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04-19-16: Lecture 7

Photosynthesis:

Photosynthesis6CO2 + 6H2O C6H12O6 + 602 ; ΔG = +686 kcal

ΔG = ΔH – TΔS

ΔG > 0 (non spontaneous)

An anabolic pathway to produce sugars Respiration in reverse!

Heterotrophs:

Autorotrophs:

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Photosynthesis: An anabolic pathway to produce sugars Respiration in reverse!

Occurs in Chloroplast

Thylakoid space

Thylakoid

Stroma

Outer membrane

inner membrane

inner membrane space

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Photosynthesis: An anabolic pathway to produce sugars Respiration in reverse!

6CO2 + 12H2O + light energy C6H12O6 + 602 + 6H2O

Balanced equation

2 Reactions to produce sugars:

Light rxns (light cycle(: in thylakoids

•Light absorbed e- is excited••H2O 2H+ + 2e- + ½ O2

•e- is transferred to NADP+(carrier enzyme)

•Makes ATP by chemiosmosis

(produce O2!)

NADP+ + 2e- + H+ NADPH

reduced

Dark rxns (dark cycle): in stroma

•Takes CO2 “Carbon Fixation” and energy from light rxns: ATP

NADPH

•Makes sugars

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Photosynthesis: An anabolic pathway to produce sugars Respiration in reverse!

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Photosynthesis: The Light Reactions (Light Cycle)

Energy is absorbed from light

Pigments:

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Photosynthesis:

Energy is absorbed from light

Pigments: absorb light at discrete wavelengths

Chlorophylls (pigments)•Light energy stored in ring structure•Cofactor is Mg2+

•Have long hydrocarbon chain: embedded in membrane•e- is excited to a higher orbital state by light•Excited e- is very unstable – wants to come back to ground state•Donates its electrons to NADP+ (carrier enzyme)

Chlorophylls are clustered in photosystems!

The Light Reactions (Light Cycle)

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Photosynthesis: The Light Reactions (Light Cycle)

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Photosynthesis:

Photosystem I and II

Photosystem I is a 700nm system Chlorophylls absorb light at 700nm

Photosystem II is a 680nm system Chlorophylls absorb light at 680nm

(Work together!)

General schemePhotosystem II (680) absorbs photonneed e- from H2O

H2O 2H+ + 2e- + ½ O2

e- goes into electron transport chain (ETC) into Photosystem I (700nm)a second photon of light is absorbeddonates e- to NADP+

NADP+ + 2e- + H+ NADPH

NEED 2 photons of lightH20NADP+

O2

NADPH

NON CYCLIC ELECTRON FLOW

The Light Reactions (Light Cycle)

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Photosynthesis:

Photosystem I and II working togetherNON CYCLIC ELECTRON FLOW

The Light Reactions (Light Cycle)

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Photosynthesis:

Chemiosmosis

H20 in thylakoid space is split – e- transferred to photosystems

H2O 2H+ + 2e- + ½ O2

e- travel through the ETC causes 2 more H+ to be pumped into thylakoid space

Thylakoid space :

e- eventually transferred to NADP+

ATP synthase (in thylakoid membrane ) uses proton motive force to make ATP

High [H+] in thylakoid space moves into stroma through AP synthase

ATPADP + PiNOW ATP and NADPH in stroma ready to use by Dark Cycle!!!

The Light Reactions (Light Cycle)

Note: 1:1 ATP:NADPH made by each H20 split

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Photosynthesis:

Chemiosmosis

The Light Reactions (Light Cycle)

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Photosynthesis: The Light Reactions (Light Cycle)

Photosystem ICYCLIC ELECTRON FLOW

(cyclic Photophosphorylation)

Photosystem I:

•H+ get pumped across into the thylakoid space•Make ATP by chemiosmosis and ATP synthase

•BIG CYCLE for e- transfer•Requires light!!!

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Photosynthesis: The Light Reactions (Light Cycle)

Photosystem ICYCLIC ELECTRON FLOW

(cyclic Photophosphorylation)

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Photosynthesis: The Light Reactions (Light Cycle)

Following the Free Energy trail!

Photons are absorbed and excite e-as the excited e- return to ground stateThey make ATP and then are stored in NADPH

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Photosynthesis: The Dark Reactions (Dark Cycle)

(also known as Calvin Benson Cycle)

Called dark cycle because it does not directly require light

But requires light indirectly-

CO2 is the carbon source

Product is G3P (3C) (glyceraldehyde-3-phosphate)

3 STEPS

Carbon fixation:

Reduction Phase:

Regeneration Phases:

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Photosynthesis: The Dark Reactions (Dark Cycle)

Carbon Fixation: 5C + 1CO2 6C 2 (3C)Rubisco

Enzyme which catalysis rxn

Reduction: 3PG (3C) G3P(3C) Glucose

NADP+NADPH + H+

ADPATP

Regeneration: 6CO2 + 6RuBP (5C) 12 G3P

RuBP

ATP NADPH

2 G3P1 Glucose

ADPATP

10 G3P

To regenerate RuBP

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Photosynthesis: The Dark Reactions (Dark Cycle)

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Photosynthesis: Linking the Light and Dark Cycles

Both in chloroplast

No light

Need H20 for light reactions

Need CO2 for the dark reactions

Many Similarities between Photosynthesis and Respiration

Both use and produce ATPATP synthaseChemiosmosisETCpumping H+ across a membrane

Both involve glucoseBoth use Redox rxns: FAD and NAD+ for respiration; NADP+ for photosynthesis

04-19-16: Lecture 7