Light-dependent reactions of photosynthesis

[Pages:142]Light-dependent reactions of photosynthesis

Tom Donald ? 2015 American Society of Plant Biologists

Lesson outline

? Photosynthesis overview ? Evolution and diversity of photosynthesis ? Light and pigments ? The light response curve and quantum efficiency ? Plastids and chloroplasts ? Structure and function of photosynthetic complexes ? Pathways of electron transport ? Damage avoidance and repair: Acclimations to light ? Monitoring light reactions ? Optimizing and improving photosynthesis ? Artificial photosynthesis ? Photosynthetic fungi and animals

? 2015 American Society of Plant Biologists

Overview: Photosynthesis captures light energy as reduced carbon

Energy input from sunlight

"High energy", reduced carbon

"Low energy", oxidized carbon

in carbon dioxide

Oxygen is released as a byproduct

6 CO2 + 6 H2O C6H12O6 + 6 O2

Light-dependent reactions The first step is the capture of light energy as ATP and reducing power, NADPH

ATP

NADPH

Light-independent reactions The second step is the transfer of

energy and reducing power from

ATP and NADPH to CO2, to produce high-energy, reduced sugars

? 2015 American Society of Plant Biologists

Photosynthesis is two sets of connected reactions

2 NADPH

e-

2 H+

2 NADP+

2 H2O

O2 + 2 H+ + 2 e-

ADP ATP H+

Chloroplast

The LIGHT reactions take place in the thylakoid membranes

The CARBON-FIXING reactions take place in the chloroplast stroma

Adapted from Kramer, D.M., and Evans, J. R. (2010). The importance of energy balance in improving photosynthetic productivity. Plant Physiol.155: 70?78. ? 2015 American Society of Plant Biologists

Light reactions (usually) take place in thylakoid membranes

Prokaryotes

Gloeobacter violaceus, the only cyanobacterium without thylakoid membranes

Eukaryotes

Chlamydomonas reinhardtii, a model green algae

PLANT

Light-induced differentiation

Synechocystis spp. PCC6803, a model cyanobacterium

Proplastid

Chloroplast

Reproduced with permission ? Annual Reviews of Plant Biology Nickelsen, J. and Rengstl, B. (2013).Photosystem II assembly: From cyanobacteria to plants. Annu. Rev. Plant Biol. 64: 609-635. ? 2015 American Society of Plant Biologists

Light reactions produce O2, ATP and NADPH

The reactions require several large multi-protein complexes: two light harvesting photosystems (PSI and PSII), the cytochrome b6f complex, and ATP synthase

Cytochrome b6f complex

ADP ATP

2 H2O O2 + 2 H+ + 2 e-

H+

2 NADPH

e-

2 H+

2 NADP+

Photosystem I (PSI)

Photosystem II (PSII)

ATP synthase

Adapted from Kramer, D.M., and Evans, J. R. (2010). The importance of energy balance in improving photosynthetic productivity. Plant Physiol.155: 70?78. ? 2015 American Society of Plant Biologists

Chlorophyll captures light energy to initiate the light reactions

First step of photochemistry

Chlorin ring captures photons

Chlorophyll is held in pigmentprotein complexes in a highly organized manner

Photon

Chl

Photon capture by chlorophyll excites the chlorophyll (Chl*). Chl* can lose an electron to become oxidized chlorolphyll (Chl+)

Chl*

e-

H+

Chl+ e-

H2O

O2

Chl+ is reduced by stripping an electron from water, releasing oxygen and protons

Buchanan, B.B., Gruissem, W. and Jones, R.L. (2015) Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists. ? 2015 American Society of Plant Biologists

Excited chlorophyll can release energy in several ways

Photon

Photochemistry (first step in photosynthesis) Chl* Fluorescence

Non-photochemical quenching (e.g., heat)

3Chl* Alternatively, it can convert to a damaging triplet state Chl

The fate of captured light energy and photosynthetic efficiency depend on many factors including temperature, water availability, nutrient availability, stress etc.

? 2015 American Society of Plant Biologists

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