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Chapter 6- Photosynthesis. Where we’re going: Get the broad picture, and the comparison to respiration Understand light dependent and dark (light independent) reactions. Broad outline of what happens in the Calvin cycle and C4 carbon fixation 1 st reaction of both of the above.
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Chapter 6- Photosynthesis • Where we’re going: • Get the broad picture, and the comparison to respiration • Understand light dependent and dark (light independent) reactions. • Broad outline of what happens in the Calvin cycle and C4 carbon fixation • 1st reaction of both of the above.
Similarities and differences to respiration • Both use energy to make ATP and NAD(P)H • Plants- sun; us- food (previously fixed organic carbon) • Both use electron transport to generate an electrochemical gradient to produce ATP; plants also use electron transport to generate NADPH. • Plants have to fix their carbon, while ours comes with the energy.
Light & Dark RXNs • Light- generate ATP and NADPH. • “Dark” (light independent- Karp doesn’t like the term, since it overlooks redox regulation, but feel free to use it)- use the ATP and NADPH to fix CO2 – many of the rxns are the reverse of glycolysis.
Types of photosynthesis: Oxygenic: typical photosynthesis, with O2 being released by the use of water to donate electrons and protons: 6CO2 + 12H20 ----> C6H12O6 + 6H2O + 6O2 Occurs in plants and cyanobacteria Anoxygenic: Bacterial photosynthesis; A compound other than water is used to donate electrons and protons: 6CO2 +12H2A + -----> C6H12O6 +12A + 6H2O; A may be sulfur, or some derivative of sulfur (e.g., S2O3). Sadly, we can’t talk more about it Light-driven proton pump: This is a unique, primitive type of photosynthesis that produces nothing but a proton gradient, that can be used to make ATP. it occurs in an archaea called Halobacterium. (like 4.47) Sadly, we can’t talk more about it
Electron transport on thylacoid membrane; Proton gradient collects in thylacoid space Stroma- Dark rxns, DNA- like cytoplasm
The light reactions- let’s follow some photons as they do their work
Major player- chlorophyll: • Porphyrin ring, w/ Mg++ center. • Hydrophobic tail, anchors in membrane
Energy transfers by resonance transfer to the reaction center. Pigments maximize the wavelengths used. This is where the light finally causes electron transfer
The quiz on Friday (ignore!) • Photosynthesis- light and dark reactions • Oxygenic and anoxygenic PS • Where things happen in the chloroplast, Chl. Structure • Δ G calculations • Photorespiration, C3, C4 plants
Non-cyclic Photophosphorylation- the electrons that begin on water end up in NADPH
Clyclic Photophos. Electrons just go around and around, pumping protons for ATP production- no NADPH made
Anoxygenic Photosynthesis • Occurs in bacteria • Source of e’s is H2S, or something like that • One photosystem. • There’s an issue with some in making NADPH, but they manage.
Fixing Carbon • We now take the ATP and NADPH and CO2, and make SUGAR!
Glycolysis product! Now we reverse glycolysis, spending ATP and NADPH to reduce to 3-phosphoglyceraldehyde!
Antiport! Stroma Cytosol (cytoplasm)-
Why Karp doesn’t like the term “dark reactions”. Plants do the intelligent thing- usually only run the “dark RXNS” when there’s plenty of reducing conditions- i.e., when there’s sunlight working on PSI Calvin cycle enzyme
Photorespiration- a BAD thing! In a complicated manner, two P-glycolates react to release CO2. So it use O2 and releases CO2.
Things to know • Terms- light and dark reactions, structure of the chloroplast, antenna pigments, location of H+, Calvin cycle, • Cyclic and non-cyclic photo-phosphorylation, major electron carriers. • Narrate the light reactions. • Recognize first rxns of C3 and C4 carbon fixation, events in mesophyll and bundle sheath cells, why cells do C4 fixation.
First rxn of C3 CO2 fixation: • CO2 + 2 molecules of 3 P-glycerate
1st reaction of C4 CO2 fixation: • CO2 + CH2=C-COO- - • -OOC-CH2-C=O PO4 Phospho-enol pyruvate Oxalacetate COO-
