complex (ATP synthetase) & as they pass through, they transfer energy to the ATP synthetase. ATP synthetase uses the energy to synthesize ATP from ADP & a phosphate. ATP synthetase uses the energy to synthesize ATP from ADP & a phosphate.

12. 4.3 The Light Reactions A Summary Energy from light forces electrons to flow from water to NADP+. Energy from light forces electrons to flow from water to NADP+. The electrons retain the energy in NADPH, which is then used to synthesize ATP. The electrons retain the energy in NADPH, which is then used to synthesize ATP. So the light reactions, convert light energy into the chemical energy found in ATP and NADPH, with the overall products being O 2, ATP, & NADPH. So the light reactions, convert light energy into the chemical energy found in ATP and NADPH, with the overall products being O 2, ATP, & NADPH.

13. Light Reactions Animation http://www.stolaf.edu/people/giannini/flashanim at/metabolism/photosynthesis.swf

14. 4.4 The Calvin Cycle General Summary Intro Saves the chemical energy produced in the light reactions in the form of sugars Saves the chemical energy produced in the light reactions in the form of sugars The Calvin Cycle occurs in the stroma of the chloroplasts. The Calvin Cycle occurs in the stroma of the chloroplasts.

15. 4.4 The Calvin Cycle 1. CO2 combines with a 5 carbon phosphate- sugar, called ribulose biphosphate (RuBP). This is called carbon fixation because carbon dioxide gas is “fixed” into an organic molcule. This produces an unstable 6 carbon molecule, which instantly breaks down into two 3 carbon molecules called phosphoglyceric acid (PGA). Catalyzed by Rubisco

16. 4.4 The Calvin Cycle 2. Each molecule of PGA is reduced to the 3 carbon sugar-phosphate molecule, phoshpoglyceraldehyde (PGAL). This requires 1 ATP and 1 NADPH (from the light reactions).

17. 4.4 The Calvin Cycle 3. A series of enzymes catalyzes the combination and rearrangement of the PGAL, producing a 5 carbon sugar-phosphate, ribulose 5-phosphate.

18. 4.4 The Calvin Cycle 4. An ATP molecule is used to add a 2 nd phosphate group to the Ribulose 5-phosphate, producing a molecule of the starting product, RuBP (completing the cycle).

19. 4.4 The Calvin Cycle Three turns of the cycle (which uses 3 CO 2 molecules) results in 6 PGAL molecules. Three turns of the cycle (which uses 3 CO 2 molecules) results in 6 PGAL molecules. 5 of those PGAL are used to regenerate RuBP & the last one is available to the organism to use for maintenance & growth. 5 of those PGAL are used to regenerate RuBP & the last one is available to the organism to use for maintenance & growth. The PGAL is removed from the Calvin Cycle to synthesize other compounds like complex carbohydrates or amino acids. The PGAL is removed from the Calvin Cycle to synthesize other compounds like complex carbohydrates or amino acids.

22. Animation of The Calvin Cycle http://www.sinauer.com/cooper/4e/animations03 05.html