From Energy, Plants & Man
(
http://www.asu.edu./clas/photosyn/books/walkerbk.html)Photosynthetic Efficiency
"In a perfect nature photosynthesis is perfect too" ....Otto Warburg
Suppose wish to determine how efficiently plants convert light energy into chemical energy and even ponder the possibility of improvement. If we know how much light energy is put in and how much chemical energy we get out we have a measure of how well a plant can convert one form of energy into another.
Let us start with a hypothetical carbohydrate with a formula CH2O. There are a number of real carbohydrates with the formula C6H12O6 and if these are burned in a calorimeter they release about 672 Kcal (2,822 Kjoules). Clearly we cannot get out more heat than it cost (in terms of light energy) to join together six molecules of carbon dioxide (to form C6H12O6) in the first place. On this basis, to form CH2O from one CO2 must have cost at least 672/6 = 112 Kcal (470 Kjoules). Moreover, red light (at 680 nm) had an energy content of about 42 Kcal (175 Kjoules) per quantum mole (Avogadro’s number of photons). So, if we only had this crude energy balance sheet to contend with, we could make one mole of CH2O. at the expense of only three quantum moles (3 x 42 Kcal) of light energy. One thing is certain; there is no way that we could get under two quanta of red light per molecule without breaking the laws of thermodynamics and these are notoriously difficult to break. Similarly nothing that we know, at least when we get into the chemical end of things, seems to work without some frictional losses, so four photons (per molecule of CH2O. formed, carbon dioxide fixed, or oxygen evolved) might seem more realistic.
Otto Warburg, a Nobel laureate and German biochemist of great distinction (the man who discovered NADP, pioneered the use of spectrophotometry in biochemistry and gave us "Warburg manometry") thought that photosynthesis was perfect and claimed that his data put the quantum requirement (the number of photons needed per CO2 fixed, or O2 evolved) at between 3 and 4. Robert Emerson, a young American who did his Ph.D. in Warburg’s laboratory in Berlin took a different view and put the minimal requirement at 8. A great controversy developed. The debate was passionate, even bitter. Chlorella was the preferred species. It was necessary, so it seemed, to grow it in special ways. A north-facing window and Berlin spring or well water seemed to confer some remarkable advantages. Measurement of light was as difficult as ever and the present generation of photon counters were yet to be invented. Warburg manometry was used to monitor gas exchange and refined to the nth degree, with flasks rectangular in cross-section and horizontal microscopes to detect minute changes in the level of manometer fluid. As always, the measurement of photosynthetic oxygen evolution was bedevilled by respiratory oxygen uptake. Whether or not respiration continues unchanged in the light was a question as impossible to answer with certainty at that time as it is now. Warburg travelled to the United States to repeat his experiments in Emerson’s laboratory. It took him months to organize apparatus and algae, eleven or twelve hours to make each determination of quantum requirement. He was unable to repeat his Berlin results, at least not in Emerson’s presence. Warburg blamed inadequate algae. The following letters, from Otto Warburg and Robert Emerson to F.C. Steward (reproduced here, verbatim, by kind permission of Geoffrey Hind), gives some inkling of the underlying currents at the time. Emerson, soon to die tragically in an air disaster, lamented the years wasted in futile argument, ironically attributing his differences with Warburg to a failure by the great man to recognise an inadequacy (a lag) in manometry. It can be scarcely doubted, given Emerson’s many other important research contributions, that his preoccupation with this controversy cost photosynthesis dear.South Coler Avenue 801
January 2nd, 1949
Dear Professor Steward !Some months ago Dr. Robert Emerson asked me if I possibly could lecture in your institute on the yield in photosynthesis. Thus I suppose that you are interested in this problem. As you know I found a requirement of 4 quanta in 1923 and the same requirement in 1945 when I repeated my experiments considering all the objectionsof Dr. Emerson. The university of Illinois invited me to decide here in Urbana which value is the right one, 4 or 12 quanta. The first thing I did here was a to simplify the procedure. I devised a chemical radiometer for the visible part of the spectrum so that it is now possible to measure quanta intensities manometrically (photo-oxidation of thiourea sensitized by chlorophyll or porphyrin, dissolved in pyridin. This quantum yield is under suitable conditions one). The determination of the yield in photosynthesis is now extremely simple. First you give in two vessels the same amounts of cells, but different volumes of acid culture medium and measure the pressure changes in both vessels, in dark and light. Thus you get for every period the oxygen development and CO2 absorption or the reverse. Then you put in the same light beam in the same position the radiometer vessel (of the same dimensions), and measure the oxygen consumption in light forthe same time. A few days ago I got cells to measure the yield in this way and I found, in the presence of two impartial observers, 4 quanta per molecule oxygen. Proceeding in exact [sic] the same way the next day two assistants of Dr. Emerson found for a different culture about 20 quanta per molecule of oxygen. So I came to the conclusion that the Chlorella cultures of Dr. Emerson are not suitable or at least unequal and I declared here that I could continue the experiments only if I had the control over the cultures of the Chlorella.
This is a long story. If here in the US you wish to decide the problem finally it seems to me necessary that I show in an impartial institute how to get four quanta, being able of course to cultivate the chlorella myself. If on the other hand I go now back to my institute in Dahlem and repeat it there I have no opportunity to show the yield to other people and the situation remains as it is. Is there a possibility to do these experiments in your institute or in the institute of the Eastman Company? I have here a lecture at January 19 and would then be free for about two months. The trouble is that I must be paid because it is not possible to change german money in dollars. It would not be advisable to ask Dr. Emerson about this plan, But Dr. Tippo, the head of the department of Botany in Urbana is acquainted with the problem and the situation. I have manometers and vessels and a prisma to produce red light. The required intensities are 0,3 to 3,0 micromoles of quanta per 10 minutes, (630 to 660 nm) or better if possible monochromatic red light.
Do not answer this letter if you see no possibilities.
I am yours sincerely
Otto Warburg.
Emerson to Steward
Professor F.C. Steward January 28th, 1950
Botany Department
University of Rochester
Rochester 3, New York.
Dear Mr Steward,
I appreciated receiving your account of Burk’s performance in New York. One of our graduate students was there, too, and gave us his impressions, but the field isso new to him that he couldn’t give us as full an account as you do. We are amused that Burk had no time for discussion of results with scientific colleagues, but had plenty of time to spill a big story for newspapers reporters. As I may have suggested in my last letter to you, we are now pretty sure that the major cause of error in the Burk-Warburg work is their assumption (stated in
Science Sept. 2nd and reiterated in the Meyerhof Festschrift of Biochemica et Biophysica Acta) that there was no "Physical lag" in their manometric system. On the basis of this unsupported assertion (which we find to be strictly contrary to fact), they based most of their photosynthesis measurements on 10-minute exposures to light, alternated with 10-minute intervals of either darkness or unmeasured light. This does not lead to correct values of photosynthesis in either phosphate or
carbonate buffers. However, there are additional factors in their work which are still obscure, and we shall not be satisfied until we can give a quantitative explanation of all the major inconsistencies. We do make progress toward this objective, but it’s disappointingly slow.
Yes, Burk gives one this impression that he is making an intentional effort to confuse issues, rather than to clarify them. I’m inclined to agree that an ethical problem is involved, as well as a question of scientific fact. I’ll appreciate advice on how to deal with the ethical issue, but I’m inclined to let it go until we have settled the facts.
With best wishes,
Sincerely
Robert Emerson.