1. PHOTOSYNTHESIS Dr. C. P. Upadhyaya

2. Outline How is solar energy captured and transformed into metabolically useful chemical energy? What are the general properties of photosynthesis? How is solar energy captured by chlorophyll? What is the molecular architecture of photosynthetic reaction centers? What is the quantum yield of photosynthesis? How does light drive the synthesis of ATP? How is carbon dioxide used to make organic molecules? How does photorespiration limit CO 2 fixation?

3. The Sun - Ultimate Energy  1.5 x 10 22 kJ of sunlight energy falls on the earth each day.  1% is absorbed by photosynthetic organisms and transformed into chemical energy.  6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2  10 11 tons of CO 2 are fixed globally per year.  Formation of sugar from CO 2 and water requires energy.  Sunlight is the energy source.

4. An anabolic, endergonic, carbon dioxide (CO 2 ) requiring process that uses light energy (photons) and water (H 2 O) to produce organic macromolecules (glucose). Definition of Photosynthesis

5. Where does the Photosynthesis takes place?  Photosynthesis occurs in thylakoid membranes of Chloroplasts.  This Chloroplast has paired folded structures called lamellae that stack to form “Grana“.  The soluble portion of the chloroplast is known as "stroma“.  The interior of the thylakoid vesicles is the "thylakoid lumen"  Chloroplasts possess DNA, RNA and ribosomes.

6. Guard Cell Carbon Dioxide (CO 2 ) Cell Wall Nucleus Chloroplast Central Vacuole Where does photosynthesis take place? Leaves A. stoma B. Mesophyll cells Oxygen (O 2 ) Location: 1. Leaves a. Stoma b.Mesophyll cells

7. The Chloroplast Schematic diagram of an idealized choloplast. Electron micrograph of a chloroplast

8. Photosynthesis Consists of Both Light Reactions and Dark Reactions  Light reactions are associated with the thylakoid membranes, capture light energy and convert it to chemical energy in the form of reducing potential (NADPH) and ATP with evolution of oxygen.  Dark reactions (also called carbon reaction) occurs in the stroma, use NADPH and ATP to drive the endergonic process of hexose sugar formation from CO 2 in a series of reactions in the stroma.

9. Chlorophyll Molecules Located in the thylakoid membranes. Chlorophyll have Mg + in the center. Chlorophyll pigments harvest energy (photons) by absorbing certain wavelengths (blue-420 nm and red-660 nm are most important). Plants are green because the green wavelength is reflected, not absorbed.  Chlorophyll is a photoreactive, isoprene-based pigment.  Mg 2+ is coordinated in the center of the planar conjugated ring structure.  A long chain alcohol, Phytol, group confers membrane solubility.  Aromaticity makes chlorophyll an efficient absorber of light.  Light absorption promotes an electron to a higher orbital, enhancing the potential for transfer of this electron to a suitable acceptor.

10. Chemical structure of Chlorophyll? Since the absorption spectra for a and b differ, plants that possess both can harvest a wider spectrum of incident energy.

11. Accessory light-harvesting pigments increase the possibility for absorption of light Structures of representative accessory light-harvesting pigments in photosynthetic cells. (a) β-Carotene, an accessory light-harvesting pigment in leaves. (b) Phycocyanobilin, a blue pigment found in cyanobacteria.

12. Question: During the fall, what causes the leaves to change colors? During the fall, what causes the leaves to change colors? In addition to the chlorophyll pigments, there are other pigments present. During the fall, the green chlorophyll pigments are greatly reduced revealing the other pigments. Carotenoids are pigments that are either red or yellow.

13. The first step in photosynthesis : the light absorption 1. Light Reactions  The photosynthetic unit consists of several hundred light-capturing chlorophylls plus a pair of special chlorophylls in the reaction center.  Light is captured by one of the "antenna chlorophylls" and routed from one to the other until it reaches the reaction center that is photochemically active.  The Reaction Center is the Place where the photoexcitation of electrons and electron transfer Occurs. The basis of photosynthesis is transduction of light energy into chemical energy. Photon absorption raises chlorophyll (Chl) to Chl*. Electron transfer from Chl* to an adjacent molecule A, producing oxidized Chl (Chl + ) and reduced A (A - ). Oxidation of A - eventually culminates in reduction of NADP + to NADPH.

14. Photosynthetic Units Consist of Many Chlorophyll Molecules but Only a Single Reaction Center The photosynthetic unit consists of several hundred light-capturing chlorophylls plus a pair of special chlorophylls in the reaction center. Light is captured by one of the "antenna chlorophylls" and routed from one to the other until it reaches the reaction center chlorophyll dimer that is photochemically active. Oxidation of chlorophyll leaves a cationic free radical, Chl +, whose properties as an electron acceptor are important to photosynthesis.

15. What Kinds of Photosystems Are Used to Capture Light Energy?  Oxygenic phototrophs have two distinct photosystems: PSI (P700) and PSII (P680). PSI systems have a maximal red light absorption at 700 nm and use ferredoxins as terminal electron acceptors. PSII have a maximal red absorption at 680 nm and use quinones as terminal electron acceptors. All chlorophyll is protein-bound – as part of either PSI or PSII or light-harvesting complexes (LHCs)

16. PSI and PSII Participate in the Overall Process of Photosynthesis What do PSI and PSII do? PSI provides reducing power in the form of NADPH. PSII splits water, producing O 2, and feeds the electrons released into an electron transport chain that couples PSII to PSI. Electron transfer between PSII and PSI pumps protons for chemiosmotic ATP synthesis. Essentially, electrons flow from H 2 O to NADP +, driven by light energy absorbed at the reaction centers. Light-driven phosphorylation of ADP to make ATP is termed photophosphorylation.

17. The Pathway of Photosynthetic Electron Transfer Is Called the Z Scheme The electron carriers are arranged as a chain, according to their standard reduction potentials Such an arrangement resembles the letter “Z” Thus the pathway name – the Z scheme Its components: PQ = plastoquinone PC = plastocyanin "F"s = ferredoxins A o = a special chlorophyll a A 1 = a special PSI quinone Cytochrome b 6 /cytochrome f complex is a proton pump

18. The Z Scheme The Z scheme of photosynthesis. (a)A diagrammatic representation. (b)The functional relationships among PSI, PSII, the cyt bf complex, and the ATP synthase in the thylakoid membrane.

19. More about Z scheme Electrons from PSII Are Transferred to PSI via the Cytochrome b6f Complex. The cytochrome b 6 f complex is a large multimeric protein possessing 26 transmembrane α-helices. This complex is homologous to the cytochrome c complex of mitochondria. The purpose of this complex is to mediate the transfer of electrons from PSII to PSI and to pump protons across the thylakoid membrane. Plastocyanin (PC in the Z scheme) is a small copper-containing protein that carries electrons from cytochrome b 6 f to PSI. The copper in PC cycles between the reduced Cu + and oxidized Cu 2+ states in this transfer.

20. How Does PSII Generate O 2 From H 2 O? The oxidation of H 2 O to O 2 is chemically difficult. The oxygen-evolving complex (OEC) is a large globular protein domain on the lumenal side of the PSII. The active site of the OEC contains a cube-like metal cluster of four Mn ions, one Ca+2 ion, and five O atoms bridging the Mn atoms. Electron removal from H 2 O molecules at the cluster (one from each Mn) facilitates O 2 formation.

21. What Is the Quantum Yield of Photosynthesis? The quantum yield of photosynthesis is defined as the amount of product formed per equivalent of light input i.e., the amount of O 2 evolved per photon. Four photons per reaction center drive the evolution of 1 O 2, reduction of 2 NADP +, and the translocation of 12 H +. Current estimates suggest that 3 ATPs are formed for every 12 H + translocated.

22. How Does Light Drive the Synthesis of ATP? Photophosphorylation: The transduction of the electrochemical gradient into the chemical energy of ATP is carried out by the chloroplast ATP synthase, more properly called the CF 1 CF 0 -ATP synthase. Electron transfer through the proteins of the Z scheme drives the generation of a proton gradient across the thylakoid membrane. Protons pumped into the lumen of the thylakoids flow back out, driving the synthesis of ATP. CF 1 -CF o ATP synthase is similar to the mitochondrial ATP synthase.

23. Cyclic Photophosphorylation Generates ATP but Not NADPH or O 2 In cyclic photophosphorylation, the photo-excited electron removed from P700 returns to P700 in a pathway indicated in picture Cyclic Photophosphorylation depends only on PSI, not on PSII P700 Primary Electron Acceptor e-e- e-e- e-e- e-e- ATP produced by ETC Photosystem I Accessory Pigments SUN Photons

24. B. Noncyclic Electron Flow  Occurs in the thylakoid membrane, Uses PS II and PS I, P680 reaction center (PSII) - chlorophyll a, P700 reaction center (PS I) - chlorophyll a, Uses Electron Transport Chain (ETC), Generates O 2, ATP and NADPH P700 Photosystem I P680 Photosystem II Primary Electron Acceptor Primary Electron Acceptor ETC Enzyme Reaction H 2 O 1/2O 2 1/2O 2 + 2H + ATP NADPH Photon 2e - SUN Photon

25. Noncyclic Electron Flow ADP +  ATP NADP + + H  NADPH Oxygen comes from the splitting of H 2 O, not CO 2 H 2 O  1/2 O 2 + 2H + (Reduced) P (Oxidized)

26. Chemiosmosis Powers ATP synthesis. Located in the thylakoid membranes. Uses ETC and ATP synthase (enzyme) to make ATP. Photophosphorylation: addition of phosphate to ADP to make ATP.

28. Next Lecture: About Calvin cycle (Synthesis of glucose using CO2 molecule)