September 16, 1997                                   MAST 634 Lecture 4: Light Reactions 

Important Points from Last Week 
        Review of CO₂ chemistry 
        CO₂/HCO₃^- uptake; carbonic anhydrydrase (CA) 
        Cavin-Bassham-Benson Cycle 
        RuBPcase Structure 
        Other carboxylations 

Photosynthesis: Light reactions 
        Basic, of course, to aquatic plants 
        Need to understand to measure plants in marine environment 
                Biomass --> chlorophyll concentrations 
                Growth rates ---> fluorescent properties 

What does a plant need to fix CO₂? 
        ATP 
        NADPH ---> reducing power to reduce CO₂ to organic C 

Remember: in chemoautotrophy, oxidization of reduced inorganic compounds produced e^- for NAD(P)H and created gradients for ATP synthesis 

We will try to answer the following questions 
        How do plants make ATP from light energy? 
        Where and how do plants get e^- to make NADPH? 
        How do marine plants get light? 

General structure of P.S. apparatus answers these questions 
 

1. Heat 
        light E ---> kinetic E = heat 
        very fast process <10^-11sec 

2. Fluorescence 
        - E goes off as another photon, i.e. light 
        - relatively slow process 10^-8sec 
        - emitted photon has less energy 

What is wave length? 
                         hc               lambda = 700 mM 
       E = hv =  lambda                                E = 171 KJ/einstein 

                                                                 Einstein is a mole of photons 

So, E excited > E emitted 
.^.. excited < emitted 

small amount (3-6%) of total absorbed light goes off as fluorescence = in vivo fluorescence very important in examining plants by remote sensing, growth rates 

3. Exciton transfer; resonance energy 

      hv pigment A ---> pigment B 
          "thing" transferred between pigments is "exciton" 
 
4. Photo-oxidation 
      "Bad"; light absorbed by other compounds, not designed to transfer energy ---> can irreversibly       destroy (oxidize the compound). 
      "Good"; excited Chl ---> photochemistry and eventually production of NADPH and ATP. 

Let's discuss reaction center and electron transfer chain. Next lecture will deal with light harvesting pigments. 
 
 
How is ATP and NADPH made? Answer comes from looking at this equation 
                        CO₂ + H₂O = CH₂O + O₂ 

      [A side: Goal here is to understand final "answer", not just give it.] 

Known since Priestly in 1772 that plants evolve O₂ 

But where does O₂ come from? 

Modern day experiment 

      use stable isotope of O, ¹⁸O 
      CO₂ + H₂ ¹⁸O ----> (CH₂O) + ¹⁸O₂ 
i.e., O₂ comes from H₂O 

C.B. van Niel at Hopkins Marine Lab (Stanford) 

                                light 
       CO₂ + 2H₂S ----------> (CH₂O) + 2S^o + H₂O 

                green photosynethic bacteria: example of anoxygenic photosynthesis only bacteria do this 

eukaryotic plants and cyanobacteria: evolve O₂ = oxygenic photosynthesis 

van Niel deduced that O₂ come from splitting of H₂O (oxidation of H₂O) 

Anoxygenic P.S. bacteria cannot split O₂ 

Why not? 
     -depends on pigment in reaction center 
     - reaction center has to become a strong oxidizer to oxidize H₂A where A is S (H₂S) or O (H₂O) 
 

     since deltaE^o´>O for deltaG<O 
          E^o´_pigment> E^o´_H2A 

          Specifically for H₂O E^o´_pigment > 0.82 

     Another redox problem 

                                                           E^o´ V  
          NADP⁺ + H⁺ + 2e^- = NADPH -0.32 

Here: Delta E^o´ = E^o´_NADP/NADPH- E^o´_pigment 
          E^o´_pigment < -0.32 

[NOTE: This web program cannot transcribe some greek symbols; this is why "delta" or other characters may be written out] 

Various "solutions" to this problem during evolution of photosynthesis 

Purple bacteria: Cyclic e^- transport version of this in oxygenic photosynthesis 
 
 

 

P₈₇₀ = not strong enough oxidizer for H₂O  

P₈₇₀ = type of bacterial chlorophyll (bchl) 

P_x Maximum absorbance at x nm 

Net Result 
      e^- flow does set up H ⁺ and charge gradient ---> ATP synthesis 
     But no NADPH synthesis 

Where do these bacteria get e^- for NAD(P)H? 
      Oxidation of 
          H₂S 
         organics (photo-organotrophs) 

Green Bacteria 

  

P₈₄₀ strong enough oxidizer for S^2- oxidation but not water 

Finally, oxygenic photosynthesis: see handout 

Three components in "Z scheme" 

      1. Photosystem II: H₂O ---> 1/2 O² + proton gradient 
      2. Photosystem I: NADP⁺ ---> NADPH 
      3. Cytochrome b₆-f: connection between PSI and II. 

Some evidence for this scheme: action of DCMU 
      - useful herbicide 
      - stops PSII (actually e- exchange at cytochrome b₆-f) 
All components are in membranes 
      - only exception in P.S. are some pigments in cyanobacteria, which we will discuss in next lecture. 

Flow of e- 
      Some cyclic ET 
      P 700* goes back to cytochrome b₆-f 
      similar process as in purple bacteria 
      but PSII is closer genetically to bacterial (anoxygenic) photosynthesis 

Balance between PSI and PSII 
      Depends on whether NH₄⁺ and NO₃^- is used as N source 
      NO₃^- ---> needs more e^- ---> more PSI 

ATP synthesis 
      Simple diagram --> see handout 
      See Voet and Voet (p 640) 

Proton-motive force, once again 
      H⁺ pumped out of stoma into thylakoid 
      H⁺ pumped out of bacterial cell 
      Creates H⁺ and charge gradient, but Mg²⁺ also pumped in and C (symbol: ell)^- pumped out of thylakoid space which removes charge gradient.