Photosynthesis Basic

8/2/2019 Photosynthesis Basic

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Photosynthesis:

harnessing solar energy


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We aim to

Understand the underlying principles of

photosynthetic production of organic carbon.

Become aware of the need for photophysical andbiochemical steps in carbon fixation.

Have an understanding of the how the nature ofmacromolecular components underlies their function.

Understand the origins, evolution, biogenesis androle in plant biology of chloroplasts and other plastid

types.


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Texts:

Sadava et al., Chapter 8

The life wire:http://bcs.whfreeman.com/thelifewire8e/

Raven et al. 2004, Chapter 7, 581 RAV

Berg et al. 2006, Chapters 19-20, 574.7 STR

Web links and a useful video in Moodle

Source of graphics: Sadava et al. andBuchanan et al. (2000), Biochemistry and Mol. Biol. of Plants.ASPB. 581.192 BIO


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Photosynthesis: Energy from the Sun

Powering the Biosphere Photosynthetic Reactants and Products

The Two Stages of Photosynthesis

The Interactions of Light and Pigments

The Light Reactions: Electron Transport, Reductions Photophosphorylation

Making Carbohydrate from CO2: The CalvinBensonCycle

Chloroplast biogenesis and genetics The evolution of chloroplasts

The evolution of photosynthesis


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Energy and matter (carbon)

Source of energy Source of carbon Photo- (light) Auto- (CO2)

or or

Chemo- (chemical) Hetero- (organic)

Organisms can be

Photo auto -trophic

Photo hetero -trophicChemo auto -trophic

Chemo hetero -trophic


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Tube worm deep-vent ecosystem

Chemotrophs


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Photosynthesis powers the biosphere

Ca. 120 Gigatonnes of C photosynthetically fixed on land/yearCa. 50 Gigatonnes fixed into oceans/yearEnough to fill 100 x 100 x 25 Km with sugar crystals.

Chlorophyll


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Identifying PhotosyntheticReactants and Products

By the 1800s, scientists had learned: Plants do not feed from soil: a tree increases in weight more

than the soil it grows in decreases. Perhaps the water!

Plants regenerate air exhausted by a burning candle, but

only in the light (Priestley). Three ingredients are needed for photosynthesis: water,

CO2, and light.

There are two products: carbohydrates and O2.

The water, which comes primarily from the soil, istransported through the roots to the leaves.

The CO2 is taken in from the air through stomata, or pores,in the leaves.


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The Ingredients for Photosynthesis


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Identifying PhotosyntheticReactants and Products

By 1804, Ingenhousz had summarized theoverall chemical reaction of photosynthesis:

CO2 + H2O + light energy sugar (CH2O) + O2

More recently, using H2O and CO2 labeled withradioactive isotopes, it has been determined thatthe actual reaction is:

6 CO2 + 12 H2O C6H12O6 + 6 O2 + 6 H2O


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The photosynthetic equation

6CO2 + 6H2O -> C6H12O6 + 6O2


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The photosynthetic equation

6CO2 + 6H2O -> C6H12O6 + 6O2

Photosynthesis tutor.http://www.life.uiuc.edu/cheeseman/JC.software.html


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Light and "dark" reactions


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In eukaryotes, photosynthesis takes place in chloroplasts.

Leaves are ultimately collections of chloroplasts (like solar cells)exposed to light


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Chloroplasts have internal membranes, the thylakoids,many forming stacks or grana, and a stroma.


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Stroma

Thylakoids

Thylakoid

Chloroplast

Lightreactions

CO2 fixationreactions

Light(photon)

Chlorophyll

Overview of Photosynthesis


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Visible light is part of the electromagneticradiation spectrum. It comes in discrete packetscalled photons.

Light also behaves as if it were a wave.

Two things are required for photons to be activein a biological process:

Photons must be absorbed by receptivemolecules.

Photons must have sufficient energy toperform the chemical work required.


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The Electromagnetic Spectrum


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When a photon and a pigment molecule meet,one of three things happens: The photon maybounce off, pass through,or be absorbed bythe molecule.

If absorbed, the energy of the photon isacquired by the molecule.

An electron in the molecule is raised from itsground state to an excited state of higher

energy.


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Exciting a Molecule


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Molecules that absorb wavelengths in thevisible range are called pigments.

When a beam of white light shines on anobject, and the object appears to be red in

color, it is because it has absorbed all othercolors from the white light except for thecolor red.

In the case of chlorophyll, plants look green

because they absorb green light lesseffectively than the other colors found insunlight.


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A molecule can absorb radiant energy of onlycertain wavelengths.

If we plot the absorption by the compound asa function of wavelength, the result is an

absorption spectrum. If absorption results in an activity of some

sort, then a plot of the effectiveness of thelight as a function of wavelength is called an

action spectrum.


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The chlorophyll molecule has two possible excitedstates, caused by absorption of blue or red light.

Ab i d A i S


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Absorption and Action Spectra


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The Molecular Structure of Chlorophyll: a tetrapyrrole


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Plants have two predominant chlorophylls:chlorophyll aand chlorophyll b.

These chlorophylls absorb blue and redwavelengths, which are near the ends of the

visible spectrum. Other accessory pigments absorb photons

between the red and blue wavelengths andthen transfer a portion of that energy to

chlorophylls. Examples of accessory pigments are thecarotenoids, such as -carotene.


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Carotenoids arephotosynthetic

accessory pigmentsthat absorbblue/green light


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Excited chlorophyll is unstable. The excitation candecay in different ways:

Photochemistry:

pigment + acceptor

pigment+ + acceptor-

pigment* + acceptor

donor+

+ pigment + acceptor-


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Resonance energy transfer allows the transfer ofthe excitation by light between antenna pigments,until it reaches the chlorophyll molecule at theheart of the reaction centre.

http://www.biologie.uni-hamburg.de/b-online/library/bio201/psunit.html


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Charge separation takes place at the reaction centre

Excited chlorophyll (Chl*) in the reactioncentre acts as a reducing agent.

The electrons of an excited molecule are lesstightly held by the nucleus, and more likely to

be passed on in a redox reaction to anoxidizing agent.

Chl* can react with an oxidizing agent in areaction such as:

Chl* + A Chl+

+ A

. Chlorophyll becomes a reducing agent and

participates in a redox reaction.

E f d El


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Energy Transfer and Electron Transport


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Electron transfer takes place from a molecule/atom with lowaffinity for electrons (the donor, that becomes oxidised) to one

with higher affinity (the acceptor, that becomes reduced)

Redox reactions consist of the transfer ofelectrons


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We can measure the affinity as electrochemicalpotential or EO

Electrons flow spontaneously from more negative to

more positive electrochemical potentials

-

+

EO

electrons


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Two distinct photosystems exist

Experiments in the 1940s used pulses of light

to observe photosynthesis.Simultaneous pulses of light of wavelengths of700 nm (~far-red) and any other light up to680 nm activated photosynthesis much more

together than given separately (far-redenhancement).

The two need to work together, because they work

in series. Between them, electron transport occurs.

The conclusion emerged that two separatephotochemical reactions took place during

photosynthesis:One up to 680 nm (in photosystem II).One up to 700 nm (in photosystem I).


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The Z scheme of photosynthesis

Light provides the boost needed for electrons to flow

uphill, from a strong oxidant to a strong reductant


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Photosystem II diagram:


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Photosystem II reaction centre shows at its heartthe arrangement of the special pair of chlorophylls

P680(a special pair of chlorophylls)


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The antenna or light-harveting complex ofphotosystem II (LHC II)


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Therefore oxygen is a result of the splitting of water

Part of photosystem II is the oxygen evolving complexFour photons are absorbed for one molecule of oxygen to evolve.

The special pair of chlorophylls in PSII is such that,when oxidised, it can collect electrons from water, anextremely reluctant donor, releasing oxygen


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The Light Reactions: Electron Transport,Reductions, and Photophosphorylation

The energized electron that leaves the Chl* inthe reaction centre immediately participatesin a series of redox reactions.

The electron flows through a series of

carriers in the thylakoid membrane, a processtermed electron transport.

Two energy rich products of the lightreactions, NADPH + H+ and ATP, are theresult.

Chemiosmotic synthesis of ATP in thethylakoid membrane is calledphotophosphorylation.


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In noncyclic electron transport, twophotosystems are required.

Photosystems are light-driven molecular unitsthat consist of many chlorophyll molecules and

accessory pigments bound to proteins inseparate energy-absorbing antenna systems.


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Photosystem I uses light energy to reduceNADP+ to NADPH + H+.

The reaction centre contains a chlorophyll amolecule called P700 because it best absorbs

light at a wavelength of 700 nm.


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Photosystem II uses light energy to oxidizewater molecules, producing electrons, protons,and O2.

The reaction centre contains a chlorophyll a

molecule called P680 because it best absorbslight at a wavelength of 680 nm.

To keep noncyclic electron transport going,both photosystems must constantly beabsorbing light.


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1. Photosystem II

uses light to oxidizewater molecules,

producing electrons,

H+, and O2.

2. The Chl molecule

in the reaction centreof photosystem II

absorbs light

maximally at 680

nm, becoming Chl*.

3. H+ from H2O and

electron transportthrough the redox

chain is captured for

the chemiosmotic

synthesis of ATP.

4. The Chl molecule

in the reaction centreof photosystem I

absorbs light

maximally at 700

nm, becoming Chl*.

5. Photosystem I

reduces an oxidizingagent, ferredoxin

(Fd), which in turn

reduces NADP+ to

NADPH + H+.

The electron transport chain uses both photosystems

The Z scheme functions through 3 main membrane


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The Z scheme functions through 3 main membranecomplexes: two photosystems and an interveningelectron-carrying cytochrome complex.

The electron transport chain is the major contributor to

the generation of an H+ gradient


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The three complexes plus the ATP synthase occupy thethylakoids, although in a heterogeneous way (the internal

membranes of grana only have photosystem II


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The type of electron transport seen so far iscalled noncyclic electron transport and produces

NADPH + H

+

and ATP and O2.There is a second type:

Cyclic electron transport produces only ATP.

The process is called cyclic because the

electron passed from an excited P700 moleculecycles back to the same P700 molecule.

Water does not enter into the cyclic electronflow reactions, and no O2 is released.

C li El t T t T Li ht E ATP l


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Cyclic Electron Transport Traps Light Energy as ATP only


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ATP is produced by a chemiosmotic mechanismsimilar to that of mitochondria, calledphotophosphorylation.

High-energy electrons move through a series

of redox reactions, releasing energy that isused to transport protons across themembrane.

Active proton transport results in the proton-motive force: a difference in pH and electriccharge across the membrane.


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If thylakoids in solution are exposed to light, theexternal solution increases in pH

and this is sufficient to synthesise ATP

pH up


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The change in pH is all that is needed to produceATP, as this can happen without light


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The electron carriers in the thylakoidmembrane are oriented so as to move protonsinto the interior of the thylakoid, and theinside becomes acidic with respect to theoutside.

This difference in pH leads to the diffusion ofH+ out of the thylakoid through specificprotein channels, ATP synthases.

The ATP synthases couple the formation ofATP to proton diffusion back across thethylakoid membrane.

The Chemiosmotic mechanism of photophosphorylation (ATP synthesis)


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The Chemiosmotic mechanism of photophosphorylation (ATP synthesis)


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The ATP synthase is a molecular device with arotary mechanism of action

ATP


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The rotary action of the ATP synthase

John Walker and Paul Boyer, Nobel in chemistry 1997.

http://www.mrc-dunn.cam.ac.uk/research/atp_synthase/movies.php


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The rotation of the ATP synthase has been visualised

Yoshida lab, http://www.res.titech.ac.jp/~seibutu/


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The rotation of the ATP synthase has been visualised


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Making Carbohydrate from CO2:The CalvinBenson Cycle

The reactions of the Calvin-Benson cycle takeplace in the stroma of the chloroplasts.

This cycle does not use sunlight directly; but itrequires the ATP and NADPH + H+ produced in

the light reactions, and these can not bestockpiled.

Thus the Calvin-Benson reactions require lightindirectly and take place only in the presence oflight.

Th h f CO


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The pathway of CO2was traced usingradiolabelled 14C

3 sec 30 sec


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Experiments that revealed that the steps ofthe CalvinBenson cycle required radioactivelylabeled carbon in CO2.

Exposure of Chlorellacells 14CO2 for 3

seconds resulted in one compound that waslabeled with 14C.

The compound was a 3-carbon sugar called 3-phosphoglycerate (3PG).

Other products of the cycle were found byincreasing the length of time of exposure in astepwise manner until the whole pathway wasrevealed.


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The initial reaction of the CalvinBenson cyclefixes one CO2 into a 5-carbon compound,ribulose 1,5-bisphosphate (RuBP).

An intermediate 6-carbon compound forms,

which is unstable and breaks down to form two3-carbon molecules of 3PG.

The enzyme that catalyzes the fixation of CO2is ribulose bisphosphatecarboxylase/oxygenase, called Rubisco.

R BP I th t b d t t i i th i !


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RuBP Is the most abundant protein in the universe!

R BP I th C b Di id A t


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RuBP Is the Carbon Dioxide Acceptor


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The CalvinBenson cycle consists of threeprocesses:

Fixation of CO2, by combination with RuBP,catalyzed by rubisco: CO2 fixation

Conversion of fixed CO2 into carbohydrate(G3P). This step uses ATP and NADPH.Reduction

Regeneration of the CO2 acceptor RuBP byATP: Regeneration

The end product of the cycle is glyceraldehyde3-phospate, G3P

The Calvin-Benson cycle


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The Calvin Benson cycle

The complete Calvin-Benson cycle


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The complete Calvin Benson cycle


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The end product of the cycle is glyceraldehyde3-phosphate, G3P.

There are two fates for the G3P:

One-third ends up as starch, which is stored

in the chloroplast and serves as a source ofglucose.

Two-thirds is converted to the disaccharidesucrose, which is transported to other organs.


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chloroplast

proplastid

elaioplastchromoplast

etioplast amyloplast

Chloroplasts are part of a family of organelles


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Chloroplasts divide, independently of mitosis


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Chloroplasts evolved from photosynthetic prokaryotes


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Chloroplasts evolved from photosynthetic prokaryotessimilar to cyanobacteria

The chloroplast genetic machinery is similarto that of bacteria

Several chloroplast division proteins aresimilar to those of bacteria

Chloroplast protein import proteins arerelated to bacterial protein secretion(export) ones.

Bacteria help us uncover the true nature of


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Bacteria help us uncover the true nature ofphotosynthesis

2H2S + CO2 -> (CH2O) + 2S + H2OPhotosynthetic sulphur bacteria

2H2A + CO2 -> (CH2O) + 2A + H2O

The generalised, Van Niel equation:photosynthesis is a light-driven redox process.

2H2O + CO2 -> (CH2O) + O2 + H2O

H2O + CO2 -> (CH2O) + O2

Oxygenic photosynthesis in cyanobacteria

is actually

O i i f h t th ti lif


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Origin of photosynthetic life

Some of the earliest

evidence for theexistence of life isthe presence ofstromatolites.

O l i h d h f h l


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Oxygen evolution changed the nature of the planet

Oxygen accumulated gradually

Aerobic (rather than anaerobic) metabolism made it possibleto extract energy from food to a much greater extent.

Eventually ozone (03) would also make life on land possible.

The huge diversity of marine phytoplankton shows


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The huge diversity of marine phytoplankton showssome of the variety of photosynthetic protists.

Plants derive specifically from single cell organisms


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Plants derive specifically from single cell organismsin the green lineage, like Chlamydomonas: (hereundergoing sexual reproduction)

Non-green photosynthetic protists are hugely


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g p y p g ysuccessful in their own right. Here a marine kelpforest (brown algae)

Among the protists, a variety of photosynthetictypes exists


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types exists


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On the origin of chloroplasts

Overwhelming molecular evidence supports the origin of

chloroplasts from cyanobacteria, most probably as a single event

The malaria parasite has a remnant plastid!

Extraordinary vestigial evidence is the nucleomorph

(remnant second eukaryotic nucleus) in the plastids ofcryptomonads.

Secondary and tertiary endosymbiosis, mostly of red algae,explain the chloroplasts with three or more membranes inhaptophytes, diatoms and brown algae, and dinoflagellates.

Extraordinary vestigial evidence is the remnant ofproteoglycan (bacterial cell wall) in the plastid of glaucophytes.

A chloroplast family tree


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p y

Remnantpeptidoglycan

Remnant

nucleomorph

Remnantnucleomorph

Photosynthesis Basic

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