CARBON DIOXIDE FIXATION 293 



time. This is a strange admission to make, but I will tell you why we don't think it 

 is important. Krebs has pointed out in his review in 1954 (Chemical Pathways of 

 Metabolism, 1, 109 (1954), D. M. Greenberg, ed., Academic Press, New York) 

 that today, fifty years after respiration was first obtained outside the living cell, 

 we can obtain only 10% of the total respiration of the j-east cell by putting all the 

 known enzymes and coenzymes together; yet one of the great contemporary bio- 

 chemical advances has been the understanding of the enzymatic apparatus of 

 respiration by studying and trying to put together the isolated enzyme system. 



Strehler: How does your rate compare with Chlorella on a chlorophj^U basis? 



Amon : We have not made that computation. \Miat we think is important is the 

 fact that we get an appreciable and reproducible complete photosj^nthesis bj^ iso- 

 lated whole, and now also by broken, chloroplasts. CO2 fixation by broken chloro- 

 plasts can be increased by 500% by adding certain substances. However, we 

 definitely have on the program an experiment to compare the rate of photosyn- 

 thesis by isolated chloroplasts with that of the same leaves from which the chloro- 

 plasts were prepared. 



Strehler: The previous question leads to the following one: Assume that the 

 ratio of CO2 fixation in the chloroplast is essentially the same as the fraction of the 

 whole cell volume the chloroplast itself occupies. If in your isolation procedure 

 you form some membrane around the chloroplast — a coagulation membrane of 

 some sort which traps these enzymes in association with the chloroplast (such as 

 has been demonstrated for isolated liver nuclei) — then one need not necessarily 

 assume that the chloroplast has all the necessary enzymes concentrated in it or 

 that it is acting as a unique photosynthesizing unit. Rather it may merely be float- 

 ing around in a cytoplasm which itself contains all the necessary enzjTnes. 



Gibbs : As a biochemist I am trying to determine what this group has accom- 

 plished that is new. Dr. Arnon has stated on various occasions that his chloroplast 

 preparations possess a pathway for the conversion of CO2 to carbohydrate other 

 than that postulated by Benson, Calvin, Horecker, et al., for the intact cell. Your 

 data do not show a new enzj-me. Thej' do not suggest a new compound. I do not 

 see in the data a new pathway for a conversion of CO2 to carbohydrate. Wliat in- 

 terests me most in these data is the elegant manner in which you have shown that 

 the chloroplast apparently possesses a complete set of enzymes for the carbon 

 cj'cle independent of those found in the cj^oplasm for carrying out similar re- 

 actions. This localization may be a clue to the manner in which the plant sepa- 

 rates carbon of photosj-nthesis from carbon of respiration. 



Amon : Let me say first that we are not defending anything we have done or its 

 significance. Our first aim was to show where photosynthetic CO2 fixation and 

 ATP formation are localized in the cell. One might say that it is unimportant to 

 have a Hill reaction or any other reaction by isolated chloroplasts. Why take a 

 chloroplast out of a leaf when we know that the whole leaf can give off oxygen? 

 Yet, as we have seen at this meeting and from the published research on photo- 

 synthesis, there is a great deal of interest in what isolated chloroplasts can do 

 when taken out of cells. 



Strehler : Do the isolated chloroplasts contain adsorbed cytoplasmic enzymes? 



Amon : Now then, may I proceed to Dr. Gibbs' point next? As far as my state- 

 ment at the last year's (1954) Botanical Congress is concerned, I am afraid that 



