Chapter 7 Photosynthesis: The light reactions

1. Chapter 7 Photosynthesis: The light reactions  synthesis using light  is the only process of biological system that can harvest sunny energy. a deceptively simple equation 6 CO2 + 6 H2O C6H12O6 + 6 O2  stored energy can be used to power other cellular processes in the plant and provide the energy source for all forms of life.  Topics: the characteristics of light, the structure of the photosynthetic apparatus, the processes of chlorophyll excitation, the synthesis of ATP and NADPH. ☼

2. Light: a wave and a particle ＊ c (the speed of wave) =λν ＊ E (the energy of photon)= hν h : Planck’s constant (6.62 × 10-34 Js) ＊ Quantum (quanta) each photon contains an amount of energy discontinuous

3. Electromagnetic spectrum Sunlight like a rain of photons of different frequencies. UV-A: 320 – 400 nm UV-B: 280 – 320 nm UV-C :  290 nm Light emitting diode (LED) blue 470 nm red 660 nm

4. Absorption spectrum: the amount of light energy taken up or absorbed by a molecule or substance as a function of the light wavelength of the light. green

5. Action spectrum光譜:the magnitude of a response of a biological system to light, as a function of wavelength Light-induced phenomenon Carotenoid absorption: 400 ~500 nm → the pigment responsible for the particular light-induced phenomenon

6. O2 evolved Late 1800s

7. Photosynthetic pigments: accessory pigments porphyrin-like group phytol chain cyanobacteria red algae xanthophyll Chl a, b, c, d

8. 2. Chlorophyll a in nonpolar solvent 3. Chlorophyll b in nonpolar solvent 4. Phycoerythrobilin in an aqueous buffer 5. β-carotene in nonpolar solvent

9. Light absorption and emission by chlorophyll Chl + h → Chl* (10-9s), discontinuous 1. heat loss; 2. fluorescence; 3. energy transfer; 4. photochemistry.

10. pigments, e- transfer proteins Light energy Chemical energy Antenna天線 complex: collect light and physical resonance transfer, no chemical reactions; high efficiency of energy transfer. Reaction center: the chemical oxidation and reduction reactions leading to long-term energy storage take place. more efficiency Core/peripheral Chl b Chl a

11. The antenna funnels注入 energy to the reaction center— fluorescene resonance energy transfer p. 138 λmax of Chl b: 650 nm Chl a: 670 nm red shif A physical phenomenon directionality and irreversibility

12. 1932 Emerson and Arnold  oxygen production vs. flash energy –a suspension of Chlorella pyrenoidosa – 10-5 s flash, 0.1 s apart – varied light intensity強度, slope saturation  many chlorophyll molecules involve in energy conversion during photosynthesis – Quantum yield () No. of photochemical products / Total No. of quanta absorbed  0.1 for O2 production Quantum requirement  10 for O2 evolution

13. – Quantum efficiency量子效率≒ 1 engage in photochemistry / absorbed photons – Energy efficiency ≒ 0.25 the stored energy of chemical compound / the energy of absorbed photons CO2 + H2O light (CH2O) + O2 G = 467 kJ/mol red light: 467×103/10 ×17.1×104 blue light: 467×103/10 ×23.95×104 E = hν O2/10 photons EE low

14. Hill reaction (1937) an artificial electron acceptors in isolated chloroplast thylakoid No CO2 condition, still O2 production 4 Fe3+ + 2 H2O → 4 Fe2+ + O2 + 4 H+  CO2 + 2 H2S → (CH2O) + 2 S + H2O H218O + CO2 → (CH2O) + 18O2 (1)CO2 + 2 H2O → (CH2O) + O2 + H2O (2) CO2 + H2O → (CH2O) + O2 ☼ [In eukaryotic photosynthetic organisms, the reaction centers and most of the antenna complexes are integral within the membrane of chloroplast; in photosynthetic prokaryotes, the site of photosynthesis is the plasma membrane or membranes derived from it. P. 132]