Photosynthesis

1. Photosynthesis Chapter 10 It’s not simple being green

2. Objectives • Understand the difference between autotroph and heterotroph • Describe the location and structure of a chloroplast. Explain how chloroplast structure is related to its function • Recognize and explain the summary equation for photosynthesis • Understand the role of REDOX reactions in photosynthesis • Understand the properties of light discussed in class • Describe the relationship between action and absorption spectrum • Explain what happens when chlorophyll or accessory pigments absorb photons

3. Objectives continued • List the function and components of a photosystem • Compare cyclic photophosphorylation and Z-scheme electron flow and explain the relationship between these components of the light reactions • Summarize the light-dependent reactions of photosynthesis • Summarize the carbon fixing reactions of the Calvin cycle (light-independent reactions) • Describe the role of NADPH and ATP in the Calvin cycle

4. Overview of Photosynthesis • Process by which chloroplast bearing organisms transform solar light energy into chemical bond energy • 2 metabolic pathways involved • Light-dependent reactions: convert solar energy into cellular energy • Calvin Cycle (Light-independent reactions): reduce CO2 to CH2O • Organisms that can perform photosynthesis are called autotrophs whereas those that cannot are called heterotrophs

5. Photosynthesis Equation • Photosynthesis6CO2 +6H20 + light  C6H1206 + 6O2 • Reduction of carbon dioxide into carbohydrate via the oxidation of energy carriers (ATP, NADPH) • Light-dependent reactions energize the carriers • Calvin Cycle reactions produce the carbohydrate PGAL (phosphoglyceraldehyde)

6. Where is all this happening?

7. Structure of the Chloroplast • Thylakoid: membranous system within the chloroplast (site of light reactions). Segregates the chloroplast into thylakoid space and stroma. • Grana stacks of thylakoids in a chloroplast • Stroma: region of fluid between the thylakoids and inner membrane where Calvin Cycle occurs

8. Light • Electromagnetic energy travelling in waves • Wavelength (): distance from peak of one wave to the peak of a second wave • inverse relationship between wavelength and energy   energy

9. Visible Spectrum • The portion of the electromagnetic spectrum that our eyes can see • White light contains all  of the visible spectrum • Colors are the reflection of specific  within the visible spectrum •  not reflected are absorbed • Composition of pigments affects their absorption spectrum

10. Why are plants green? • Pigments contained within the chloroplast absorb most  of light but absorb the green  the least • Pigments include • Chlorophyll a • Chlorophyll b • Carotenoids • Carotenes • Xanthophylls

11. Absorption vs. Action • Absorbption spectrum is the range of wavelengths that can be absorbed by a pigment • Action spectrum means the wavelengths of light that trigger photosynthesis

12. Chlorophyll a • Is only pigment that directly participates in the light reactions • Other pigments add energy to chlorophyll a or dissipate excessive light energy • Absorption of light elevates an electron to a higher energy orbital (increased potential energy)

13. Photosystems • Collection of pigments and proteins found associated with the thylakoid membrane that harness the energy of an excited electron to do work • Captured energy is transferred between photosystem molecules until it reaches the chlorophyll molecule at the reaction center

14. What Next? • At the reaction center are 2 molecules • Chlorophyll a • Primary electron acceptor • The reaction-center chlorophyll is oxidized as the excited electron is removed through the reduction of the primary electron acceptor • Photosystem I and II

15. Photosystem II • When sufficient light energy is captured, the reaction center chlorophyll loses a pair of electrons • These electrons must be replaced to continue the process • Electrons from the splitting of water molecules replace those lost from photosystem II • Electrons used to make ATP

16. Photosystem I • Works in a similar fashion to photosystem II but does not receive electrons from water, rather, electrons from photosystem II are added to photosystem I • Ferredoxin received the electrons and passes them to an enzyme NADP+ reductase to make NADPH

17. Electron Flow When Working Together • Two routes for the path of electrons stored in the primary electron acceptors • Both pathways • begin with the capturing of photon energy • utilize an electron transport chain with cytochromes for chemiosmosis • Z-scheme electron flow • uses both photosystem II and I • electrons from photosystem II are removed and replaced by electrons donated from water • synthesizes ATP and NADPH • 4 electrons donated to Photosystem 2 through the conversion of 2 molecules of water into O2 and 2H+ • Cyclic (Photophosphorylation) flow • Uses photosystem I only • electrons from photosystem I are recycled • synthesizes ATP only

18. Z-scheme Electron Flow • Electron at reaction-center of photosystem II is energized • 2H2O split via enzyme catalysed reaction forming 4H+, 4e, and O2. Electrons move to fill orbital vacated by removed electrons 3 Each excited electron is passed along an electron transport chain fueling the chemiosmotic synthesis of ATP

19. Z-scheme Electron Flow 4 The electrons are now lower in energy and enters photosystem I via plactocyanin (PC) where they are re-energized 5 The electrons are then passed to a different electron transport system that includes the iron containing protein ferridoxin. The enzyme NADP+ reductase assists in the oxidation of ferridoxin and subsequent reduction of NADP+ to NADPH

20. Cyclic Photophosphorylation • Electrons in Photosystem I are excited and transferred to ferredoxin that then shuttles the electron to the cytochrome complex. • The electron then travels down the electron chain and re-enters photosystem I

21. Where are the photosystems found on the thylakoid membrane?

22. Chemiosmosis in 2 Organelles • Both the Mitochondria and Chloroplast generate ATP via a proton motive force resulting from an electrochemical inbalance across a membrane • Both utilize an electron transport chain primarily composed of cytochromes to pump H+ across a membrane. • Both use a similar ATP synthase complex • Source of “fuel” for the process differs • Location of the H+ “reservoir” differs

23. Calvin Cycle • Starts with CO2 and produces Glyceraldehyde 3-phosphate • Three turns of Calvin cycle generates one molecule of product • Three phases to the process • Carbon Fixation • Reduction of CO2 • Regeneration of RuBP

24. A molecule of CO2 is converted from its inorganic form to an organic molecule (fixation) through the attachment to a 5C sugar (ribulose bisphosphate or RuBP). • Catalysed by the enzyme RuBP carboxylase (Rubisco). • The formed 6C sugar immediately cleaves into 3-phosphoglycerate

25. Each 3-phosphoglycerate molecule receives an additional phosphate group forming 1,3-Bisphosphoglycerate (ATP phosphorylation) • NADPH is oxidized and the electrons transferred to 1,3-Bisphosphoglycerate cleaving the molecule as it is reduced forming Glyceraldehyde 3-phosphate

26. The final phase of the cycle is to regenerate RuBP • Glyceraldehyde 3-phosphate is converted to RuBP through a series of reactions that involve the phosphorylation of the molecule by ATP

27. Variations Anyone? • In hot/arid regions plants may run short of CO2 as a result of water conservation mechanisms • C4PhotosynthesisCO2 may be captured by conversion of PEP (Phosphoenolpyruvate) into oxaloacetate and ultimately malate that is exported to cells where the Calvin cycle is active • CAM Photosynthesis CO2 may be captured as inorganic acids that my liberate CO2 during times of reduced availability

28. Why are CAM and C4 versions necessary?