FEATURE ARTICLE
How Plants Produce Dioxygen
At its core, oxygen production comes down to the chemistry of a poorly understood manganese-containing complex in the membranes of plant chloroplasts
Veronika Szalai, Gary Brudvig
Charge Separation, Electron Transfer
When light hits a plant leaf, chlorophylls in the chloroplast thylakoid membrane harvest the light energy and deliver it to the PSII reaction center. What happens to the light energy when it reaches PSII, and how does the light energy get converted to chemical energy? Like chlorophylls in the thylakoid membrane, the chlorophyll molecules in PSII accept the light energy transferred to them. However, one chlorophyll species in PSII differs from other chlorophylls in two respects. First, the chlorophyll species in PSII "traps" light energy as soon as it arrives by initiating a charge-separation reaction, prohibiting escape of the light energy to surrounding chlorophylls. Second, the chlorophyll species in PSII is not a spatially isolated chlorophyll molecule like those found in other regions of the thylakoid membrane. Instead, it is composed of two adjacent and identical chlorophyll molecules. For this reason, it is called a special pair.
The special pair of chlorophylls in PSII traps light energy in order to turn it into chemical energy. The first step through which photosynthetic organisms convert light energy to chemical energy is called photoexcitation (Figure 9). When a chlorophyll pair, or dimer, absorbs light energy, it has more energy than it originally possessed and is referred to as being in an excited state. In this excited state, electrons associated with the chlorophyll dimer are not as tightly bound and can be easily removed. The chlorophyll dimer is called a donor species because it donates an electron to another molecule. The recipient of these electrons is called an acceptor and it is reduced on acceptance of the electrons released by the chlorophyll dimer. The electrons that are passed from one molecule to another are "unpaired," since they exist as single negative charges. Creation of a pair of charges, one positive and one negative, following photoexcitation is the way that plants convert light energy into chemical energy.
Donor and acceptor molecules work together to stabilize opposite charges. Figures 8 and 9 show that after light is absorbed, an initial donor, a pair of chlorphyll molecules called P680, is photoexcited. P680 then donates an electron to the first acceptor molecule, which is a chemical relation of chlorophyll called Pheo, through a process known as charge separation. Spatial separation of the positive charge on P680 from the negative charge on Pheo stabilizes the opposite charges. When close together in space, the likelihood that two opposite charges will combine to form a neutral species is high. Annihilation of the positive charge by reaction with the negative charge wastes the absorbed light energy. However, if the positive and negative charges can be quickly separated over a distance, the probability of forming a neutral species through recombination drops significantly.
One way to envision how photosynthetic organisms separate charges by a distance is to imagine that the two charged species migrate away from each other, a behavior similar to that of two charged species in a solution. Unfortunately the two charged species in a photosynthetic reaction center cannot move appreciably because both are tightly held in place by the protein to which they are bound. The remedy that photosynthetic organisms have adopted is one in which the charges are passed in rapid succession by electron transfer from one molecule to another molecule. Photoexcitation, charge separation, electron transfer and charge stabilization are crucial components of how plants convert solar energy to chemical energy.
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