Third Report of the Committee on Photochemistry Reprint and Circular Series of the National Research Council (1938) / Chapter Skim
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Photosynthesis
Photosynthesis
Pages 117-156
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From page 117... ...
factor may influence photosynthesis indirectly through its effect in the plant on internal factors other than those directly connected with the photosynthetic mechanism. This is particularly true for experiments with the higher plants.
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From page 118... ...
In either case the material to be investigated is usually enclosed in a lighttransm~tting chamber.2 The change in carbon dioxide or oxygen composition during a period of illumination or darkness may then be determined. In practice a flow method is often employed in order to maintain a nearly constant carbon dioxide concentration in the reaction chamber.
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From page 119... ...
A notable characteristic of this method is its rapid response to changes in photosynthetic rate. Photosynthesis measurements in algae and other aquatic plants Photosynthesis measurements in aquatic plants are usually carried out with the plant material suspended in water.
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From page 120... ...
Hoover's results were obtained with young wheat plants, using equal incident light intensities (less than 300 foot-candles) at the various wave lengths.
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From page 121... ...
The formaldehyde hypothesis The occurrence of formaldehyde as an intermediate product of photosynthesis has been neither proved nor disproved (589. In any event its concentration must be very low, even during rapid photosynthesis, since
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From page 122... ...
2. Rate of photosynthesis as a function of wave length (low light intensity)
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From page 123... ...
For both species of diatoms Barker found a quotient close to 0.95, with a tendency for the value to increase slightly with increasing light intensity. He concluded that 10 per cent or less of the photosynthetic products appeared as stored fat and that this fat production was probably the result of a secondary reaction.6 The eject of light intensity and temperature on the rate of photosynthesis Figure 3 shows diagrammatically the relation between the rate of photosynthesis and light intensity at high carbon dioxide concentration.
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From page 124... ...
As shown in figure 3, the rate of photosynthesis is nearly independent of temperature at low light intensities, but becomes temperature-dependent at higher intensities. Thus, for Chlorella Warburg (82)
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From page 125... ...
From the type of behavior shown in figure 3, as well as from other evidence to be discussed below, it must be concluded that photosynthesis is a complex cyclic reaction, involving at least one thermal reaction, usually known as the Blackman reaction,9 in addition to whatever photoreactions are necessary. According to this view, at low light intensities, where the rate of photosynthesis is approximately proportional to the intensity, the time between the absorption of successive photons, by that portion of the mechanism involved in the reduction of a single carbon dioxide molecule, will be sufficient to permit the Blackman reaction to be completed.
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From page 126... ...
(The heavy water experiments will be discussed below, page 828.) The decrease in photosynthetic rate at high temperatures is probably due to injury to the plant material.
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From page 127... ...
. Light saturation was not reached in this work; at higher light intensities still higher carbon dioxide concentrations would have been required.
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From page 128... ...
Moreover, the improvement in yield depended upon the frequency of the flashing. At low light intensities, intermittent light produced no improvement in yield.
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From page 129... ...
by Pratt and Trelease (64~. Using Chlorella cells suspended in heavy water as well as in ordinary water, they studied the effect of flashing light on the rate of photosynthesis.
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, where the rate in HERO falls more rapidly than the rate in D2O. At low light intensities the rates in D20 and H20 become nearly equal.
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From page 131... ...
] 4 In continuous light, narcotics such as phenylurethan and thymol inhibit photosynthesis at both high and low light intensities, in contrast to hydrogen cyanide, which inhibits only at high intensities (83~.
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From page 132... ...
Although the behavior illustrated in figure 8 may not give very definite evidence concerning the order in which certain steps in the reaction cycle may occur, it probably indicates, as Emerson has pointed out (reference 23, page 319) , that carbon dioxide enters into a reaction in the photosynthetic cycle other than the Blackman reaction, since, if the latter reaction alone were affected by carbon dioxide concentration, the maximum yield of photosynthesis per flash should remain constant.
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From page 133... ...
One is the occurrence of the Blackman reaction during the dark periods (see page 8261; the other is the probability that, after a short dark interval, the induction period is less pronounced than after a longer interval. No very convincing mechanism has so far been advanced for the induction process in photosynthesis.
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From page 134... ...
To a certain extent, the influence of these other factors may be minimized by using low light intensities, high carbon dioxide concentration, and a relatively high temperature. In this range, since the rate of photosynthesis is nearly proportional to light intensity, the quantum efficiency should be nearly constant and relatively independent of other external variables.
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From page 135... ...
On the other hand, McAlister (53) failed to find any stimulation in wheat plants after illumination at high light intensities, despite the fact that a very sensitive method was used.
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From page 136... ...
Light intensities varied from 830 to 24,000 ergs per cm.2 per second. A mercury arc was used with a monochromator to give monochromatic light of wave lengths 5461 and 4360 A
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lain both investigations it was evident that the previous conditions of growth played an important part in determining quantum efficiency values. Warburg and Negelein found that a week of growth at high light intensity, followed by a week of growth at low light intensity, gave the most favorable results for their strain of Chlorella.
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From page 138... ...
The higher quantum efficiencies estimated from Briggs' data are in approximate agreement with those determined by Burns. The present data concerning the problem of the quantum efficiency of photosynthesis are in serious disagreement.
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From page 139... ...
After making corrections for light absorbed by red pigments, he concluded that the maximum quantum efficiency is approximately 0.25 molecule of carbon dioxide per t6 The purple bacteria referred to here are members of the Thiorhodaceae. Another group of photosynthetically active purple bacteria, the Athiorhodaceae, apparently require organic compounds instead of sulfur compounds as reducing agents.
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From page 140... ...
The maximum slope occurred at various light intensities, depending upon the type of pretreatment of the bacteria. By extrapolation French found that as the position of the maximum slope approached zero light intensity, the quantum efficiency appeared to approach a value of 0.25 (or 0.5 on the basis of hydrogen)
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From page 141... ...
The production of chlorophyll is, of course, also dependent upon the proper supply of mineral nutrients; perhaps the most conspicuous of these are magnesium, which enters into the chlorophyll molecule, and iron, which apparently acts as a catalyst in some step in the formation process. Chemical properties of chlorophyll The structure and chemical properties of extracted chlorophyll are now known rather completely, largely as a result of the investigations of Willstatter, Stoll, Fischer, and Conant.
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From page 142... ...
) AH HCH3 CH efficiency measurements with wave lengths chosen to correspond to the respective absorption maxima of the two pigments.
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From page 143... ...
have indicated that the temperature dependence at low carbon dioxide concentrations may be only apparent, and due to complications resulting from the use of buffer mixtures. These observations weaken somewhat the evidence for participation of a carbonic acid-chlorophyll complex in photosynthesis.
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From page 144... ...
Therefore, assuming the quantum efficiency of the bleaching process to be 1, they estimated an average lifetime for the bleached state of about 10-3 sec. The degree of bleaching was approximately proportional to the square root of light intensity.
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From page 145... ...
No observations were made at wave lengths below 6300 A Kautsky and coworkers (49, 50)
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From page 146... ...
, it would appear that, at low light intensities, the percentage yield of fluorescence should increase with increasing intensity of the incident light, approaching a maximum value after the attainment of a maximum photosynthetic rate. If this type of relation could actually be established, it would constitute strong evidence for a connection between photosynthesis and fluorescence.
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From page 147... ...
Emerson and Arnold calculated the ratio between the total number of chlorophyll molecules present in the reaction vessel and the number of carbon dioxide molecules reduced per single light flash. Instead of finding a value of 1 (or of 3 or 4 or 5, as would be expected in case a series of three or four or five alternate photochemical and thermal reactions were required for the photosynthetic cycle)
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From page 148... ...
, would thus be very difficult to explain. The photosynthetic Unit One way of reconciling the small yield of carbon dioxide reduced per flash with the relatively high quantum efficiencies observed at low light intensities is to assume that light absorbed by a large number of chlorophyll molecules can be made available to a single carbon dioxide molecule.
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From page 149... ...
If one or more of t.~.~n intermediate products were sufficiently unstable to decompose · 1 ~ ·1 1 · _ ~ _1 _~ ~ ~ AL ~~ 1~~ d CAN ~ ~1~ ~ Am ~~ appreciably wlt~m a period of a minute or less, men a plant SIlOUl~ O~ greatly benefited at low light intensities by the cooperation of more than one chlorophyll molecule in the reduction of a carbon dioxide molecule, since the time between successive photoreactions would be shortened in proportion to the number of chlorophyll molecules per unit. As Gaflron and Wohl (40)
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From page 150... ...
It has also been generally assumed that the photosynthetic process includes ~ series of four successive photochemical steps, AH for a single step being limited to the energy supplied by a single quantum. In van Niel's mechanism, however, the four photochemical steps are assumed to be identical, resulting in each case in the formation of a hydrogen atom; the four hydrogen atoms then presumably bring about the reduction of carbon dioxide to formaldehyde.
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From page 151... ...
with a quantum efficiency as high as 0.25, since reaction 2 is exothermic to the extent of about 150,000 cal. The energy thus wasted would be nearly equivalent to the total energy supplied by four photons at 7000 A
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From page 152... ...
Until the accumulated respiratory products were exhausted, this oxygen consumption would largely counterbalance the photosynthetic oxygen production, thus producing an induction period. Increased probability of oxidation at high light intensities would cause a more noticeable underproduction of oxygen at high intensities than at low intensities, in agreement with experimental results (53~.
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From page 153... ...
But Franck and Herzfeld explained the induction period by assuming that, in the dark, a considerable fraction of the chlorophyll molecules become attached to intermediate respiratory products. The experiments of McAlister with wheat (53)
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From page 154... ...
With a mechanism of the type just proposed, a relatively high quantum efficiency could be maintained at very low light intensities without necessitating stable intermediate substances, such as would be necessary with a mechanism like that of Franck and Herzfeld. Perhaps the principal value of this discussion has been to give an indication of how little actually is known concerning the chemistry of the photosynthetic process.
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From page 155... ...
V., AND ALBERS, v. M.: Cold Spring Harbor Symposia Quant.
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From page 156... ...
VAN NIEL, c. B.: Gold Spring Harbor Symposia Quant.
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