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Path of Electrons in Photosynthesis Proc. Natl. Acad. Sci. USA Vol. 73, No. 12, pp. 4502-4505, December 1976 Botany Path of electrons in photosynthesis (energy levels of chlorophyll/delayed light/semiconductors/carotene diode/system I and II) WILLIAM ARNOLD Biology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 Contributed by William Arnold, September 20, 1976 ABSTRACT Electrons, from the oxidation of water, inside The redox level for the oxidation of water to 02 is at +0.815 the grana disks (thylakoids) are transferred across the membrane V. It is generally found that to make the reaction go, an over- to the outside, to the Calvin cycle or the Hill oxidant. The span voltage of about 0.5 V is needed. in redox level may be 2.3 V. Part of the system II chlorophyll is on the inside of the membrane and part on the outside. An The chlorophyll apparatus must be able to lift an electron electron trap is embedded in the membrane. Alternately, an from +1.3 to -1.0 V, a total span of about 2.3 V. excited chlorophyll on the inside gives an electron to the trap, Enhancement. Emerson discovered that with monochro- and an excited chlorophyll on the outside gives a hole to the trap. as one goes through the spectrum the 02 production Two quanta move an electron from inside to outside. The matic light, charging of this condenser drives the redox levels on the inside goes to zero at a shorter wavelength than the absorption of positive and those on the outside negative. The final voltages chlorophyll does (3). That is, some of the longer wavelength depend upon the electron flow and a carotene diode. A voltage light absorbed by chlorophyll cannot give 02- Nevertheless, the of 0.3 is involved. Delayed light is the exact reverse of the light mixing of this non-02-producing light with light that can pro- reaction. System I makes ATP. duce 02 gives a higher quantum yield than either light alone. Photosynthesis is the process by which green plants reduce CO2 This unexpected and baffling phenomenon, now known as to carbohydrates and oxidize water to 02. This process makes "enhancement," has been the subject of much research since the food we eat and the 02 we breathe, and in past times made its discovery. It is believed that enhancement shows that there the coal and oil we now use. Energy, which comes from sunlight are two photochemical reactions in green plants (4). The absorbed by chlorophyll, is stored to the extent of some 5.1 eV long-wavelength (6850 A; 1.81 eV) reaction, system I, does not per carbon atom. produce 02, and from the measurement of Weaver has about Research with radioactive carbon has made it possible to 110 chlorophyll molecules in the unit (5). System II, the short- follow "The Path of Carbon in Photosynthesis" in detail (1). wavelength (6700 A; 1.85 eV) reaction, does produce 02, and CO2 does not take part in a photochemical reaction. Carbon from general arguments about photosynthetic units must have reduction is a series of enzyme reactions known as the Calvin about 500 chlorophyll molecules. cycle. This cycle can take place in the dark, and is driven by In a number of the attempts to explain the operation of the electrons at -0.4 V and by ATP. The chlorophyll apparatus (in chlorophyll apparatus, the assumption has been that system II the chloroplasts) must be able to use light energy to oxidize and system I are connected in series for the electron flow. Sys- water to 02, lift electrons from the level of water (+0.8 V) to tem II has 4.5 times as many chlorophyll molecules as system -0.4 V, and make ATP. I. So an additional assumption has to be made-that system II Exactly how the chlorophyll apparatus works is not known. can transfer excitation to system I-for the rate of electron Many attempts to explain its operation have been published, transfer to be equal in the two systems. The energy difference but none, including two that I have written, are satisfactory. (0.04 V) between the two systems is not large enough to prevent The present paper gives a new map for the flow of electrons the transfer of excitation energy from system I to system II. So in the chlorophyll apparatus. The paper is not a complete theory we should expect to see 02 production [exp(-0.04/kT) = 0.2] of photosynthesis, but an attempt to explain how the energy of at wavelengths where only system I can absorb light. This we light, absorbed by chlorophyll, is made available for chemical do not see. reaction. In this paper, it is assumed that system II and system I are not There are four major constraints that we use in drawing the in series. map. Energy Levels of Chlorophyll. It has been known since the Delayed Light. After illumination, green plants emit, in the time of Stokes that the chlorophyll in green plants fluoresces. dark, a dim glow that we call delayed light (6). The emission It is known that the intensity of the fluorescence depends on the spectrum of this light is the same as that for the fluorescence rate of photosynthesis. We assume that the excitation of chlo- of chlorophyll in the plants (7). This means that excited chlo- rophyll to the singlet state (1.8 eV) is the first step in photo- rophyll must be regenerated in the dark. After bright illumi- synthesis. nation, the intensity of delayed light is proportional to the re- The Hill Reaction: The Span in Redox Potential. Hill dis- ciprocal of the time in the dark, over the range of a few milli- covered that chloroplasts could be removed from plant cells and seconds to 1 1k hr. made to produce 02 in the light if they were provided with The intensity of the delayed light, at time t in the dark, be- some reducible substance (2). These Hill oxidants seem to take comes saturated with respect to the intensity of the exciting the place of the entire Calvin cycle. Many Hill oxidants are now light. That is, we can find an exciting intensity beyond which in redox some are much there is no increase in the delayed light. If t is small, the in- known, covering a wide range levels; If t is more negative than the -0.4 V of the Calvin cycle, extending tensity of exciting light to give saturation is very high. V. large, the intensity of exciting light to give saturation is very to about -1.0 low. Abbreviation: PSU, photosynthetic unit. Experiments show that most, if not all, of the delayed light 4502 Downloaded by guest on October 1, 2021 Botany: Arnold Proc. Natl. Acad. Sci. USA 73 (1976) 4503 Insideizing Outside ch-- ch Oxidizing~_ LI]Reducing -1.0-1 ch*f- chi -5 -0.5- - eV 1. 8eV 150 A JK TRAP Q x 0 0 -____ uJ ~~Protein r +05- (nsulator) ctf-- ch* ch - ch + 1.0- Chlorophyll Lipid Membrane- (250 molecules) (insulator) FIG. 1. Left side shows the four levels of chlorophyll on the redox scale. Right side is a schematic of a possible cross section of system II PSU. is made by system II. Experiments also show that the delayed After the reaction the reducing level ch--ch is left. The light is much the same in chloroplasts in which the electron flow transfer of electrons from this level is labeled as slow. has been stopped by poisons. Fig. 1 (left) is a plot of the four levels on the redox scale. The fundamental assumption of this paper is that delayed I light is made by the reverse of the fast reaction that stabilizes System the energy of excited chlorophyll so that it can be used for It is generally believed that the illumination of system I reduces photosynthesis. ferredoxin (-0.4 V) and oxidizes cytochromef (+0.4 V). A fast Energy levels of chlorophyll transfer of an electron from the ch*-ch+ level (-0.85 V) to ferredoxin, followed by a slow transfer of a hole from the To draw the map of the flow of electrons in the chlorophyll ch-ch+ level (+0.95 V) to cytochrome f, will do nicely. apparatus, I must plot the various chlorophyll levels on the redox scale. Nelson measured the ionization energy and the electron System 1I affinity of solid ethyl.chlorophyllide (8). In a later paper, Azzi I think that the system II photosynthetic units (PSUs) are located and I used his data to estimate where the chlorophyll levels are on or in the membranes of the grana disks. Each disk is a small on the redox scale (9). These values will be used here. vesicle (thylakoid) shaped like a coin some 0.5 AsM in diameter. Nelson emphasized that if excited chlorophyll is the first actor If we take the diameter of one PSU to be 150 A, then there will in photosynthesis and if only one electron at a time is trans- be room for about 800 PSUs on each face of the disk. I think that ferred, then there are four different redox levels. the oxidation of water to 02 takes place inside of the vesicle. The The four levels will be denoted with symbols like "ch--ch," system II PSUs transfer electrons across the membrane, from which means that a chlorophyll molecule with an extra electron inside to outside, when the electrons are used by the Calvin can give the electron to some substance and reduce it, thereby cycle or the Hill reaction.
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