CHLOROPLAST PREPARATIONS FROM DIFFERENT PLANTS 1537 



More recently, Tolbert and Zill (1954) found that squeezed-out, plastide-bearing 

 contents of giant Chara and Nitella cells can fix C*02 in light in typical "photosyn- 

 thetic" products (PGA, sugar phosphates, cf. chapter 36), to an amount of about 10% 

 of that formed by whole cells. Conspicuous was the lack of tagged pentose and heptose 

 phosphates, indicating that their formation may be the most easily disrupted link in the 

 reaction cycle of photosj'nthesis. 



Arnon, Allen and Whatley (1954) found that washed whole (but not fragmented!) 

 spinach chloroplasts can fix C*02 in light at a rate up to 160 times that of dark fixation. 

 However, ascorbate had to be provided to achieve this rate, and as long as oxygen libera- 

 tion has not been proved, it remains possible that ascorbate, rather than water, is used 

 as reductant (Krasnovsky reaction, cf. p. 1583). 



The same chloroplasts proved able to fix P* from inorganic phosphate (more about 

 this "photosynthetic phosphorylation" in chapter 36, part B). At high phosphate con- 

 centration, the C*02 fixation was suppressed (as if the two processes were competitive). 

 Since combination of photochemical H-transfer from H2O (e. g., to TPN) with carboxyl- 

 ation and ATP-formation seems to provide all the necessary "feed materials" for sugar 

 synthesis, Arnon considered these experiments proof that the complete photochemical 

 and enzymic apparatus of photosynthesis is present in the chloroplasts. 



That whole chloroplasts behave differently from fragmented ones — in particular 

 in that they tend to reduce preferentially weak oxidants — was derived by Punnett 

 (1954) from analysis of published data. 



Gerretsen (1951) noted, in studying the redox potential and the pH of crude chloro- 

 plast suspensions from Avena saliva, that under anaerobic conditions, the acidity, which 

 grew slowly in darkness (probably because of CO2 production hy fermentation), grew 

 more rapidly in fight (at the same time the redox potential slowly changed). If, how- 

 ever, a small amount of carbon dioxide was introduced, the pH increased in 1 ght and 

 the redox potential changed much faster. Gerretsen interpreted these results as indi- 

 cating participation of carbon dioxide as hydrogen acceptor in such crude preparations. 

 The amount of CO2 used up was about 2-3% of that consumed during the same time by 

 an equal area of an illuminated leaf. 



Boyle (1948) asserted that the Hill reaction (with quinone as oxidant) does not oc- 

 cur at all unless small quantities of carbon dioxide are present. However, this had 

 not been confirmed by other investigators (Warburg, and others, unpublished; Clen- 

 denning and Gorham, 19500. 



2. Chloroplast Preparations from Different Plants 



The original experiments by Hill (1937, 1939) and Hill and Scarisbrick 

 (1940) were carried out with chloroplast preparations obtained by macera- 

 tion of leaves from Stellaria media, Lamium album, and a few other plants. 

 Since then, numerous species have been investigated; experiments have 

 been made on preservation of the chloroplasts, the effects of various oxi- 

 dants, and the methods for determining their reduction to supplement the 

 measurement of liberated oxygen. Some kinetic studies have been made, 

 including determinations of the quantum yield; but wide variations in the 

 efficiency of individual preparations, and the rapid deterioration of the 

 latter, have impeded systematic measurements. 



Material for the preparation of chloroplast suspensions must fulfil the 



