Biochemistry Online: An Approach Based on Chemical Logic

Biochemistry Online





Learning Goals/Objectives for Chapter 8D: 

After class and this reading, students will be able to

  • described the mechanistic similarities between mitochondrial oxidative/phosophorylation in which NADH and FADH2 are regenerated on reduction of O2 and the light reaction of photosynthesis in which O2 and a reducing agent, NADPH are produced;
  • describe similarities in fluorescence resonance energy transfer and exciton transfer;
  • describe the difference in properties between chlorophylls acting as antennae and chlorophylls at the reaction center;
  • describe how sunlight driven excitation of chlorophyll molecules at the reaction center produces an oxidzing agent strong enough to oxide water and form O2, itself a powerful oxidizing agent;
  • explain the general flow of electrons from dioxgen to NADP+ through a series of mobile and membrane protein bound electron carriers in the Z scheme of electron transport in the chloroplast thylacoid membranes;
  • explain with picture diagrams how oxidation of H2O and phosphorylation reactions (to produce ATP) are coupled in in the Z scheme;

D2. Photoexcitation and Electron Transfer

Obviously, the energy to power the light reactions comes directly from sunlight. Clue two is that plants have an organelle that animal cells don't - the chloroplast. Its structure is in many ways similar to a mitochondria in that it has internal membranes (thylacoid membranes) surrounding enclosed compartments.

Plants have many pigments (chlorophyll, phycoerthryins, carotenoids, etc.) whose absorption spectra overlap that of the solar spectra. The main pigment, chlorophyll, has a protophorphryin IX ring (same as in heme groups) with Mg instead of Fe. When the chlorophyll absorbs light, the excited electrons must relax eventually to their ground state. It can do this by either radiative or nonradiative decay. In radiative decay, a photon of lower energy is emitted (after some energy has already been lost by vibrational transitions) in a process of either fluorescence or phosphorescence.  In nonradiative decay, the energy of an excited electron can be transferred to another similar molecule (in this case other chlorophyll molecules) in a process which excites the energy of an electron in the second molecule to the same excited state. (It is as if a photon is released by the first excited molecule which then is absorbed by an electron in a second molecule to excite it to the same exited state, although the excitation occurs without photon production).   In this fashion, energy is transferred from one chlorophyll to another. This type of energy transfer is called resonance energy transfer or exciton transfer.

Figure:   resonance energy transfer

One type of chlorophyll has slightly different characteristics, however. Because of its unique environment, the energy level of the excited state electron is lower than in the rest of the chlorophyll molecules, in much the same way that pKa's of amino acid side chains differ with environment, and the standard reduction potential of FADs that are tightly bound to enzymes differ due to the different environment of FAD/FADH2  These unique chlorophyll centers are called reaction centers.

Figure:   reaction centers

The rest of the chlorophyll molecules act as antennas which transfer energy to the reaction centers.

Figure:  Antennae Proteins

(reprinted with permission from Kanehisa Laboratories and the KEGG project: )

An electron in an adjacent excited state chlorophyll (which is at a higher level than the excited state energy of the reaction center) can then be transferred to this lower energy state level in the reaction center, in a process which forms a positively charged ion from the first excited state molecule and an anion from the recipient reaction center. This process of energy transfer is called electron transfer.

Figure:  electron transfer


Return to Chapter 8D:  The Light Reactions of Photosynthesis Sections

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Archived version of full Chapter 8D:  The LIght Reaction of Photosynthesis


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