Biochemistry Lect 9 2011 colour 2 slides per page.pdf

8/13/2019 Biochemistry Lect 9 2011 colour 2 slides per page.pdf

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LECTURE 9: Conversion of Light Energy to Chemical

Energy - Photosynthesis

Key Concepts (the big picture):Key Concepts (the big picture):

Photosynthesis takes place in the chloroplasts.

Photoreceptive compounds (eg. chlorophyll) within thechloroplast are grouped together with other compounds, to

form photosystems, and it is these complexes which

absorb light.


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Key Concepts (the big picture) cont:Key Concepts (the big picture) cont:

The processes within photosystems are called the

light reactions. These reactions include:

- splitting of water to produce O2 and H+

- NADP+ reduction to produce NADPH

+-

WHAT YOU NEED TO KNOW!! You should:WHAT YOU NEED TO KNOW!! You should:

describe the two hotos nthetic sta es and the

overall reaction of photosynthesis

explain the structure of the chloroplast and the

molecules involved in light absorption

understand the reactions taking place during the

light reactions

discuss how the absorption of light is coupled to ATP

synthesis


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10 x 1017 kcal/year of energy is stored by photosynthesis

= 10 x 1011 tons/year carbon fixed(~5 x 109 tons petroleum consumed in 2005)


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0

500

1000

Multicellular algae

Cambrian explosion

O2 @ 5-18% present

O2 @ 100% present +

2000

2500

3000

Cyanobacteria?

Stromatolite fossils

First eukaryotes?

Sterols

Mitochondria

3500

4000

4500

Meteors stop

Carbon signatures

Greenland

2

x million years


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Jean Baptist van Helmont

1579-1644

Grew 2.5 kg tree in 91

kg soil. Five years later,only water added 76.5 kg

soil only 56 g lighter

Priestly Lavoisier

Scheele Ingenhouz


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Mint leaves in chamber sustained mouse,

That the air would neither extinguish a candle, nor was it all

inconvenient to a mouse which I put into it

Priestly

Phlogistron?

Evil air

http://s.ngeo.com/wpf/media-live/photos/000/002/cache/jar-mouse_247_600x450.jpg

IngenhouzRepeated Priestly's work

Showed that light cleaned the air


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Lavoisier's work revealed the

Lavoisier

conservat on o matter

Very precise measurements

combustion

Reactants mass = products

Priestley's theory of phlogistation

Scheele

Unfortunate habits


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Antoine Lavoisier

And lost his head for issues with his taxes


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Senebier & Saussure

Showed that the increase in plant

mass was due to the uptake of

CO2 and H2O

Julius Mayer 1814-1878

German sur eon/ h sician

Discovered that light powered reactions

Showed the conservation of energy

1st law of thermodynamics

Light transferred to chemical energy

But where was the Oxygen coming from?

H2O or CO2?


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Cornelius van Niel (1897-1985)

Showed that H2O was split not CO2

Sulphur Bacteria CO2 + 2H2S (CH2O) + 2S + H2O

Plants CO2 + 2H2O (CH2O) + O2 + H2O


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Using labeled oxygen isotopes 18O

in CO2 or H2O

van e s owe

CO2 + H218O (CH2O) +

18O2

6CO2 + 6H2O (C6H12O6) + 6O2

The site of photosynthesis in a plant


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The thylokoid membrane is impermeable to most ions and molecules

(not Mg2+ and Cl- however)

Fig 9.1 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle.

More tomorrow on this


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Photosynthesis

b has -CHO

Chloroplast thylakoid

membranes have chlorophyll a

and b

Chlorophyll


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Absorbance of Chlorophyll a & bnote lack of absorbance around 500-600 nm

Fig. 9.2

Excitedstate

ron

e

ea

PhotonGround

Photon(fluorescence)

Energy

ofelect

(a) Excitation o f isolated chlorophyll molecule (b) Fluorescence

statemolecule


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Fig. 9.3 How a photosystem harvests light

1

Photosystem II


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Near perfect quantum yield (PSI, 1.0

PSII not so good at 0.85)

Nature Reviews 2004 Volume 5

Note that there are two photosystems II and I


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e

e

e

ATP

Fig. 9.4 A

mechanical

analogy for

the light

reactions:

Mill

makes

ATP

NADPH

e

e

e

Photosystem II Photosystem I

e

P680*

Photosystem 2

Pheophytin chlorophyll minus Mg2+

e

Electron transfer and proton translocation

Plasto uinone/ol

Q QH2 Q QH2

PCQ

e

P680

Cytbf

complex

2H+ in to thylakoid space adds to proton gradient


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P680*

Photosystem 2 P680

e

Splitting of water and O2release

H2O

Mn Mn

Mn Mn

O

e

e

P680

H+ in Thylakoid space - Adds to proton gradient

H2

Bacterial Photosystem IIScience 303, 1831, 2004

Photosystem 11 (Nature 2004 Vol 5)

Splitting O2 involves some of the nastiest reactions in

the biosphere


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e

e

e

ATP

Fig. 9.4 A

mechanical

analogy for

the light

reactions:

Mill

makes

ATP

NADPH

e

e

e


e

Cytochrome-b6f

Homology to Complex III

of mitochondria


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e

e

e

ATP

Fig. 9.4 A

mechanical

analogy for

the light

reactions:

Mill

makes

ATP

NADPH

e

e

e


e

P700*

Photosystem 1 P700

A0

Acceptor chlorophylls

Ae

PC

Fe-S

Ferredoxin

Ferredoxin

reductase

NADP+NADPH

e

P700

Note that NADPH removes 2H+ removed from thylakoid space

Cytbf

complex e


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Photosystem II

Fig. 9.5 How non-cyclic electron flow during the light reactions

generates ATP and NADPH


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Mitochondrion ChloroplastFig. 9.6Comparison of

chemiosmosis in

mitochondria

and

chloroplasts

CHLOROPLAST

STRUCTURE

MITOCHONDRION

STRUCTURE

Intermembrane

space

Inner

membrane

Electrontransport

chain

H+ DiffusionThylakoid

space

Thylakoid

membrane

KeyMatrix

Higher [H+]

Lower [H+]

Stroma

ATP

synthase

ADP + Pi

H+ATP

Chloroplast ATPase


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Difference between mitochondria and

chloroplastsChloro lasts are more reliant on H roton radient

Chloroplast inner membranes permeable to Cl- and Mg2+

So mitochondria use net charge potential and pH gradient

Larger pH difference required in chloroplasts.

Difference between mitochondria andchloroplasts

In c orop asts PSII removes H+ rom stroma, an t e

cytochrome complex translocates into thylakoid space

In mitochondria Complex I, and IV pump and Complex

III translocates protons out of matrix

Mitochondria brown purple!

Chloroplasts green!


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Light

Fd

Cytochromecomplex

STROMA(low H+ concentration)


4 H+Light

NADP+

reductase

NADP+ + H+3

Fig. 9.7 A tentative model for the organization of the thylakoid membrane

ToCalvinCycle

THYLAKOID SPACE(high H+ concentration) 4 H+

Pq

Pc

NADPH

+2 H+

H2OO2

e

e

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2

ADP+

iH+

ATPP

ATPsynthase

STROMA

(low H+

concentration)

Thylakoidmembrane


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Photosystem II and I separated?

Cyclic electron flow?


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Fig. 9.8 Cyclic electron flow

Why have cyclic electronflow?

-This produces ATP only

-Some bacteria only have PSI

-Mutant plants without cyclic flow

still grow, but not in bright light?


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P700*

Photosystem 1 P700

A0

A1

Cyclic electron flow

e

PC

Fe-S

Ferredoxin

Cyt

bf

e

P700

comp ex

H+Calvin Cycle appears to require more ATP than first

thought! PS1 can make extra ATP

Tomorrow Melvin Calvin

Biochemistry Lect 9 2011 colour 2 slides per page.pdf

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