BCH/PPA 503 -- Plant
Biochemistry
Lecture Four

Electron Transport and Energy Capture

Outline

Pigments and Energy Transfer

 

 

Life on earth requires a constant input of energy.  The principal source of this energy is radiation reaching the earth from the sun.  This radiation has wavelengths differing by more than 106, ranging from <0.1 nm to >1 m.

 

 

 

 

 

 

The shorter the wavelength of the radiation, the more energy it has. Light Energy-caused e- excitation can: When a quantum of light energy (hn) is adsorbed by a pigment molecule (P) it causes e- to jump to a higher energy level forming an excited state (P*):

 

 

 

 

 

 

 

 

The light adsorbing pigment molecules that harvest light energy for photosynthesis are chlorophylls and carotenoids in higher plants. Adsorption and action spectra (see image):

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Chlorophylls are Mg+2 containing substituted tetrapyrroles structurally related to hemes, Fe+2 containing porphyrins.  The structure of chlorophyll is given in Fig. 1 of the article by Wettseien et al. (1995) of the Plant Cell.

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:

[Image of 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:

2H2O + 2NADP+ + xADP + xPi + nhv -> O2 + 2NADPH + 
2H+ + xATP + xH2O

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:

 

 

 

 

 

 

Thus, PSI and II can be linked in series and e- flow from H2O to NADP+, driven by light energy adsorbed at the reaction centers.  O2 is a by-product of the photolysis of H2O.  Phosphorylation of ADP is caused by a proton gradient that is established across the thylakoid membrane as a consequence of the light-induced e- transfer reactions.  This light-driven phosphorylation is known as photophosphorylation and is similar to the chemiosmotic e- transport occurring in mitochondria.  This process has classically been represented by the 'Z scheme' of e- transfer:

[Image of Z scheme]
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.

 


  • All materials © 1998, 1999, 2000, Dr. David Hildebrand or Dr. Bob Houtz, unless otherwise noted.
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    This page was last modified January 17, 2000.