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© 2015 American Society of Plant Biologists

Tom Donald

Light-dependent reactions

of photosynthesis

https://www.flickr.com/photos/clearwood/9058871983/in/album-72157626284715720/


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


Overview: Photosynthesis captures light

energy as reduced carbon

“Low energy”,

oxidized carbon

in carbon

dioxide

Energy input

from sunlight

“High

energy”,

reduced

carbon

Oxygen is

released as

a byproduct

6 CO2 + 6 H2O C6H12O6 + 6 O2

The first step is the capture of

light energy as ATP and

reducing power, NADPH

The second step is the transfer of

energy and reducing power from

ATP and NADPH to CO2, to produce

high-energy, reduced sugars

ATP NADPH

Light-dependent reactions

Light-independent reactions


Photosynthesis is two sets of connected

reactions

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

Chloroplast 2 H2O O2 + 2 H+ + 2

ADP H+

ATP

The LIGHT reactions take place in

the thylakoid membranes

The CARBON-FIXING

reactions take place in

the chloroplast stroma

2 NADP+

2 NADPH

2 H+ e−

e−

http://www.plantphysiol.org/content/155/1/70.full




Light reactions (usually) take place in

thylakoid membranes

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.

Chloroplast Proplastid

Light-induced

differentiation

Gloeobacter violaceus,

the only cyanobacterium

without thylakoid

membranes

Chlamydomonas

reinhardtii, a model

green algae

Synechocystis spp. PCC6803,

a model cyanobacterium

PLANT

Prokaryotes Eukaryotes

http://www.annualreviews.org/doi/abs/10.1146/annurev-arplant-050312-120124




Light reactions produce O2, ATP and

NADPH

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

2 H2O O2 + 2 H+ + 2

ADP

H+

ATP

ATP synthase Photosystem II (PSII)

Photosystem

I (PSI)

Cytochrome

b6f complex The reactions

require several

large multi-protein

complexes: two

light harvesting

photosystems (PSI

and PSII), the

cytochrome b6f

complex, and ATP

synthase

2 NADP+

2 NADPH

2 H+ e−

e−





Chlorophyll captures light energy to

initiate the light reactions

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

First step of

photochemistry

Chlorophyll is

held in pigment-

protein

complexes in a

highly organized

manner

Chlorin ring

captures photons

Photon capture by

chlorophyll excites the

chlorophyll (Chl*). Chl*

can lose an electron to

become oxidized

chlorolphyll (Chl+)

Chl

Chl* e−

Photon

Chl+

H2O

e−

H+

O2

Chl+ is reduced by

stripping an electron

from water, releasing

oxygen and protons

http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0470714220.html


Excited chlorophyll can release energy in

several ways

Chl

Chl* Photon

Photochemistry (first step in photosynthesis)

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

The fate of captured light energy and photosynthetic efficiency

depend on many factors including temperature, water availability,

nutrient availability, stress etc.

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


Heat, drought, & other stresses affect

photosynthetic efficiency

Light reactions

Carbon-fixing reactions

ATP NADPH ADP NADP+

The light reactions

and carbon-fixing

reactions are linked

through pools of

ATP/ADP and

NADPH/NADP+

For example….

• High temperature affects protein complex stability and thylakoid membrane fluidity

• Low temperatures slow enzyme-catalyzed reactions

• Drought causes stomata to close, lowering CO2 uptake and carbon-fixing reactions

• Nutrient deficiency or toxicity can affect electron transfer machinery


Oxygenic photosynthesis requires TWO

photosystems

P680+

P680

En

erg

y

Redox p

ote

ntial eV

Strong

reductant

Strong

oxidant

−1.5

−1.0

−0.5

0.0

0.5

1.0

Oxidizers remove

electrons from other

species

Reductants donate

electrons to other

species

2 H2O

4 H+ O2

4

P700*

PSII

P680*

P700+

P700

PSI

P680+ is a very strong

oxidant – strong enough to

pull electrons from H2O

NADP+

e−

NADPH

P700* is a very strong

reductant – strong

enough to donate

electrons to NADP+

e−


PSI & PSII are connected by an electron

transport chain

P680+

P680

En

erg

y

Redox p

ote

ntial eV

Strong

reductant

Strong

oxidant

−1.5

−1.0

−0.5

0.0

0.5

1.0

Oxidizers remove


species

Reductants donate

electrons to other

species

2 H2O e−

4 H+ O2

4

P700*

PSII

P680*

P700+

P700

PSI

NADP+

e−

NADPH

Cyt b6f

PQ

PC

The electron transport chain

generates proton-motive force that

drives ATP production


This diagram is known as a Z-scheme

P680+

P680

En

erg

y

Redox p

ote

ntial eV

Strong

reductant

Strong

oxidant

−1.5

−1.0

−0.5

0.0

0.5

1.0

Oxidizers remove


species

Reductants donate

electrons to other

species

2 H2O e−

4 H+ O2

4

P700*

PSII

P680*

P700+

P700

PSI

NADP+

e−

NADPH

Cyt b6f

PQ

PC





PSI can function without PSII, but it

doesn’t produce oxygen or NADPH

P700*

P700+

P700

PSI

Cyt b6f

PQ

PC




Cyclic electron transport:

• Involves PSI

• Does not involve PSII

• Involves the electron transport chain

• Results in ATP production

• Does not liberate O2

• Does not produce NADPH


The photosystems are embedded in

thylakoid membranes

PSI

Cyt b6f

PC

2 NADP+

2 NADPH

2 H+ e−

2 H2O e−

4 H+ O2

4 PSII

PQ

Cytochrome b6f

(Cyt b6f) is a multiprotein

membrane-embedded

complex

Plastocyanin (PC) is a

small protein and

mobile electron carrier

Plastoquinone (PQ) is

a small molecule and

mobile electron carrier

Thylakoid

Membrane

LUMEN

STROMA

STROMA

LUMEN

Plastid

PQ


Electrical and H+ gradients drive ATP

synthesis

ADP + Pi

PSI

Cyt b6f

2 NADP+

2 NADPH

2 H+ e−

PSII

H+

ATP

[H+]

Proton

gradient

from high

(in) to low

(out)

2 H2O e−

4 H+ O2

4

Thylakoid

Membrane

ATP

Synthase


Products of the light-dependent reactions

drive carbon-fixation

ADP

H+

ATP

NADP+

NADPH

H+

Light-dependent reactions Carbon-fixing reactions

Adapted from: Buchanan, B.B., Gruissem, W. and Jones, R.L. (2000) Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists.

Carboxylation

CO2

ATP

ADP + Pi

NADP+ + H+

NADPH

Energy

input

Reducing

power input

Glyceraldehyde 3-

phosphate (GAP)

For every 3 CO2 fixed, one

GAP is produced for

biosynthesis and energy

1 x GAP

Calvin-

Benson

Cycle

Each CO2 fixed

requires 3 ATP

and 2 NADPH Rubisco

Ribulose-1,5-

bisphosphate

Regeneration

http://www.aspb.org/publications/biotext/


Lesson outline














Evolution and diversity of

photosynthesis

There are two types of reaction centers, Type I & Type II

Each type is found in various photosynthetic bacteria

Both types are found in cyanobacteria and chloroplasts

Type I are iron-sulfur

reaction centers

Type II are pheophytin-

quinone reaction centers

PSII

PSI

Note that

oxygenic

photosynthesis

requires Type I

& Type II

reaction centers

working in series O2

Most

photosynthetic

prokaryotes use

only a single

type of reaction

center and do

not release

oxygen

Reprinted from Allen, J.P. and Williams, J.C. (1998). Photosynthetic reaction centers. FEBS Letters. 438: 5-9 with permission from Elsevier.

http://www.febsletters.org/article/S0014-5793%2898%2901245-9/abstract




Type I & Type II reaction centers are

broadly distributed in prokaryotes

Reprinted from Macalady, J.L., Hamilton, T.L., Grettenberger, C.L., Jones, D.S., Tsao, L.E. and Burgos, W.D. (2013). Energy, ecology and the distribution of microbial life. Phil. Trans. Roy. Soc. B: 368: 20120383. by

permission of the Royal Society. See also Blankenship, R.E. (2010). Early evolution of photosynthesis. Plant Physiol. 154: 434–438;

Type I

Type II

Type II

Type I

Type I

Cyanobacteria,

chloroplasts

Type I + Type II

Several bacterial lineages

have some photosynthetic

members, indicating that

lateral gene transfer has

played an important role in the

evolution of photosynthesis.

http://rstb.royalsocietypublishing.org/content/368/1622/20120383





Prokaryotic photosynthetic diversity

Adapted from Raymond, J. (2008). Coloring in the tree of life. Trends Microbiology. 16: 41-43.

Phylum Colloquial name

Discovery Reaction center

Pigments

Cyanobacteria 1800s Type I + Type II Chl a,b,c,d

Proteobacteria Purple sulfur / nonsulfur bacteria

1800s Type II

BChl a,b

Chlorobi Green sulfur bacteria

1906 Type I BChl a,c, d, e; Chl a

Chloroflexi Filamentous anoxygenic phototrophs

1974 Type II

BChl a,c

Firmicutes Heliobacteria

1983 Type I BChl g; Chl a

Acidobacteria 2007 Type I BChl a,c

http://www.ncbi.nlm.nih.gov/pubmed/18182293




Type I may be the ancestral reaction

center

With kind permission from Springer Science+Business Media Nelson, N. (2013). Evolution of photosystem I and the control of global enthalpy in an oxidizing world. Photosynth. Res. 116: 145-151.

A proposed scenario for the initial evolution

of photosynthetic reaction centers

Gene duplication: PsA → PsaB

tim

e

pa

st

pre

se

nt

Chlorobi Cyanobacteria

Fe-S cluster as electron acceptor

Ancestral

reaction

center

Proteobacteria

Cyanobacteria Rhodopseudomonas

http://link.springer.com/article/10.1007%2Fs11120-013-9902-6




Oxygenic photosynthesis evolved at

least 2.5 billion years ago

Adapted from Des Marais, D.J. (2000). When did photosynthesis emerge on Earth? Science. 289: 1703-1705 and Xiong, J. and Bauer, C.E. (2002). Complex evolution of photosynthesis. Annu. Rev. Plant Biol. 53: 503-521. NASA; Ruth Ellison

Eukaryotes

4 3 2 1 0 Billion years before present

Life

Phototropic bacteria?

Cyanobacteria

Algae

Plants

Mammals

Origin of

Earth

Stromatolites are nearly 3

billion years old and may

have been formed by oxygen-

producing cyanobacteria

Oxygenic

photosynthesis

http://www.sciencemag.org/content/289/5485/1703.summary



https://astrobiology.nasa.gov/nai/reports/annual-reports/2010/mit/environmental-oxygen-and-the-rise-and-fall-of-metazoans/

https://astrobiology.nasa.gov/nai/reports/annual-reports/2010/mit/environmental-oxygen-and-the-rise-and-fall-of-metazoans/

http://de.wikipedia.org/wiki/Stromatolith/media/File:Lake_Thetis-Stromatolites-LaRuth.jpg


Eukaryotic photosynthesis is derived

from endosymbiosis

Reprinted with permission from Rumpho, M.E., Pelletreau, K.N., Moustafa, A. and Bhattacharya, D. (2011). The making of a photosynthetic animal. J. Exp. Biol. 214: 303-311.

A single primary

endosymbiotic event* about

1.5 billion years ago gave rise

to chloroplasts in

chlorophytes (green algae

and plants), red algae, and

glaucophytes *A second more recent event that

gave rise to Paulinella

chromatophora is described later

This lesson focuses on

photosynthesis as it

occurs in plants, green

algae and cyanobacteria

Ancestral

cyanobacterium

Secondary, tertiary

endosymbiosis

Brown algae,

diatoms,

dinoflagellates,

euglenoids….

Glaucophytes Rhodophytes Chlorophytes

Green plants

http://jeb.biologists.org/content/214/2/303.abstract




Secondary

endosymbiosis

led to more

photosynthetic

eukaryotes

Reproduced with permission © Annual Reviews from Keeling, P.J. (2013). The number, speed, and impact of plastid endosymbioses in eukaryotic evolution. Annu. Rev. Plant Biol. 64: 583-607.

A huge variety of photosynthetic

euglenoids, diatoms, dinoflagellates

and other algae are products of

secondary (or tertiary) endosymbiosis;

the engulfment of a primary (or

secondary) endosymbiont





1.5 billion years ago

Glaucophytes

Rhodophytes

(Red algae)

Chlorophytes

(Green algae) Plants

Plastid Primary eukaryotic host

Nucleus

Free-living

β-Cyanobacterium

Mitochondrion

0.06 billion years ago

Free-living

α-Cyanobacterium

Eukaryotic

amoeba host

Photosynthetic amoeba

Paulinella chromatophora

chromatophores

5 μm

(Gould, Nature 2012) (Gould, Nature 2012)

(Gould, Nature 2012)

Reprinted by permission from Macmillan Publishers Ltd: from Gould, S.B. (2012). Evolutionary genomics: Algae's complex origins. Nature. 492: 46-48. Reproduced with permission © Annual Reviews Gould, S.B., Waller, R. F., and McFadden, G. (2008).

Plastid evolution. Annu. Rev. Plant Biol. 59: 491 – 517; See also Mackiewicz, P., Bodył, A. and Gagat, P. (2012). Possible import routes of proteins into the cyanobacterial endosymbionts/plastids of Paulinella chromatophora. Theory in Biosciences. 131: 1-18.

A recent independent endosymbiotic

event: Paulinella chromatophora

http://www.nature.com/nature/journal/v492/n7427/full/nature11759.html



http://www.annualreviews.org/doi/abs/10.1146/annurev.arplant.59.032607.092915




http://link.springer.com/article/10.1007/s12064-011-0147-7




Summary: Evolution and diversity of

photosynthesis

Photosynthetic bacteria

Oxygenic (H2O as

electron donor)

Anoxygenic

Purple

nonsulfur

bacteria

Green

nonsulfur

bacteria

Purple

sulfur

bacteria

Green

sulfur

bacteria

Cyanobacteria

Photosynthetic

eukaryotes

Secondary, tertiary

endosymbiosis

Primary

endosymbiosis

Rhodophytes,

glaucophytes,

Viridiplantae (plants,

chlorophytes)

Brown algae,

diatoms,

dinoflagellates,

euglenoids….


Lesson outline














Light and pigments

Gamma rays

X rays

IR

Visible

UV

Microwaves

Radio waves

1 nm 1 μm 1 mm 1 m Wavelength

500

nm

400

nm

600

nm Wavelength

Light travels at a fixed speed,

c (3 x 108 m/sec).

Frequency (ν) is inversely

proportional to wavelength (λ):

ν = c / λ Energy is proportional to frequency: E = hν and

inversely proportional to wavelength.

Therefore, shorter wavelength light (e.g., UV)

has higher energy than longer wavelength light

High energy

High frequency

Short wavelength

Low energy

Low frequency

Long wavelength


Light that hits a leaf is mainly light in the

visible spectrum (400 – 700 nm)

NASA

300 400 500 600 700 800

Wavelength (nm)

Spectr

a o

f lig

ht hitting leaf

(μm

ol p

hoto

s /

m2-s

ec)

The sun emits light at a

range of different

wavelengths, but much of

the very short wavelength

light is absorbed by Earth’s

atmosphere

Ultraviolet

The earth’s

atmosphere blocks

much of the short

wavelength light

http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/irwindows.html


Absorption spectra of photosynthetic

pigments

All chlorophyll-based

photosynthesis systems use

chlorophyll a

Different antenna systems use

different subsets of accessory

pigments which expand the range of

light absorbed

• Chlorophyll b is found in land

plants, green algae and

cyanobacteria

• Carotenoids are found in all

chlorophyll-based photosynthesis

systems

• Phycoerythrin are found in

cyanobacteria and non-green

algae, and phycocyanin in

cyanobacteria

300 400 500 600 700 800

Wavelength (nm)

Ab

so

rptio

n s

pe

ctr

a o

f

ph

oto

syn

thetic p

igm

en

ts

(no

rma

lize

d)

Chlorophyll a

Chlorophyll b

β-carotene

Phycoerythrin

Phycocyanin Acce

sso

ry

pig

me

nts


Accessory pigments are in antenna

complexes next to reaction centers

Govindjee and Shevela, D. (2011). Adventures with cyanobacteria: a personal perspective. Frontiers in Plant Science. 2: 28; Reprinted by permission from Macmillan Publishers Ltd: Scholes, G.D., Fleming, G.R.,

Olaya-Castro, A. and van Grondelle, R. (2011). Lessons from nature about solar light harvesting. Nat. Chem. 3: 763-774.

In cyanobacteria,

accessory pigments

are arranged in

phycobilisomes Antenna pigments

transfer light

energy to the

reaction center

Antenna complex Antenna complex Photosystem

In green algae and

plants accessory

pigments are

embedded in the

thylakoid membranes

http://journal.frontiersin.org/article/10.3389/fpls.2011.00028/full

http://www.nature.com/nchem/journal/v3/n10/abs/nchem.1145.html




Absorbance spectrum of photosynthesis

in a green plant

300 400 500 600 700 800 Wavelength (nm)

Absorbance

spectra

Chlorophyll a

Chlorophyll b

β -carotene

Absorbance

spectrum of a

green plant

The photosynthetic action

spectrum shows the rate of

photosynthesis that occurs

when a single wavelength

of light shines on a plant

A different action spectrum can be

measured in other organisms as a

function of their accessory pigments

Reaction center chlorophylls absorb maximally at 680

and 700 nm. Longer wavelength light does not have

sufficient energy to drive photosynthesis in plants


Pigments are characterized by networks

of double bonds

300 400 500 600 700 800

Wavelength (nm)

Ab

so

rptio

n s

pe

ctr

a o

f

ph

oto

syn

thetic p

igm

en

ts Chlorophyll a

Chlorophyll b

β-carotene

Phycoerythrin

Phycocyanin Acce

sso

ry

pig

me

nts

Phycocyanobilin

(linear tetrapyrrole) β-carotene

Chlorophyll a

Tetrapyrrole ring

with Mg in center


Bacteriochlorophylls are related but have

different absorption spectra

300 400 500 600 700 800

Wavelength (nm)

Spectr

a o

f lig

ht hitting leaf

(μm

ole

photo

ns /

m2-s

ec) Bacteriochlorophyll a

Bacteriochlorophyll b

Mg

Small changes in side chains are

sufficient to change the absorption

spectra of (bacterio)chlorophylls


Chlorophyll biosynthesis

Rissler, H.M., Collakova, E., DellaPenna, D., Whelan, J. and Pogson, B.J. (2002). Chlorophyll biosynthesis. Expression of a second Chl I gene of magnesium chelatase in Arabidopsis supports only limited chlorophyll

synthesis. Plant Physiol. 128: 770-779; Yamazaki, S., Nomata, J. and Fujita, Y. (2006). Differential operation of dual protochlorophyllide reductases for chlorophyll biosynthesis in response to environmental oxygen

levels in the cyanobacterium Leptolyngbya boryana. Plant Physiol. 142: 911-922.

Protochlorophyllide Chlorophyllide

Protoporphyrin IX is a precursor

of Mg-containing chlorophyll and

Fe-containing heme

Chlorophyll

synthase

Attaches

phytyl tail

http://www.plantphysiol.org/content/128/2/770







Lesson outline














The light response curve and quantum

efficiency

Skillman, J.B. (2008). Quantum yield variation across the three pathways of photosynthesis: not yet out of the dark. J. Exp. Bot. 59: 1647-1661 by permission of Oxford University Press .

At low light intensities,

why is there is a

production (negative

consumption) of CO2?

At low light

intensities the

relationship

between CO2

consumption

and light

intensity is

linear. Why?

Why does the rate of

CO2 consumption level

off at higher light

intensities?

http://jxb.oxfordjournals.org/content/59/7/1647.full




Quantifying photosynthesis: The light

response curve


Plants have mitochondria

and respire, consuming O2

and producing CO2. In the

light they are net CO2

consumers, but in the dark

production is greater than

consumption

At low light intensities,

photosynthesis is light

limited, so as more

photons are absorbed

more CO2 is fixed

As light intensity increases

above the light saturation

point, photosynthetic reaction

rate is determined by light-

independent reactions

This is the light compensation point:

The amount of light needed to balance photosynthetic CO2

consumption to respiratory CO2 production





Quantum Yield: Moles CO2 fixed or O2

produced per moles photons

Note that the quantum

yield can only be

measured in light-

limiting conditions where

the relationship between

absorbed light and CO2

fixation is linear

In this study, the quantum

yield is 0.05 mol CO2 fixed

per mol absorbed photons

(the slope of the line)

What factors

influence

quantum yield?






Quantum Yield: Moles CO2 fixed or O2

produced per moles photons

Note that the quantum

yield can only be

measured in light-

limiting conditions where

the relationship between

absorbed light and CO2

fixation is linear

In this study, the quantum

yield is 0.05 mol CO2 fixed

per mol absorbed photons

(the slope of the line)

What factors

influence

quantum yield?

• Light absorbance by

photosynthetic vs

non-photosynthetic

pigments

• Balance in excitation

energy between PSI

and PSII

• (For CO2 yield,

activities of

downstream

processes)

• Temperature






Lesson outline














Plastids and chloroplasts: Essential

organelles for most plant cells

Kristian Peters; Louisa Howard, Dartmouth microscopy facility; and3k and caper437

Besides photosynthesis,

several metabolic pathways

occur in plastids including N

and S assimilation, and the

synthesis of secondary

metabolites, pigments, and

hormones

Envelope (double

membrane, resembles

prokaryotic membranes) with

specialized transporters

Stroma

Plant cell (outlined) with

many green chloroplasts A single chloroplast

Thylakoid membranes

http://de.wikipedia.org/wiki/Rosettiges_Rosenmoos/media/File:Rhodobryum_roseum.jpeg

http://remf.dartmouth.edu/Botanical_TEM/

http://commons.wikimedia.org/wiki/File:Chloroplast_in_leaf_of_Anemone_sp_TEM_85000x.png


In plants, plastids divide by fission and

differentiate

Immage credit LadyofHats; see also Sakamoto W., Miyagishima S., and Jarvis P. (2008). Chloroplast Biogenesis: Control of Plastid

Development, Protein Import, Division and Inheritance. The Arabidopsis Book 6:e0110. doi:10.1199/tab.0110

Leaf

Flower,

fruit

Dark grown

photosynthetic

tissue

Seeds, embryonic,

meristems and

reproductive

tissues

Storage of starch, oils and proteins

https://en.wikipedia.org/wiki/Amyloplast/media/File:Plastids_types_en.svg

http://www.bioone.org/doi/full/10.1199/tab.0110


Chlamydomonas cells have a single large

chloroplast

These images show the thylakoids (green),

starch grains (brown) and a special region

called the pyrenoid (py) in which carbon-

fixing reactions take place

flagella

chloroplast

nucleus

py

Engel, B.D., Schaffer, M., Kuhn Cuellar, L., Villa, E., Plitzko, J.M. and Baumeister, W. (2015). Native architecture of the Chlamydomonas chloroplast revealed by in situ cryo-electron tomography. eLife. 4: e04889.

http://elifesciences.org/content/4/e04889


Light induces conversion from etioplast

to chloroplast

Pribil, M., Labs, M. and Leister, D. (2014). Structure and dynamics of thylakoids in land plants. J. Exp. Bot. 65: 1955-1972 by permission of Oxford

University Press . Von Wettstein, D., Gough, S. and Kannangara, C.G. (1995). Chlorophyll biosynthesis. Plant Cell. 7: 1039-1057..

Prolamellar body

in etioplast

Transition to

lamellar layers

Primary lamellar

layers

Grana layers

DARK

LIGHT

http://jxb.oxfordjournals.org/content/65/8/1955



http://www.plantcell.org/content/7/7/1039.full.pdf+html




Land plants have distinctive grana stacks

in the thylakoids

Grana stacks

Membranes at the margins are non-

appressed (green) and those within the

grana stacks (red) are appressed. Different

complexes are found in appressed vs non-

appressed regions of the thylakoids

Grana

Louisa Howard, Austin, J.R., et al and Staehelin, L.A. (2006). Plastoglobules Are lipoprotein subcompartments of the chloroplast that are permanently coupled to thylakoid membranes and contain biosynthetic enzymes.

Plant Cell. 18: 1693-1703; see also Staehelin, L.A. (1976). Reversible particle movements associated with unstacking and restacking of chloroplast membranes in vitro. J. Cell Biol. 71: 136-158.

http://remf.dartmouth.edu/Botanical_TEM/

http://www.plantcell.org/content/18/7/1693.abstract



http://jcb.rupress.org/content/71/1/136.long




Appressed and non-appressed

thylakoids have different functions

Daum, B., et al., (2010). Arrangement of Photosystem II and ATP synthase in chloroplast membranes of spinach and pea. Plant Cell. 22: 1299-1312; Nagy, G., et al. and Minagawa, J. (2014). Chloroplast remodeling during

state transitions in Chlamydomonas reinhardtii as revealed by noninvasive techniques in vivo. Proc. Natl. Acad. Sci. USA. 111: 5042-5047.

lumen

Grana

PSII packing into appressed membranes

State 1

ATP synthase

PSI-LHCI

supercomplex

Cyt b6f

LHCII Cyt b6f

PSII

PSII is mainly found in

appressed regions,

ATP synthase and PSI in

unappressed regions, and

Cyt b6f is distributed throughout




http://www.pnas.org/content/111/13/5042.long




Summary: Light, pigments, quantum

efficiency and chloroplasts

The first step of photosynthesis is light capture by

pigments in thylakoid membranes of chloroplasts.

β-carotene


Lesson outline














Structure and function of photosynthetic

complexes

Photosystem II Cytochrome

b6f complex Photosystem I

Stroma (electro-

negative side)

Lumen (electro-

positive side)

Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex†. Photochemistry and Photobiology. 84: 1349-1358.

http://onlinelibrary.wiley.com/doi/10.1111/j.1751-1097.2008.00444.x/full




Linear electron transport involves three

complexes, PSII, Cyt b6f & PSI

H2O e−

H+

O2

Photosystem II

e−

e−

e−

e−

Cytochrome


Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex†. Photochemistry and Photobiology. 84: 1349-1358.





Structure and function of

Photosystem II – LHCII complex

Reprinted by permission from Calderone, V., Trabucco, M., Vujičić, A., Battistutta, R., Giacometti, G.M., Andreucci, F., Barbato, R. and Zanotti, G. (2003). Crystal structure of the PsbQ protein of Photosystem II from

higher plants. EMBO reports. 4: 900-905; Reprinted with permission © Annual Reviews Nickelsen, J. and Rengstl, B. (2013).Photosystem II assembly: From cyanobacteria to plants. Annu. Rev. Plant Biol. 64: 609 – 634.

Conserved reaction

center core

PSII is a multi-protein

complex that functions as a

dimer. This diagram

represents a monomer

Divergent extrinsic proteins

D1 D2

CP43 CP47

Dimer structure of PSII The conserved

reaction center

core is made of

up proteins D1

and D2, and

inner-antenna

proteins CP43

and CP47

The oxygen-evolving

Mn4CaO5 cluster is on the

luminal side and shielded

by the more divergent

extrinsic proteins

H2O

1/2 O2 + 2 H+

http://embor.embopress.org/content/4/9/900.long








Conserved cores, variable light

harvesting structures

Reprinted with permission © Annual Reviews Nickelsen, J. and Rengstl, B. (2013).Photosystem II assembly: From cyanobacteria to plants. Annu. Rev. Plant Biol. 64: 609 – 634.

The peripheral

antenna and light-

harvesting complex

(LHC) are different

between

cyanobacteria and

chloroplasts

LHCII

Same core

Cyanobacteria & red

algae harvest light

through peripheral

antenna systems

Plants and green algae

harvest light through

membrane-embedded light

harvesting complexes






Proteins in PSII can be characterized by

SGC, SDS-PAGE and EM

Reprinted with permission from Caffarri, S., Kouřil, R., Kereïche, S., Boekema, E.J. and Croce, R. (2009). Functional architecture of higher plant

Photosystem II supercomplexes. EMBO J. 28: 3052-3063.

Protein complexes can

be separated by sucrose

gradient centrifugation

Hea

vie

r L

igh

ter

The subunit composition of each

band is determined by SDS-PAGE

EM can also reveal complex composition

http://emboj.embopress.org/content/28/19/3052




Proteins’ roles are to orient and position

pigment molecules Numerous

chlorophylls,

β carotenes and

other small

molecules are held in

position by PSII

proteins

Neveu,Curtis

http://en.wikipedia.org/wiki/Photosynthetic_reaction_centre/media/File:Photosystem-II_2AXT.PNG


Electron transfer in PSII

(1) Light converts reaction center chlorophyll (P680)

to excited form P680*

(2) Electron leaves P680*, forming P680+ (photo-

oxidation, charge separation)

(3) The electron is transferred to Pheophytin

(Pheo), forming Pheo−

(4) Pheo− passes the electron to QA to produce QA−

(5) QA− passes the electron to QB to produce QB

−

P680 → P680* → P680+

Pheo → Pheo−

e- e−

(1) (2)

(3)

QA → QA−

e- e−

(4)

PSII

(4)

(3)

STROMA

LUMEN

(5)

QB → QB−

(5)

e- e−


Plastoquinone/ plastoquinol is a carrier

of electrons and protons

See Cramer, W. A., Hasan, S.S., and Yamashita, E. (2011). The Q cycle of cytochrome bc complexes: A structure perspective. Biochim. Biophys. Acta - Bioenerg. 1807: 788–802.

Plastoquinone (PQ) at the QB site

is reduced to PQ2 − which picks

up to protons from the stroma to

form PQH2 (plastoquinol)

Plastoquinone (PQ)

at QB site of PSII

Plastoquinonol (PQH2)

2 e−

2 H+ Diffusion through lipid

bilayer to Cyt b6f

PQH2 diffuses

through the lipid

bilayer, carrying

protons and

electrons.

http://www.sciencedirect.com/science/article/pii/S0005272811000375




The oxygen-evolving complex (OEC)

resides on PSII luminal surface

Reprinted from Vogt, L., Vinyard, D.J., Khan, S. and Brudvig, G.W. (2015). Oxygen-evolving complex of Photosystem II: an analysis of

second-shell residues and hydrogen-bonding networks. Curr. Opin. Chem. Biol. 25: 152-158 with permission from Elsevier. Neveu,Curtis

The OEC’s catalytic center core is an inorganic

Mn4CaO5 cluster which performs the

mechanistically-challenging reaction of

removing four tightly-bound electrons and four

protons from water to form dioxygen

LUMEN







PSII

Chl / Chl+

Pheo / Pheo−

PQA / PQA−

PQB / PQB−/ PQB

2−

H2O e−

H+

O2

e−

e−

e−

To Cyt c6f

Luminal side

Electron transport in PSII: Electrons

move from luminal to stromal side

Reprinted from Senge, M. O., Ryan, A. A., Letchford, K. A., MacGowan, S. A., & Mielke, T. (2014). Chlorophylls, symmetry, chirality, and photosynthesis. Symmetry. 6: 781-843

http://www.mdpi.com/2073-8994/6/3/781/htm




In plants, LHCII assembles as trimers

Each monomer of LHCII

from spinach includes one

polypeptide, 14 chlorophylls

and four carotenoids

LHCII trimers

(green)

surrounding

PSII dimers

(grey)

The rate of energy

transfer from LHCII

to the reaction

center is regulated

in response to light

environment,

metabolic state etc.

Reprinted by permission from Macmillan Publishers Ltd Liu, Z., Yan, H., Wang, K., Kuang, T., Zhang, J., Gui, L., An, X. and Chang, W. (2004). Crystal structure of spinach major light-harvesting complex at 2.72 Å

resolution. Nature. 428: 287-292; see also Drop, B., Webber-Birungi, M., Yadav, S.K.N., Filipowicz-Szymanska, A., Fusetti, F., Boekema, E.J. and Croce, R. (2014). Light-harvesting complex II (LHCII) and its

supramolecular organization in Chlamydomonas reinhardtii. Biochimica et Biophysica Acta (BBA) - Bioenergetics. 1837: 63-72..

http://www.nature.com/nature/journal/v428/n6980/abs/nature02373.html









Q cycle and Cytochrome b6f complex

Stroma (electro-

negative side)

Lumen (electro-

positive side)

PSII Reaction Center

Cytochrome

b6f complex

PSI Reaction Center

Mobile plastoquinone

(PQH2) carries electrons

from PSII to Cyt b6f

Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex†. Photochem. Photobiol 84: 1349-1358.





H+

Reprinted from Hasan, S.S., Yamashita, E., Baniulis, D. and Cramer, W.A. (2013). Quinone-dependent proton transfer pathways in the photosynthetic cytochrome b6f complex. Proc. Natl. Acad. Sci. USA 110: 4297-

4302; See also Tikhonov, A. (2013). pH-Dependent regulation of electron transport and ATP synthesis in chloroplasts. Photosynth. Res. 116: 511-534.. Stroebel, D., Choquet, Y., Popot, J.-L. and Picot, D. (2003). An

atypical haem in the cytochrome b6f complex. Nature. 426: 413-418.

Cyt b6f structure

(from cyanobacteria)

Stroma

Lumen

Lumen

Stroma

Electrons are transferred

from PQH2 through Cyt b6f

and ultimately passed to

plastocyanin (PC). This is

accompanied by the transport

of H+ from stroma to lumen

Cyt b6f is made

from eight

polypeptides

including Cyt

b6 and Cyt f

From

PSII

To

PSI

Redox centers include four

hemes and an Fe2S2 cluster

e−

e−

PQH2

PC

Cyt b6f

http://www.pnas.org/content/110/11/4297.full



http://link.springer.com/article/10.1007/s11120-013-9845-y?no-access=true







Plastoquinone is a two-electron carrier

that delivers e− to Cyt b6f

Plastoquinone (PQ) is reduced to

plastoquinol (PQH2), then oxidized

to plastoquinone (it cycles). This

involves oxidation–reduction and

deprotonation–protonation, which

couples proton translocation and

electron transfer

PQ PQH2


e−

Cyt b6f

First half Q cycle Second half Q cycle

PQH2

2 H+

e−

PC

PQ

PS

II

e−

PQH2

2 H+

e−

PC

PQ P

SII

e−

Cycle completion

e− PQ

e−

H+

PQH2

PQH2 delivers two protons to

lumen, one electron to PC.

PQ returns to PSII

Another PQH2 delivers two protons to lumen, one

electron to PC. PQ, two electrons and two

protons regenerate PQH2 which cycles again

Electrons & protons pass through Cyt b6f

through the Q cycle


Structure and function of

Photosystem I – LHCI complex

Stroma (electro-

negative side)

Lumen (electro-

positive side)

PSII Reaction Center

Cytochrome

b6f complex

PSI Reaction Center

Electrons are passed from Cyt b6f

to PSI by plastocyanin (PC) or

sometimes in algae Cyt c6 Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex†. Photochemistry and Photobiology. 84: 1349-1358.





PSI is a large multi-protein, multi-pigment

complex

Å

Å

PSI is a complex of

17 protein subunits,

dominated by PsaA

and PsaB, and 178

prosthetic groups,

mainly chlorophyll

Reprinted from Amunts, A. and Nelson, N. (2009). Plant photosystem I design in the light of evolution. Structure. 17: 637-650 with permission from Elsevier.

http://www.cell.com/structure/abstract/S0969-2126(09)00151-8




Structure and electron transport chain of

Photosystem I

Reprinted by permission from Macmillan Publishers Ltd Jordan, P., Fromme, P., Witt, H.T., Klukas, O., Saenger, W. and Krausz, N. (2001). Three-dimensional structure of cyanobacterial photosystem I at 2.5 Å resolution.

Nature. 411: 909-917, © 2015 David Goodsell & RCSB Protein Data Bank See also . Amunts, A., Drory, O. and Nelson, N. (2007). The structure of a plant photosystem I supercomplex at 3.4 Å resolution. Nature. 447: 58-63

Stroma

Lumen Chlorophyll

Ferredoxin

e−

To NADPH

Plastocyanin e−

e−

Phylloquinone

Fe4S4 (x3)

PSI complex showing

proteins and electron

transport chain

Electron transport chain

e−

http://www.nature.com/nature/journal/v411/n6840/full/411909a0.html







In plants, PSI is surrounded by a

crescent of LHCI complexes

Reprinted from Mazor, Y., Borovikova, A. and Nelson, N. (2015). The structure of plant photosystem I super-complex at 2.8 Å resolution. eLife. 4: e07433. Minagawa, J. (2013). Dynamic reorganization of

photosynthetic supercomplexes during environmental acclimation. Front. Plant Sci. 4: 513.

PSI core

LHCI

Proteins Pigments

Looking at the complex from

the lumen; the membrane is

in the plane of the slide

Chlamydomonas

Two rows of LHCI

Pea complex

Single row of LHCI

http://elifesciences.org/content/early/2015/06/15/eLife.07433



Ferredoxin transfers electrons via

ferredoxin:NADP+ reductase (FNR)

Reprinted from Mulo, P. (2011). Chloroplast-targeted ferredoxin-NADP+ oxidoreductase (FNR): Structure, function and location. Biochim. Biophy. Acta (BBA) - Bioenergetics. 1807: 927-934 with permission from Elsevier.

From

PSI

e-

NADP+ NADPH

e− H+ + 2

Ferredoxin is

a small iron-

containing

Fe2S2 protein

Ferredoxin (Fd) Ferredoxin:

NADP+ reductase

Fe2S2

Reduced Fd can also pass

electrons to other enzymes





Structure and function of ATP synthase

Daum, B., Nicastro, D., Austin, J., McIntosh, J.R. and Kühlbrandt, W. (2010). Arrangement of Photosystem II and ATP synthase in chloroplast membranes of spinach and pea. Plant Cell. 22: 1299-1312.

Thylakoid lumen

ADP + Pi

H+

ATP

H+

ATP

Synthase

H+

H+

H+ H+ H+

H+ H+

H+

H+

ATP couples the dissipation of the proton gradient to ATP synthesis





ATP synthase is a multi-subunit rotary

machine

Reprinted by permission from Macmillan Publishers Ltd: from Abrahams, J.P., Leslie, A.G.W., Lutter, R. and Walker, J.E. (1994). Structure at 2.8 A resolution of F1-ATPase from bovine heart mitochondria. Nature. 370:

621-628.; Seelert, H., Poetsch, A., Dencher, N.A., Engel, A., Stahlberg, H. and Muller, D.J. (2000). Structural biology: Proton-powered turbine of a plant motor. Nature. 405: 418-419. See also Groth, G. and Pohl, E. (2001).

The structure of the chloroplast F1-ATPase at 3.2 Å resolution. J. Biol. Chem. 276: 1345-1352.

Rotary mechanism of ATP

synthase Video link

2 nm Stalk

The F1 subunit

has three α and

three β subunits,

and one each of

γ, δ and ε

F0 subunit: 10 – 15

copies of subunit c

assembled as a ring

within the thylakoid

membrane

Electron flow through F0 turns the

central stalk subunit generating

torque to energize ATP synthesis

http://www.nature.com/nature/journal/v370/n6491/abs/370621a0.html






http://www.jbc.org/content/276/2/1345.short



https://www.youtube.com/watch?v=PjdPTY1wHdQ


Summary: Structure and function of

photosynthetic complexes

ATP Synthase

The efforts of hundreds of scientists across several decades have

revealed detailed structures and mechanisms for each of the unusual,

important, and beautiful photosynthetic complexes, comprising dozens

of proteins and hundreds of pigments and other cofactors

Video link

Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex. Photochemistry and Photobiology. 84: 1349-1358.

https://www.youtube.com/watch?v=PjdPTY1wHdQ





Lesson outline














Pathways of electron transport

Linear electron transport Cyclic electron transport Water-water cycle

• Electrons transferred

from H2O to NADPH

• Involves PSII, Cyc b6f,

and PSI

• Generates NADPH and

pmf for ATP synthesis

• Electrons cycle with no

net production of NADPH

• Involves Cyc b6f and PSI

• Generates pmf for ATP

synthesis but not NADPH

H2O

NADPH H+

H+

H+

H+

H2O

H+

H+

H2O

• Electrons transferred

from H2O to H2O

• Involves PSII, Cyc b6f,

and PSI

• Generates pmf for ATP

synthesis but not NADPH


Pathways of electron transport

LET (diagrammed by the Z-scheme) is only one possible electron transport pathway. It is the only scheme

that results in NADPH, but other schemes can produce ATP and protect against photodamage

Cyclic electron transport

(CET) balances production of

ATP and NADPH and helps

alleviate photodamage

The Mehler reaction or

water-water cycle is

thought to contribute to

photoprotection

Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex†. Photochem. Photobiol. 84: 1349-1358.





LET: Flow of electrons from H2O to PSII

to Cyt b6f to PSI to NADPH

2 H2O e-

4 H+

O2

Photosystem II

e−

e−

e−

e−

Cytochrome


Plastoquinone

Plastocyanin

Ferredoxin

H2O PSII PQ Cyt b6f PC PSI Fd NADP+ NADPH






Stoichiometry of electron transport

yields

Minagawa, J. (2013). Dynamic reorganization of photosynthetic supercomplexes during environmental acclimation. Front. Plant Sci. 4: 513.

1. Assuming a quantum

efficiency of 0.8, for

each 5 photons

captured by PSII, 4 e−

enter LEF from PSII…

3… and 4 H+ are

released by the

oxidation of H2O

4. The 4 e− from PSII lead

to 8 H+ released into the

lumen during the Q cycle

7. Therefore, at least 2 H+

must be contributed by

CEF to meet the

requirements of the

Calvin-Benson cycle

2… 4 e− are

required to

reduce

2 NADP+

+ 2 H+ to

NADPH…

5. The Calvin-Benson

cycle requires 3 ATP

for each 2 NADPH

6. ATP synthase

requires 14 H+ to

produce 3 ATP



In cyclic electron transport, electrons

pass from PSI to Cyt b6f

As a result, protons

are moved into the

lumen (powering

ATP synthesis) but

electrons are not

passed to NADP+.

Thus, cyclic

electron transport

alters the ratio of

ATP to NADPH

produced as

compared to linear

electron transport.

It also alleviates

photoinhibition.

Reprinted from Shikanai, T. (2014). Central role of cyclic electron transport around photosystem I in the regulation of photosynthesis. Curr. Opin. Biotechnol. 26: 25-30 by permission from Elsevier..





Reprinted from Shikanai, T. (2014). Central role of cyclic electron transport around photosystem I in the regulation of photosynthesis. Curr. Opin. Biotechnol. 26: 25-30 by permission from Elsevier..

There are two routes of cyclic electron

transport (CET)

(1) PSI Fd PQ Cyt b6f PC PSI

(2) PSI Fd NADP+ PQ Cyt b6f PC PSI

Scheme 1 requires PGR5

(PROTON GRADIENT

REGULATED5) and PGRL1

(PGR-LIKE1) Scheme 2 involves NAD(P)H

dehydrogenase (NDH)





The water-water cycle is another form of

electron flow

2 H2O e-

4 H+

O2

Photosystem II

e−

e−

e−

Cytochrome


e−

O2 O2·− H2O2 H2O

SOD APX

SOD = superoxide

dismutase

APX = ascorbate

peroxidase

Like CET, the water-water cycle

produces ATP but not NADPH. It

usually is restricted to the early period

after the transition from dark to light

when reductant cannot be used

because the intermediates of the Calvin

Benson cycle have not built up.

H2O PSII PQ Cyt b6f PC PSI O2 H2O2 H2O






Plastid terminal oxidase oxidizes

reduced PQH2 by reducing O2

Reprinted with permission from Rumeau, D., Peltier, G. and Cournac, L. (2007). Chlororespiration and cyclic electron flow around PSI during photosynthesis and plant stress response. Plant Cell Environ. 30: 1041-1051. See

also Nawrocki, W.J., Tourasse, N.J., Taly, A., Rappaport, F. and Wollman, F.-A. (2015). The plastid terminal oxidase: Its elusive function points to multiple contributions to plastid physiology. Annu. Rev. Plant Biol. 66: 49 -74.

Water-

water

cycle Chloro-

respiration

Chlororespiration

alleviates excitation

pressure on electron

transport

http://onlinelibrary.wiley.com/wol1/doi/10.1111/j.1365-3040.2007.01675.x/full







Flavodiiron proteins provide

photoprotection in cyanobacteria

Reprinted with permission from Allahverdiyeva, Y., Suorsa, M., Tikkanen, M. and Aro, E.-M. (2015). Photoprotection of photosystems in fluctuating light intensities. J. Exp. Bot. 66: 2427-2436. Zhang, P., Eisenhut, M., Brandt,

A.-M., Carmel, D., Silén, H.M., Vass, I., Allahverdiyeva, Y., Salminen, T.A. and Aro, E.-M. (2012). Operon flv4-flv2 provides cyanobacterial photosystem II with flexibility of electron transfer. Plant Cell. 24: 1952-1971.

FLV1/FLV3 carry

out a Mehler-like

reaction that

regenerates NADP+

H2O

H+

H+

FLV1/FLV3

FNR

NADP+ NADPH

O2 H2O

FLV2/FLV4

FLV2/FLV4 function

in alternative

electron transfer

and alleviate PSII

excitation pressure








Summary: Variations in photosynthetic

electron transport

In linear electron transport,

water (H2O) is the electron

donor and NADP+ the

electron acceptor.

In cyclic electron transport,

electrons transferred from

PSI are returned to PSI.

The water-water cycle is a

variation in which electrons

transferred from water are

passed to O2, reducing it to

H2O.

The relative contributions of each

are determined by metabolic

supply and demand

RUBISCO

3 GAP


Lesson outline














Damage avoidance and repair:

Acclimations to light stress

Adapted from Li, Z., Wakao, S., Fischer, B.B. and Niyogi, K.K. (2009). Sensing and responding to excess light. Annu. Rev.Plant Biol. 60: 239-260.

Light intensity

Rate

of

ph

oto

syn

thesis

At low light

intensities, light

intensity is the

dominant factor

limiting

photosynthesis

At high light intensities,

excess light can damage

photosynthetic machinery

The metabolic and

physiological state of

the plant or cell

determines how much

light is “too much”

Optimal conditions

Stressed conditions





Excess excitation energy can lead to

photo-oxidative damage

Reprinted by permission from Macmillan Publishers Ltd Demmig-Adams, B. and Adams, W.W. (2000). Photosynthesis: Harvesting sunlight safely. Nature. 403: 371-374.

Photon-excited chlorophyll forms

excited singlet chlorophyll 1Chl*

1Chl* can return to its ground

state through:

Photochemistry

Fluorescence

Dissipation

(e.g., heat, NPQ)

Alternatively, it can convert to the

excited triplet state 3Chl* which

can transfer energy to oxygen to

produce singlet oxygen with

subsequent damage due to

reactive oxygen species

P

F

D

NPQ





There are protective strategies to avoid

high-light induced damage

Li, Z., Ahn, T.K., Avenson, T.J., Ballottari, M., Cruz, J.A., Kramer, D.M., Bassi, R., Fleming, G.R., Keasling, J.D. and Niyogi, K.K. (2009). Lutein accumulation in the absence of zeaxanthin restores nonphotochemical

quenching in the Arabidopsis thaliana npq1 mutant. Plant Cell. 21: 1798-1812. Havaux, M. and Niyogi, K.K. (1999). The violaxanthin cycle protects plants from photooxidative damage by more than one mechanism.

Proc. Natl. Acad. Sci. USA. 96: 8762-8767; Adapted from Li, Z., Wakao, S., Fischer, B.B. and Niyogi, K.K. (2009). Sensing and responding to excess light. Annu. Rev.Plant Biol. 60: 239-260

Decrease light incidence through

dynamic changes to antenna complex

Reaction

center

Antenna complex

Release excess

energy as heat or

fluorescence

Detoxify reactive oxygen side-

products of excess excitation energy

(e.g., antioxidant production)

lutein

A plant defective in energy dissipation

is susceptible to high-light induced

bleaching and death

zeaxanthin




http://www.pnas.org/content/96/15/8762.abstract




Movements to optimize light interception

Oikawa, K., Kasahara, M., Kiyosue, T., Kagawa, T., Suetsugu, N., Takahashi, F., Kanegae, T., Niwa, Y., Kadota, A. and Wada, M. (2003). CHLOROPLAST UNUSUAL POSITIONING1 is essential for proper

chloroplast positioning. Plant Cell. 15: 2805-2815. Donald Hoburn; Walter Siegmund; Kristian Peters

Lo

w lig

ht

Hig

h lig

ht

Top view Side view

Chloroplasts

move to sides

of cells to

decrease light

interception Selaginella

lepidophylla

Leaf reorientation

or curling are

adaptations to

minimize light

damage. Leaf

reorientation can

also decrease

heat damage

Arctostaphylos patula

Eucalyptus rossii




https://commons.wikimedia.org/wiki/File:Eucalyptus_rossii_leaves_1.jpg

https://commons.wikimedia.org/wiki/File:Arctostaphylos_patula_08399.JPG

http://en.wikipedia.org/wiki/File:Selaginella_lepidophylla_trocken.jpeg


Acclimation via stoichiometric changes

in complex abundance

Reprinted by permission from Walters, R.G. (2005). Towards an understanding of photosynthetic acclimation. J. Exp. Bot. 56: 435-447.

Low light: More

light harvesting

complexes (LHCII)

High light: Greater

photosynthetic

capacity (PSII etc)

Also more

photoprotection

Arabidopsis grown in low (black bars) or high (white bars).

Chloroplast components relative to total leaf chlorophyll

http://jxb.oxfordjournals.org/content/56/411/435.short




Excess light energy is dissipated via non-

photochemical quenching

“Non-photochemical quenching”

encompasses several components:

qE = Energy-dependent quenching: The xanthophyll cycle

qT = State transition: Conformational changes in LHCII

qI = Photoinhhibition: Light-induced reduction in quantum

yield as a consequence of damage

Chl

1Chl*

Photon Non-photochemical

quenching (NPQ)


Energy-dependent quenching (qE)

usually is dominant form of NPQ

PSII

[H+]

LHC PSII LHC

Unquenched:

High efficiency

transfer of light

energy to PSII

reaction center

[H+]

[H+]

[H+] [H+]

VDE Energy-dependent

quenching:

1. Lumen acidification

activates Violaxanthin

De-epoxidase

2. VDE converts

violaxanthin to zeaxanthin,

leading to light energy

dissipation in LHCII


Xanthophyll cycle: reversible

interconversion of carotenoids

Violaxanthin

de-epoxidase

(VDE)

Zeaxanthin

epoxidase

(ZE)

High light / low luminal pH

induces VDE, which catalyzes

the conversion of violaxanthin

to zeaxanthin

In low light / high luminal pH,

the reaction is reversed by ZE

Hieber, A.D., Kawabata, O. and Yamamoto, H.Y. (2004). Significance of the lipid phase in the dynamics and functions of the xanthophyll cycle as revealed by PsbS overexpression in tobacco and in-

vitro de-epoxidation in monogalactosyldiacylglycerol micelles. Plant Cell Physiol 45: 92-102 by permission of Oxford University Press.

http://pcp.oxfordjournals.org/content/45/1/92.full




Zeaxanthins promotes structural

changes & heat dissipation

Reprinted with permission from Ruban, A.V. (2015). Evolution under the sun: optimizing light harvesting in Photosynthesis. J. Exp. Bot. 66: 7 – 23; Müller-Moulé, P., Conklin, P.L. and Niyogi, K.K. (2002). Ascorbate

Deficiency Can Limit Violaxanthin De-Epoxidase Activity in Vivo. Plant Physiol. 128: 970-977.

Zeaxanthin accumulation (due to VDE

activation) causes a rearrangement of

LHCII and RCII, which decreases light

transfer to RCII

The structural changes

cause more light energy

to be dissipated as heat









Zeaxanthin and lutein also have roles as

antioxidants and in photoprotection

Niyogi, K.K., Björkman, O. and Grossman, A.R. (1997). The roles of specific xanthophylls in photoprotection. Proc. Natl. Acad. Sci. USA 94: 14162-14167.

Chlamydomonas mutants deficient in

zeaxanthin and lutein production are

more susceptible to photo-oxidation

Interestingly, these two xanthophyll

pigments (obtained from dietary

sources) also protect human eyes from

phototoxic damage by accumulating in

the macula (orange color)





The redox state of PQ pool contributes to

state transitions (qT)

Rosso, D., Bode, R., Li, W., Krol, M., Saccon, D., Wang, S., Schillaci, L.A., Rodermel, S.R., Maxwell, D.P. and Hüner, N.P.A. (2009). Photosynthetic redox imbalance governs leaf sectoring in the Arabidopsis thaliana

variegation mutants immutans, spotty, var1, and var2. Plant Cell. 21: 3473-3492.

When PSII, PSI and

downstream metabolism are

balanced, the PQ pools is

distributed between PQ

(oxidized) and PQH2 (reduced)

High light (or light that

favors PSII) or conditions

that decrease

downstream metabolism

lead to an over-reduction

of the PQ pool





Reduced PQH2 activates LHCII kinase

and promotes state transition

Minagawa, J. (2013). Dynamic reorganization of photosynthetic supercomplexes during environmental acclimation. Front. Plant Sci. 4: 513.

Accumulation of PQH2

activates LHCII kinase

LHCII kinase

phosphorylates

LHCII. Some LHCII

relocates to PSI



State 1

ATP synthase

State 2 PSI-LHCI

supercomplex

Cyt b6f

LHCII

trimer

(quenched)

LHCII

trimer

A current model indicates

that state transitions

balance PSII and PSI

mainly by quenching LHCII

energy transfer to PSII

LHCII

phosphorylation also

prevents light energy

from being passed to

PSII

Nagy, G., Ünnep, R., et al.. and Minagawa, J. (2014). Chloroplast remodeling during state transitions in Chlamydomonas reinhardtii as revealed by noninvasive

techniques in vivo. Proc. Natl. Acad. Sci. USA 111: 5042-5047. See also Ünlü, C., Drop, B., Croce, R. and van Amerongen, H. (2014). State transitions in

Chlamydomonas reinhardtii strongly modulate the functional size of Photosystem II but not of photosystem I. Proc. Natl. Acad. Sci. USA 111: 3460-3465.








Photoinhibition (qI) is caused by light

damage to PSII

Reprinted by permission from Takahashi, S. and Murata, N. (2008). How do environmental stresses accelerate photoinhibition? Trends Plant Sci 178-182.

The D1 protein of

PSII is susceptible

to photodamage,

and when its rate

of damage

exceeds the rate

of repair,

photosynthesis is

inhibited.





Damage and repair of PSII are stress and

environmentally sensitive

Reprinted by permission from Nath, K., Jajoo, A., Poudyal, R.S., Timilsina, R., Park, Y.S., Aro, E.-M., Nam, H.G. and Lee, C.H. (2013). Towards a critical understanding of the Photosystem II repair mechanism and its

regulation during stress conditions. FEBS Lett. 587: 3372-3381.

http://www.febsletters.org/article/S0014-5793%2813%2900700-X/abstract




Metabolic demand for NADPH and ATP

feed back into light harvesting

Reprinted with permission from Schöttler, M.A., Tóth, S.Z., Boulouis, A. and Kahlau, S. (2015). Photosynthetic complex stoichiometry dynamics in higher plants: biogenesis, function, and turnover of ATP synthase and

the cytochrome b6f complex. J. Exp. Bot. 66: 2373-2400.

When supply > demand,

elevated NADPH & ATP levels

feed back and induce

photoprotection (red arrows) NPQ

pH-induced promotes

conversion of violaxanthin (V)

into zeaxanthin (Z)

Photosynthetic

control of

plastoquinol

reoxidation

Metabolic imbalances, drought,

cold, pathogen infection and

other factors can decrease flux

through the Calvin-Benson cycle

http://jxb.oxfordjournals.org/content/66/9/2373.abstract




Timescales of high light responses

Seconds Minutes Hours Days

Electron transport affected

Δ pH

Reduced PQ pool

Protein degradation and PSII repair

Changes in antenna size

Increased xanthophyll pool Changes in PSI/ PSII stoichiometry

Photochemistry

Metabolism

Translatome

Transcriptome

Growth

Dark qE

qT

qI 10 – 200 s

10 – 60 s

10 – >30 min

10 – 60 min

NPQ kinetics

Adapted from Jahns, P. and Holzwarth, A.R. (2012). The role of the xanthophyll cycle and of lutein in photoprotection of Photosystem II. Biochim. Biophys. Acta Bioenergetics. 1817: 182-193. Dietz, K.-J. (2015). Efficient high light

acclimation involves rapid processes at multiple mechanistic levels. J. Exp. Bot. 66: 2401-2414. Erickson, E., Wakao, S. and Niyogi, K.K. (2015). Light stress and photoprotection in Chlamydomonas reinhardtii. Plant J. 82: 449-465.




http://jxb.oxfordjournal/content/66/9/2401.abstract



http://onlinelibrary.wiley.com/doi/10.1111/tpj.12825/abstract




Light reactions

Carbon-fixing reactions

ATP NADPH ADP NADP+

High temperatures

increase membrane

permeability and

decrease proton-

motive force

Drought-induced

stomatal closure

prevents CO2 uptake

Cold temperature

slows enzyme-

catalyzed reactions

Nutrient deficiency

or toxicity affects

electron transport

Heat, drought, & other stresses affect

photosynthetic efficiency


Retrograde signaling from plastid to

nucleus is critical for homeostasis

Reprinted with permission from Macmillan Publishing Ltd from Jarvis, P. and López-Juez, E. (2013). Biogenesis and homeostasis of chloroplasts and other plastids. Nat Rev Mol Cell Biol. 14: 787-802; see also Dietz, K.-J.

(2015). Efficient high light acclimation involves rapid processes at multiple mechanistic levels. J. Exp. Bot. 66: 2401-2414l Chi, W., Sun, X. and Zhang, L. (2013). Intracellular signaling from plastid to nucleus. Annu. Rev.

Plant Biol. 64: 559-582; Estavillo, G.M., Chan, K.X., Phua, S.Y. and Pogson, B.J. (2013). Reconsidering the nature and mode of action of metabolite retrograde signals from the chloroplast. Frontiers Plant Sci. 3: 300.

Retrograde signals

are biogenic (during

development) and

operational (during

function)

Possible signals include compounds

derived from heme biosynthesis,

ROS, nucleotide derivatives and

many more

http://www.nature.com/nrm/journal/v14/n12/abs/nrm3702.html






http://journal.frontiersin.org/article/10.3389/fpls.2012.00300/abstract


Reprinted from Takahashi, S. and Badger, M.R. (2011). Photoprotection in plants: A new light on Photosystem II damage. Trends Plant Sci. 16: 53–60. with permission from Elsevier

Summary of photosynthetic acclimation

mechanisms

Although the

photosynthetic

machinery is

susceptible to

photoinhibitory

damage, it also

has several

strategies for

photoprotection

http://www.cell.com/trends/plant-science/abstract/S1360-1385(10)00211-6




Lesson outline














Methods to monitor light reactions

NASA; Steveadcuk

• PSII activity can be measured by

chlorophyll fluorescence

• PSI activity can be measured by the

absorbance change at 810~830 nm

• Transthylakoid proton motive force

(pmf) and proton flux can be measured

by electrochromic shift (ECS)

Alternatively, photosynthesis can be

monitored spectroscopically

Photosynthesis can be monitored by O2

production and CO2 consumption

http://climate.nasa.gov/news/956/

http://en.wikipedia.org/wiki/Chlorophyll_fluorescence/media/File:OS30p%2B_fluorometer_being_used_to_measure_plant_stress_in_the_field.jpg


Quantifying photosynthesis by gas

exchange measurements

Closed system

Light

Light on

O2

CO2

Light off

Co

nc

en

tra

tio

n

in c

ha

mb

er

Open system

Light

Light on

O2

CO2

Light off

Co

nc

en

tra

tio

n

in e

xit

ing

air

Air in Air out

Ambient [O2]

Ambient [CO2]

Gas exchange

in a leaf can be

measured in a

closed or open

(flow through)

system

[CO2] is

measured by

infrared gas

analysis (IRGA)

and [O2] by an

oxygen

electrode


Principle of fluorescent measurements of

photosynthesis

PSII

chlorophyll

Incident

light

Fluorescence

(longer

wavelength)

Photochemistry

NPQ (heat)

When a chlorophyll a molecule absorbs light, it is excited

from its ground state to its singlet excited state (Chl*), which

can return to the ground state via one of three pathways:

(1) The excitation energy can be transferred to reaction

centers to drive photosynthesis (photochemistry, qP)

(2) Energy can be re-emitted as chlorophyll fluorescence

(3) It can be released as heat (non-photochemical

quenching, NPQ)

fluorescence

Chl

Chl* Photon

qP

NPQ

The practical application is

that fluorescence can be

quantified under various

conditions to measure

photosynthetic activities


Reaction centers “close”, maximizing

fluorescence after pulse

Murchie, E.H. and Lawson, T. (2013). Chlorophyll fluorescence analysis: a guide to good practice and understanding some new applications. J. Exp. Bot. 64: 3983-3998 by permission of Oxford University Press.

P680 → P680* → P680+

Pheo → Pheo−

e- e−

QA → QA−

e- e−

QB → QB−

e- e−

Reaction

center

transiently

closed

A pulse of saturating light

leads to a transient pulse of

fluorescence as reaction

centers close (become

reduced) and light is emitted

as fluorescence

Reaction centers open with

measuring beam on, closed

after saturating pulse





Determining max Fm, min Fo, and Fv:

Dark-acclimated tissues (no NPQ)

A very bright light

pulse is mainly

emitted as

fluorescence;

reaction centers

close until they

can be re-oxidized

PSII

chlorophyll

Saturating

pulse

Photochemistry

PSII

chlorophyll Very low

level light

Photochemistry In dim light

(measuring light),

the energy is

nearly all used for

photochemistry

Flu

ore

sc

en

ce

Fo

Fm

Fo

0

Fm

Saturating pulse

Measuring light on

Fv =

Fm

- F

o X

A saturating flash transiently closes reaction centers, resulting in maximum Chl fluorescence.

With measuring light only, the reaction centers are fully opened, resulting in minimum Chl fluorescence.


Fv / Fm is an indicator of maximum

quantum yield of PSII

Fo = baseline

fluorescence, reaction

centers open

Fm = maximal

fluorescence when

reaction centers closed

(Fm− Fo) = Fv

= variable fluorescence

Fv / Fm = maximum

quantum efficiency of

PSII chemistry

(values less than ~0.8

indicate stress or

photoinhibition)






Light-induced fluorescence is quenched

by qP + NPQ

F ' is steady state

fluorescence in the

light

NPQ and qP can be distinguished

by the difference in fluorescence

when reaction centers are open or

closed (Fm′)

Due to NPQ

Due to photochemistry

(photochemical quenching, pQ)

Fq′ / Fm′ = ΦPSII

ΦPSII = quantum efficiency of PSII

Fm′ – F′ = Fq′






Different NPQ components relax at

different rates

• Fm increases as NPQ

reverses

• Different components

of NPQ have different

relaxation kinetics

(e.g., qE reverses

more rapidly than qI.)






With kind permission from Springer Science+Business Media from Zhang, R. and Sharkey, T. (2009). Photosynthetic electron transport and proton flux under moderate heat stress. Photosynthesis Research. 100: 29-43.

PSI redox status can be determined by

810nm absorbance

P700 oxidation ratio = A / B

P700+ under actinic light

Max P700+ induced by far red and flash

P700

e-

e- P700+

Reduced P700 does not

absorb 810 nm light, but

oxidized P700+ does

P700+ Fully

oxidized

P700 Fully

reduced

An increase in

reduction rate of

P700+ can indicate

an induction of cyclic

electron transport





Trans-thylakoid pmf can be measured by

electrochromic shift

Witt, H.T. (1979). Energy conversion in the functional membrane of photosynthesis. Analysis by light pulse and electric pulse methods: The central role of the electric field. Biochim. Biophy. Acta Bioenergetics. 505: 355-427.

The proton-motive

force (electrical and

pH gradient) across

the thylakoid can be

measured by its

effect on pigment

absorbance spectra

Ab

sorb

an

ce

Wavelength

Without

electric

field

With

electric

field

Electrochromic shift (ECS) is

the change in light absorbing

properties undergone by

pigments when subjected to a

changing electric field

http://www.sciencedirect.com/science/article/pii/0304417379900089




Electrochromic shift (ECS) monitors pmf

and ATP synthase activity

-400 -200 0 200 400

0.000

0.001

0.002

0.003

-10 0 10 20 30 40 50

-0.005

0.000

0.005

0.010

0.015

Op

tic

al

de

ns

ity

, re

lati

ve

Op

tic

al

de

ns

ity

, re

lati

ve

Time, ms

ECS

light off

ECS

light on

pH

Time, s

light off

ECS

1/EC: proton conductance,

proportional to ATP synthase

activity. In other words, the faster

the ECS decays, the more active

ATP synthase With kind permission from Springer Science+Business Media from Zhang, R. and Sharkey, T. (2009). Photosynthetic electron transport and proton flux under moderate heat stress. Photosynth. Res. 100: 29-43; see also

Baker, N.R., Harbinson, J., and Kramer, D.M. (2007). Determining the limitations and regulation of photosynthetic energy transduction in leaves. Plant Cell Environ. 30: 1107–1125..

Light induces pmf,

therefore ECS

ΔECS: light-induced transthylakoid

pmf (proton motive force)

ECS: relaxation

time constant








With kind permission from Springer Science+Business Media from Bailleul, B., Cardol, P., Breyton, C., and Finazzi, G. (2010). Electrochromism: a useful probe to study algal photosynthesis. Photosynth. Res. 106: 179–189.

The kinetics of ESC can separate

activities of different components

EC

S

Light pulse

VERY fast = charge separation through PSI & PSII

Fast = Cyt b6f electron flow

Slow decay = H+ flow

through ATP synthase

Inhibitors can

distinguish PSI

from PSII

PSII inhibitors

http://link.springer.com/article/10.1007%2Fs11120-010-9579-z




Chlorophyll fluorescence: Imaging and

scaling up

Murchie, E.H. and Lawson, T. (2013). Chlorophyll fluorescence analysis: a guide to good practice and understanding some new applications. J. Exp. Bot. 64: 3983-3998; NASA; NASA/Caltech; Guanter, L., et al. (2014).

Global and time-resolved monitoring of crop photosynthesis with chlorophyll fluorescence. Proc. Natl. Acad. Sci. USA 111: E1327-E1333; Joiner, J., et al., (2011). First observations of global and seasonal terrestrial

chlorophyll fluorescence from space. Biogeosciences. 8: 637-651..

F’

Photosynthetic efficiency of

PSII photochemistry (F q′/F m′)

Fluorescence measurements

can be coupled to imaging

systems to measure

photosystem efficiency across

many size scales – within a

leaf or within a plant.

Technologies are

being developed to

monitor solar-induced

fluorescence from

space




https://www.nasa.gov/press/goddard/2014/march/satellite-shows-high-productivity-from-us-corn-belt/.VkCtjCu96i8

http://www.caltech.edu/news/tracking-photosynthesis-space-46627

http://www.pnas.org/content/111/14/E1327.full



http://www.biogeosciences.net/8/637/2011/bg-8-637-2011.html




Summary: Spectrophotometric

measurements of photosynthesis

The light-absorbing and emitting properties of

the photosynthetic complexes allow their

activities to be monitored with great

precision. These methods have been

developed using intact leaves and cells, but

to some extent can be applied to field- and

global-level monitoring






Lesson outline














Optimizing and improving

photosynthesis

Crop plants grow in

very unnatural

conditions, different

than those in which

natural selection has

acted. Can

photosynthetic light-

dependent reactions

be turbo-charged for

higher yields?


Harvest other light wavelengths?

Ort, D.R., et al. (2015). Redesigning photosynthesis to sustainably meet global food and bioenergy demand. Proc. Natl. Acad. Sci. USA 112: 8529–8536 See also Croce, R. and van Amerongen, H. (2014). Natural

strategies for photosynthetic light harvesting. Nat Chem Biol. 10: 492-501 Blankenship, R.E., et al. and Sayre, R.T. (2011). Comparing photosynthetic and photovoltaic efficiencies and recognizing the potential for

improvement. Science. 332: 805-809. .

A hypothetical system in which PSI

is replaced by a bacterial reaction

center that uses longer wavelength

light. (Note that the system would

have to be extensively engineered to

bridge the gap between water

oxidation and NADPH reduction in

the absence of PSI.)

Another suggestion is to

increase green-light

absorption by introducing

phycoerythrin from red algae

(who wants black leaves?)

Current oxygenic photosynthesis

Hypothetical long-wavelength oxygenic photosynthesis




http://www.nature.com/nchembio/journal/v10/n7/full/nchembio.1555.html



http://www.sciencemag.org/content/332/6031/805




Can single-celled organisms be

optimized for culture conditions?

Reprinted from Kirst, H., Formighieri, C. and Melis, A. (2014). Maximizing photosynthetic efficiency and culture productivity in cyanobacteria upon minimizing the

phycobilisome light-harvesting antenna size. Biochim. Biophys. Acta - Bioenergetics. 1837: 1653-1664; IGV; Biotech

Light penetration into culture vessels is

a problem for algal biofuels production.

By engineering cyanobacteria with

truncated light antennas (phycocyanin

deletion mutants), productivity

increased (due to improved light

penetrance and less excess light

dissipated from surface cells).




https://en.wikipedia.org/wiki/Photobioreactor/media/File:Photobioreactor_PBR_500_P_IGV_Biotech.jpg




Can shading be decreased in field

conditions?

Ort, D.R., et al. (2015). Redesigning photosynthesis to sustainably meet global food and bioenergy demand. Proc. Natl. Acad. Sci. USA 112: 8529–8536.

In a typical canopy,

upper leaves

intercept too much

light and must

dissipate some as

heat, meanwhile

shading lower

leaves

In a “smart

canopy”, upper

leaves would have

smaller antenna

complexes and a

more vertical

orientation,

permitting more

light to reach lower

leaves for greater

overall efficiency





Can photoprotection be optimized for

less photooxidative damage?

Murchie, E.H. and Niyogi, K.K. (2011). Manipulation of Photoprotection to Improve Plant Photosynthesis. Plant Physiol. 155: 86-92.

Enhance PSII repair

Increase NPQ responsiveness

Increase flow of electrons through CEF or water-water cycle

ROS production can induce damage that is metabolically costly to repair

Manipulating photoprotective

pathways can enhance stress

resistance and photosynthetic

productivity of crop plants.

What are the metabolic

costs of enhanced

photoprotection?





Lesson outline














Applying photosynthetic insights

towards solar electricity and fuels

The energy that reaches the earth from the sun in

an hour is equivalent to “all the energy humankind

currently uses in a year”

Solar panels make electricity that is hard to

store

The goal of artificial

photosynthesis is to

develop cheap,

durable ways to

make fuels from

sunlight (like a leaf)

Barber, J. and Tran, P.D. (2013). From natural to artificial photosynthesis. J. Roy. Soc. Interface. 10: 20120984; Photos by Tom Donald

http://dx.doi.org/10.1098/rsif.2012.0984

https://www.flickr.com/photos/clearwood/8535378620


Developing biomimetic systems for

photosynthesis

Cogdell, R.J., Gardiner, A.T., Molina, P.I. and Cronin, L. (2013). The use and misuse of photosynthesis in the quest for novel methods to harness

solar energy to make fuel. Phil. Trans. Royal Soc. A: Mathematical Physical Engineering Sci 371: 20110603 by permission of the Royal Society.

The four fundamental steps that

comprise photosynthesis can be

engineered using catalysts 1. Light harvesting

2. Charge

separation

3. Water

oxidation

4. Proton

reduction

http://rsta.royalsocietypublishing.org/content/371/1996/20110603.short


Semiconducting materials can be used

as photocatalysts

Barber, J. and Tran, P.D. (2013). From natural to artificial photosynthesis. J. Roy. Soc. Interface. 10: 20120984 by permission of the Royal Society.

Semiconducting

materials can

carry out charge

separation

They can be

augmented with

electrocatalysts

They can be wired

together as a Z-

scheme

http://dx.doi.org/10.1098/rsif.2012.0984


Hybrid and bio-inspired systems are

being explored for fuel production

Torella, J.P., Gagliardi, C.J., Chen, J.S., Bediako, D.K., Colon, B., Wray, J.C., Silver, P.A., and Nocera, D.G. (2015). Efficient solar-to-fuels production from a hybrid microbial–water-splitting catalyst system. Proc. Natl.

Acad. Sci. USA. 112: 2337- 2342. Reproduced from Duan, L., Tong, L., Xu, Y. and Sun, L. (2011). Visible light-driven water oxidation-from molecular catalysts to photoelectrochemical cells. Energy Environ. Sci. 4:

3296-3313 with permission of The Royal Society of Chemistry. .

A bioelectrochemical cell. Water-

splitting and proton-reducing

electrodes are immersed into a

culture of bacteria that convert H2

to isopropanol A photoelectrical cell that mimics the

electron transfer reactions of PSII

Torella, J.P., Gagliardi, C.J., Chen, J.S., Bediako, D.K., Colon, B., Wray, J.C., Silver, P.A., and Nocera, D.G. (2015). Efficient solar-to-fuels production from a hybrid microbial–water-splitting catalyst system. Proc. Natl. Acad. Sci. USA. 112: 2337- 2342.




http://pubs.rsc.org/en/Content/ArticleLanding/2011/EE/c1ee01276b#!divAbstract




Bacteriorhodopsin: non-chlorophyll

based phototrophic system

Bacteriorhodopsin uses light

energy to pump protons (or

other ions) which can be used

for ATP synthesis Structure of

bacteriorhodopsin

Retinal is the

chromophore

As an alternative to chlorophyll based

systems, bacteriorhodopsins are being

explored for solar-driven chemistry.

See for example Claassens, N.J., Volpers, M., Martins dos Santos, V.A.P., van der Oost, J., and de Vos, W.M. (2013). Potential of

proton-pumping rhodopsins: Engineering photosystems into microorganisms. Trends Biotechnol. 31: 633 – 642.

http://dx.doi.org/10.1016/j.tibtech.2013.08.006





Lesson outline














Photosynthetic fungi and animals

This lesson focuses on photosynthesis

in cyanobacteria and the products of

primary endosymbiosis

Secondary, tertiary

endosymbiosis Brown algae,

diatoms,

dinoflagellates,

euglenoids….

Other, unrelated

eukaryotic organisms

can serve as hosts to

photosynthetic bacteria

and algae:

Fungi

• Lichen

Animals

• Corals

• Flatworms

• Sea slugs

• Salamanders

Reprinted with permission from Rumpho, M.E., Pelletreau, K.N., Moustafa, A. and Bhattacharya, D. (2011). The making of a photosynthetic animal. J. Exp. Biol. 214: 303-311.





Photosynthetic fungi: Lichen

Photos by Vernon Ahmadjian courtesy of BSA; jdurant

Lichen are mutualistic associations

of fungi (blue) and green algae or

cyanobacteria (green).

The photosynthetic partner

(phycobiont) resides outside the fungal

cells (mycobiont). The phycobiont

provides the mycobiont with reduced

carbon in return for shelter, nutrients

and water.

Lichen have many forms

Algal cell Fungus

http://pix.botany.org/media/collection/id.21.html

https://en.wikipedia.org/wiki/Lichen/media/File:Lichen_Cross_Section_Diagram.svg


Photosynthetic animals:

Reef-building corals

Credit: Todd LaJeunesse, Penn State University. Fournier, A. (2014). The story of symbiosis with zooxanthellae, or how they enable their host to thrive in a nutrient poor environment. BioSciences Master Reviews.

Weis, V.M. (2008). Cellular mechanisms of Cnidarian bleaching: stress causes the collapse of symbiosis. J. Exp. Biol. 211: 3059-3066. Allisonmlewis

Symbiodinium

dinoflagellates live

within the bodies of

corals and provide

them with fixed

carbon

Various species of

coral and their

dinoflagellates

Coral bleaching is

the loss of the

symbiont.

Bleaching is

caused by stress,

particularly heat,

and often leads to

coral death

Bleached

http://science.psu.edu/news-and-events/2012-news/dna-analysis-aids-in-classifying-single-celled-algae

http://biologie.ens-lyon.fr/ressources/bibliographies

http://biologie.ens-lyon.fr/ressources/bibliographies/m1-12-13-biosci-reviews-fournier-a-1c-m.xml

http://jeb.biologists.org/content/211/19/3059.long



https://en.wikipedia.org/wiki/Symbiodinium/media/File:HostTissue_section.png


Bailly, X., Laguerre, L., Correc, G., Dupont, S., Kurth, T., Pfannkuchen, A., Entzeroth, R., Probert, I., Vinogradov, S., Lechauve, C., Garet-Delmas, M.-J., Reichert, H. and Hartenstein, V. (2014). The chimerical and

multifaceted marine acoel Symsagittifera roscoffensis: from photosymbiosis to brain regeneration. Front. Microbiol. 5: 498.

Lif

e c

ycle

of

the p

ho

tosyn

theti

c

wo

rm S

ym

sag

itti

fera

ro

sco

ffen

sis

http://journal.frontiersin.org/article/10.3389/fmicb.2014.00498/full



Spotted salamanders

Kerney, R., Kim, E., Hangarter, R.P., Heiss, A.A., Bishop, C.D. and Hall, B.K. (2011). Intracellular invasion of green algae in a salamander host. Proc. Natl. Acad. Sci. USA 108: 6497-6502. Camazine

Egg capsule

Spotted salamander

Ambystoma maculatum

The algae provides the

animal with oxygen and

photosynthate, and

benefits from the animal’s

nitrogenous waste

The single-celled algae

Oophila amblystomatis lives

inside the egg capsule of

spotted salamander

Recently, algae (red fluorescent

spots) were found within the

animal’s cells, the first evidence

for endosymbiosis in a vertebrate




https://en.wikipedia.org/wiki/Spotted_salamander/media/File:SpottedSalamander.jpg

https://en.wikipedia.org/wiki/Spotted_salamander/media/File:SpottedSalamander.jpg



Plastid-stealing sea slugs

Pelletreau, K.N., Bhattacharya, D., Price, D.C., Worful, J.M., Moustafa, A. and Rumpho, M.E. (2011). Sea slug kleptoplasty and plastid maintenance in a metazoan. Plant Physiol. 155: 1561-1565. Rumpho, M.E.,

Pelletreau, K.N., Moustafa, A. and Bhattacharya, D. (2011). The making of a photosynthetic animal. J. Exp. Biol. 214: 303-311.

Vaucheria litorea (algae)

Elysia

chlorotica

(sea slug)

The plastids ingested from

the algae stay viable for

several months within the

sea slug – this “plastid

stealing” is known as

kleptoplasty Chloroplasts in sea slug

Ingested chloroplasts are

stored in animal’s cells








Summary: Light-dependent reactions are

billions of years old

The core chemical reactions

have remained essentially

the same for > 2.5 billion

years, since the days of the

first cyanobacteria

Core reactions are conserved,

but variation in tolerances and

capacitieis of each of the

reactions allows for dynamic

responses and adaptations to

variable light, temperature etc.

The Z-scheme is a

familiar way to

depict the light-

harvesting

reactions of

photosynthesis

Ruth Ellison

http://de.wikipedia.org/wiki/Stromatolith/media/File:Lake_Thetis-Stromatolites-LaRuth.jpg


Summary: Photosynthetic reactions are

variable and responsive

Variable

Light intensity,

wavelength,

angle, duration

Responses

Light harvesting

Chloroplast and leaf

movements, accumulation of

pigments and proteins

Variable Chloroplast chemistry

Transthylakoid pH/ pmf

PQ / PQH2

Energy dissipation and photon balancing

Photoprotection

qE, quenching in LHC

* *

qT, state transition

Variable Whole-plant physiology

environmental stress

Plastid terminal oxidase

Antioxidant production

Variable Light and stress-

induced reactive oxygen

species

Protecting plants from

photo-oxoidative damage,

particularly when they are

stressed, may be a fruitful

approach to improving

photosynthesis


Summary: Photosynthesis research may

lead to better crops and fuels

Ort, D.R., et al. (2015). Redesigning photosynthesis to sustainably meet global food and bioenergy demand. Proc. Natl. Acad. Sci. USA 112: 8529–8536.; IGV Biotech; Caltech; Joint Center for Artificial Photosynthesis




https://en.wikipedia.org/wiki/Algae_fuel/media/File:Photobioreactor_PBR_4000_G_IGV_Biotech.jpg

http://www.caltech.edu/news/artificial-leaf-harnesses-sunlight-efficient-fuel-production-47635

http://solarfuelshub.org/research/index.html


Photosynthesis has to be integrated with

stress & development

Tom Donald; Tom Donald; See Allahverdiyeva, Y., Battchikova, N., Brosché, M., Fujii, H., Kangasjärvi, S., Mulo, P., Mähönen, A.P., Nieminen, K., Overmyer, K., Salojärvi, J. and Wrzaczek, M. (2015). Integration of

photosynthesis, development and stress as an opportunity for plant biology. New Phytol. 208: 647–655; FEMA

We need to understand photosynthesis

within the context of a whole plant,

considering the effects of development,

stress, age, demand for photosynthate and

all other factors that affect it.

STRESS

DEVELOPMENT

PHOTOSYNTHESIS



http://onlinelibrary.wiley.com/doi/10.1111/nph.13549/abstract



https://commons.wikimedia.org/wiki/File:FEMA_-_36365_-_Flooded_corn_field_in_Illinois.jpg

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