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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