Pigments and Energy Transfer

1)Chemical and Physical considerations

a)membrane location

b)electronic orbitals

2)Mechanisms of excitation transfer

3)Light and Light absorption

a)Absorption/Action Spectra

4)Mechanisms for increasing/decreasing light absorption

5)Macro and Micro environmental effects

a)chloroplast membrane components

• Wavelengths 300 nm and less have sufficient energy to break H-bonds
• Wavelengths 100 nm and less have sufficient energy to generate free radicals
• Visible light (~370-750 nm) has sufficient energy to excite e^-, but not enough to affect H-bonding
• Water and CO₂ filter out long wavelengths
• Ozone filters out short wavelengths
• It is estimated that ~1.5 x 10²² kJ of light energy reaches the earth each day from the sun
• About 1% of this is absorbed by photosynthetic organisms and transduced into chemical energy

• be released as heat (thermal dissipation)
• be released as other light (fluorescence)
• be absorbed and then released after a finite period of time (phosphorescence)
• excite other proximal molecules (exciton transfer)
• be used (trapped) by chemical reactions

Chlorophylls are excellent light adsorbers because of their aromaticity and planar structure.  They possess delocalized p electrons above and below the planar ring.  The energy differences between electronic states in these p orbitals correspond to the energies of visible light photons.  Every chlorophyll is synthesized in the chloroplasts from 8 molecules of 5-aminolevulinic acid.  Chlorophyll serves 2 roles in photosynthesis.  It 1) is involved in light harvesting or the transfer of light energy to photoreactive sites by exciton transfer and 2) it participates directly in the photochemical events whereby light energy becomes chemical energy.  These processes take place in integral membrane-protein complexes known as light-harvesting (or antenna) complexes and reaction centers.  Oxidation of the chlorophyll molecule in the reaction center leaves a cationic free radical, Chl^·+.  The Mg⁺² does not change in valence during redox reactions of chlorophyll.  A photosynthetic unit is like an antenna or satellite dish of several hundred light-harvesting chlorophyll (& carotenoid) molecules (usually ~300 chlorophylls per reaction center with a chlorophyll a/b ratio of 2:3) in a pigment-protein complex in the thylakoid membranes of chloroplasts plus a special pair of photochemically reactive chlorophyll a molecules within the unit called the reaction center:

This image is from a University of Minnesota page.

There are actually 2 reaction centers whereby light energy is converted into chemical energy, photosystems II (PSII) and I (PSI).  The 2 photosystems are connected via an electron transport chain that includes another integral membrane complex, the cytochrome b6f complex.

Photosynthesis can be represented as the reduction of NADP⁺ by e^- derived from H₂O using light energy, hn, with some ATP being generated in the process.  The overall equation of the light reactions of photosynthesis is:

where n is the # of photons, h is Planck's constant and n is the frequency of the light.

The separation of the oxidizing and reducing aspects of this equation is accomplished by devoting PSI to NADP⁺ reduction and PSII to water oxidation.  PSI and PSII are linked via an electron transport chain wherin the weak reductant generated by PSII provides the electron to reduce P700⁺, a weak oxidant, restoring it for another cycle of photochemical activity:

This image is from M.J. Farabee's web site.

This so-called Z scheme or non-cyclic photophosphorylation begins with the removal of an e^- from the special pair of photochemically reactive chlorophyll a molecules of P680 in PSII by light energy.  The P680⁺ abstracts an e^- from H₂O breaking it into H⁺ ions and O^-2 ions.  These O^-2 ions combine to form diatomic O₂ that is released.  The light energy accumulated at the P680 reaction center boosts the e^- to a higher energy state and it is attached to a primary e^- acceptor, plastoquinone, which begins a series of redox reactions, passing the e^- through cytochrome b6, the Rieske iron-sulfur center, cytochrome f, plastocyanin and finally to P700⁺.  Photons accumulating in P700 cause an e^- to be boosted to a still higher redox potential after which it is passed to ferredoxin.  The e^- from PSII replaces the excited e^- in the P700 molecule.  Ferredoxin donates the e^- to a flavoprotein, ferredoxin-NADP⁺ reductase which catalyzes the reduction of NADP⁺ to NADPH.