Photosynthesis ΔG > 0 ΔG = ΔH –TΔS

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1 04-19-16: Lecture 7 Photosynthesis: Photosynthesis 6CO 2 + 6H 2 O C 6 H 12 O 6 + 60 2 ; 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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Transcript of Photosynthesis ΔG > 0 ΔG = ΔH –TΔS

Slide 1ΔG = ΔH – TΔS
Heterotrophs:
Autorotrophs:
2
Photosynthesis: An anabolic pathway to produce sugars Respiration in reverse!
Occurs in Chloroplast
Photosynthesis: An anabolic pathway to produce sugars Respiration in reverse!
6CO2 + 12H2O + light energy C6H12O6 + 602 + 6H2O
Balanced equation
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
•Takes CO2 “Carbon Fixation” and energy from light rxns: ATP
NADPH
Photosynthesis: An anabolic pathway to produce sugars Respiration in reverse!
04-19-16: Lecture 7
Energy is absorbed from light
Pigments:
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)
04-19-16: Lecture 7
04-19-16: Lecture 7
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 scheme Photosystem II (680) absorbs photon need 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 absorbed donates e- to NADP+
NADP+ + 2e- + H+ NADPH
O2
NADPH
04-19-16: Lecture 7
The Light Reactions (Light Cycle)
04-19-16: Lecture 7
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 + Pi NOW 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
04-19-16: Lecture 7
04-19-16: Lecture 7
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!!!
04-19-16: Lecture 7
Photosystem ICYCLIC ELECTRON FLOW
Following the Free Energy trail!
Photons are absorbed and excite e- as the excited e- return to ground state They make ATP and then are stored in NADPH
04-19-16: Lecture 7
Called dark cycle because it does not directly require light
But requires light indirectly-
3 STEPS
Carbon fixation:
Reduction Phase:
Regeneration Phases:
Carbon Fixation: 5C + 1CO2 6C 2 (3C) Rubisco
Enzyme which catalysis rxn
NADP+ NADPH + H+
RuBP
04-19-16: Lecture 7
Both in chloroplast
Need CO2 for the dark reactions
Many Similarities between Photosynthesis and Respiration
Both use and produce ATP ATP synthase Chemiosmosis ETC pumping H+ across a membrane
Both involve glucose Both use Redox rxns: FAD and NAD+ for respiration; NADP+ for photosynthesis
04-19-16: Lecture 7