7. Photosynthesis: Light Reactions Read till next time (incl. this lesson): Biology of Plants 6th ed. pp. 126-153 7th ed. pp. 115-139 8th ed. pp. 122-149 [Plant Physiology (Taiz & Zeiger) pp. 195-224]

Autotrophs & Heterotrophs PHOTOSYNTHESIS CO2 + H2O >>> C(H2O) + O2 6CO2 + 6H2O >>> C6H12O6 + 6O2   (glucose) Other high- energy products RESPIRATION Autotrophs & Heterotrophs

Carbohydrate (energy rich) + O2 Light energy Respiration CO2 + H2O (energy poor) Energy for Biosynthesis, Active transport, Movement etc. Photosynthesis

‘Light energy’ = energy of photon = h c / λ Quanta Photons Photosynthetically Active Radiation (PAR) = radiation (400-700 nm)

So, all photons in PAR can give rise to photosynthesis… But mostly blue and red photons are used in photosynthesis! (Why?)

So, all photons in PAR can give rise to photosynthesis… But mostly blue and red photons are used in photosynthesis! However SOME green (and any other) photons are absorbed too, And IF ABSORBED, they too give rise to photosynthesis……

Fig. 2.3 http://www.doctortee.com/dsu/tiftickjian/bio100/photosynthesis.html

So, all photons in PAR can give rise to photosynthesis… But mostly blue and red photons are used in photosynthesis! However SOME green (and any other) photons are absorbed too, And IF ABSORBED, they too give rise to photosynthesis……… AS MUCH AS THE BLUE AND RED PHOTONS. So, the quantum yield* for photosynthesis is the same for all photons IRRESPECTIVE of their energy. (How can this be?... Later!) *quantum yield = = photosynthesis performed per photon ABSORBED (In percentage: can theoretically be 0-100% but is in reality 0 to 84%)

Quantum Yield 1 Quantum Yield

Quantum Yield (= photosynthesis per quantum absorbed)

An Overview of Photo-Synthesis Light Reactions “Dark” Reactions CO2 Fixation

The Structure of the Chloroplast Stroma Granum

The PHOTO-reactions of photosynthesis Light Reactions “Dark” Reactions CO2 Fixation

The Chlorophyll Structure Chlorophyll b Chlorophyll a Hydrophobic

Chlorophyll a MW ~ 950

The PHOTO-reactions of photosynthesis Light Reactions “Dark” Reactions CO2 Fixation

Photoexcitation / De-Excitation of Chlorophyll 1 (~<1%) 2 (~<10%) 3 Energy Transfer to Photosynthesis (~80-90%)

Photoexcitation / De-Excitation of Chlorophyll 1 (~<1%) 2 (~<10%)

So, all photons in PAR can give rise to photosynthesis… But mostly blue and red photons are used in photosynthesis! However SOME green (and any other) photons are absorbed too, And IF ABSORBED, they too give rise to photosynthesis……… AS MUCH AS THE BLUE AND RED PHOTONS. Therefore the quantum yield* for photosynthesis is the same for all photons IRRESPECTIVE of their energy. How can this be? *quantum yield = = photosynthesis performed per photon ABSORBED (In percentage: can theoretically be 0-100% but is in reality 0 to 84%)

(Heat) Fluorescence Energy transfer to Photosynthesis

Energy transfer and photochemistry Fluorescence Ph Fig. 2.4 Chlorophyll Heat Energy transfer and photochemistry Fluorescence Ph Blue-photon excitation level Red-photon excitation level Ground state red orange yellow green blue Electron energy level

The Chlorophyll Structure Chlorophyll b Chlorophyll a Hydrophobic

Phycobilisome Phycoerythrin Phycocyanin (Allo-phycocyanin) Thylakoid membrane

Tail T H Fig. 2.2 Head H

Fig. 4.3 Ch PSII APC PC PE Thylakoid membrane

The ‘cluster’ of pigment molecules = photosystem Chlorophyll b Chlorophyll a Hydrophobic

TWO photosystems (PS), PS I and PS II Quinone

How light is harvested 3 1 4 2 Resonance Energy Transfer

Photosystem ‘Antenna’ Resonance Energy Transfer Electron transfer Reaction Centre 4 Electron acceptor

TWO photosystems (PS), PS I and PS II Quinone

Primary Q Quinone P 680 / P 680+ In PS II H2O

Primary Fd Feredoxin P 700 / P700+ In PS I PS II (H2O)

-1.0V -0.8V Quinone Red-Ox Potential 0.8V

(mid-point) Redox potential (V) +0.8 +0.4 -0.4 -0.8 -1.0 PSII (Pheophytin) QA QB PSI “A” PQ Cyt b6/f PC P700+ Strong oxidant Strong reductant Fd (mid-point) Redox potential (V) (Thylakoid membrane) Ph Stroma Lumen Fig. 5.3 NADP+ NADPH H2O

NADP+ + 2e- + 2H+ > NADPH + H+ To OXIDISE to take away an electron(s) from another compound לחמצן To become oxidised to lose an electron(s) to another compound להתחמצן H2O > 2e- + 2H+ + ½ O2 2H2O > 4e- + 4H+ + O2 To REDUCE to give away an electron(s) to another compound לחזר To become reduced to gain an electron(s) from another compound להתחזר NADP+ + e- > NADPH NADP+ + 2e- + 2H+ > NADPH + H+

2 4 4

Why TWO Photosystems? -1.0V -0.8V Red-Ox Potential 0.8V

A mechanical analogy for the Light Reaction

Complexes -1.0V -0.8V Quinone Red-Ox Potential 2 3 1 0.8V

PS-I Cyt PS-II Complex 1 Complex 3 Complex 2 4e- 2H2O O2 +4H+ 2H2O

(mid-point) Redox potential (V) +0.8 +0.4 -0.4 -0.8 -1.0 PSII (Pheophytin) QA QB PSI “A” PQ Cyt b6/f PC P700+ Strong oxidant Strong reductant Fd (mid-point) Redox potential (V) (Thylakoid membrane) Ph Stroma Lumen Fig. 5.3 NADP+ NADPH H2O

A mechanical analogy for the Light Reaction

-1.0V -0.8V Quinone Red-Ox Potential 0.8V

To Calvin Cycle 10nm

PQ > PQH2 > P680+ “Splitting of Water”, “Photolysis”

Complex 4

pH ~ 8 pH ~ 4, Proton Motive Force (PMF)

Pi PSII PSI H2O 2e- 2H+ ½ O2 Cyt b6/f PQ H+ PQH2 H+ H+ H+ H+ H+ ADP+Pi ATP NADP+ NADPH+H+ PMF pH ~4 Fig. 5.5 Stroma pH ~8 Fd Thylakoid membrane ATP synthase PC Lumen

ATPase / ATP Synthase