1. Chapter 15 (part1)
Photosynthesis

2. The Sun - Ultimate Energy
1.5 x 1022 kJ falls on the earth each day
1% is absorbed by photosynthetic organisms and transformed into chemical energy
6CO2 + 6H2O  C6H12O6 + 6O2
1011 tons (!) of CO2 are fixed globally per year
Formation of sugar from CO2 and water requires energy
Sunlight is the energy source!

3. Photosynthesis: Light Reactions and Carbon Fixation
The light reactions capture light energy and convert it to chemical energy in the form of reducing potential (NADPH) and ATP with evolution of oxygen
During carbon fixation (dark reactions) NADPH and ATP are used to drive the endergonic process of hexose sugar formation from CO2 in a series of reactions in the stroma
Light: H2O + ADP + Pi + NADP+ + light  O2 + ATP + NADPH + H+
CF: CO2 + ATP + NADPH + H+  Glucose + ADP + Pi + NADP+
Sum: CO2 + light  Glucose + O2

4. Chloroplast
Inner and outer membrane = similar to mitochondria, but no ETC in inner membrane.
Thylakoids = internal membrane system. Organized into stromal and granal lammellae.
Thylakoid membrane - contains photosynthetic ETC
Thylakoid Lumen – aqueous interior of thylkoid. Protons are pumped into the lumen for ATP synthesis
Stroma – “cytoplasm” of chloroplast. Contains carbon fixation machinery.
Chloroplasts possess DNA, RNA and ribosomes

6. Conversion of Light Energy to Chemical Energy
Light is absorbed by photoreceptor molecules (Chlorophylls, carotenoids)
Light absorbed by photoreceptor molecules excite an electron from its ground state (low energy) orbit to a excited state (higher energy) orbit .

7. The high energy electron can then return to the ground state releasing the energy as heat or light or be transferred to an acceptor.
Results in (+)charged donor and (–)charged acceptor = charge separation
Charge separation occurs at photocenters.
Conversion of light NRG to chemical NRG

8. Photosynthetic Pigments

9. Chlorophyll Photoreactive, isoprene-based pigment
A planar, conjugated ring system - similar to porphyrins
Mg in place of iron in the center
Long chain phytol group confers membrane solubility
Aromaticity makes chlorophyll an efficient absorber of light
Two major forms in plants Chl A and Chl B

10. Accessory Pigments Carotenoid Phycobilin
Absorb light through conjugated double bond system
Absorb light at different wavelengths than Chlorophyll
Broaden range of light absorbed

11. Absorption Spectra of Major Photosynthetic Pigments

12. Harvesting of Light and Transfer of Energy to Photosystems
Light is absorbed by “antenna pigments” and transferred to photosystems.
Photosystems contain special-pair chlorophyll molecules that undergo charge separation and donate e- to the photosynthetic ETC

13. Resonance Transfer
Energy is transfer through antenna pigment system by resonance transfer not charge separation.
An electron in the excited state can transfer the energy to an adjacent molecule through electromagnetic interactions.
Acceptor and donor molecule must be separated by very small distances.
Rate of NRG transfer decreases by a factor of n6 (n= distance betwn)
Can only transfer energy to a donor of equal or lower energy

14. Photosynthetic Electron Transport and Photophosphorylation
Analogous to respiratory ETC and oxidative phosphorylation
Light driven ETC generates a proton gradient which is used to provide energy for ATP production through a F1Fo type ATPase.
The photosynthetic ETC generates proton gradient across the thylakoid membrane.
Protons are pumped into the lumen space.
When protons exit the lumen and re-enter the stroma, ATP is produced through the F1Fo ATPase.

15. Photosynthetic ETC

16. Eukaryotic Photosystems
PSI (P700) and PSII (P680)
PSI and PSII contain special-pair chlorophylls
PSI absorbs at 700 nm and PSII absorbs at 680 nm
PSII oxidizes water (termed “photolysis")
PSI reduces NADP+
ATP is generated by establishment of a proton gradient as electrons flow from PSII to PSI

17. Z-Scheme

18. The Z Scheme
An arrangement of the electron carriers as a chain according to their standard reduction potentials
PQ = plastoquinone
PC = plastocyanin
"F"s = ferredoxins
Ao = a special chlorophyll a
A1 = a special PSI quinone
Cytochrome b6/cytochrome f complex is a proton pump

19. P680(PSII) to PQ Pool

20. Excitation, Oxidation and Re-reduction of P680
Special pair chlorophyll in P680 (PS II) is excited by a photon
P680* transfer energy as a e- to pheophytin A through a charge separation step.
The oxidized P680+ is re-reduced by e- derived from the oxidation of water

21. Oxygen evolution by PSII
Requires the accumulation of four oxidizing equivalents
P680 has to be oxidized by 4 photons
1 e- is removed in each of four steps before H2O is oxidized to O2 + 4H+
Results in the accumulation of 4 H+ in lumen

23. Electrons are passed from Pheophytin to Plastoquinone
Plastoquinone is analagous to ubiquinone
Lipid soluble e- carrier
Can form stable semi-quinone intermediate
Can transfer 2 electrons on at a time.

24. Transfer of e- from PQH2 to Cytbf Complex (another Q-cycle)
Electrons must be transferred one at a time to Fe-S group.
Another Q-cycle
First PQH2 transfers one electron to Fe-S group, a PQ- formed. 2 H+ pumped into lumen
A second PQH2 transfers one electron to Fe-S group and the one to reduce the first PQ- to PQH2. 2 more H+ pumped into lumen
4 protons pumped per PQH2. Since 2 PQH2 produced per O2 evolved 8 protons pumped

26. Terminal Step in Photosynthetic ETC
Electrons are transferred from the last iron sulfur complex to ferredoxin.
Ferredoxin is a water soluble protein coenzyme
Very powerful reducing agent.
Ferredoxin is then used to reduce NADP+ to NADPH by ferredoxin-NADP+ oxidoreductase
So NADP+ is terminal e- accepter