PHOTOCHEMISTRY OF LIVE CELLS 1623 



tent and how rapidly quinone — or any of the other oxidants used in the 

 study of Hill reaction — are taken up by cells (or by colloidal chloroplast 

 dispersions). Perhaps the drop in the rate and in total yield of the Hill 

 reaction, which occurred when the amount of quinone was increased above 

 3 X 10 ~^ mole/liter, could be due to the incapacity of the cells to take up so 

 much quinone and protect it from photochemical decomposition. 



Quinone is not only a self-inhibitor of the Hill reaction but it also in- 

 hibits photosynthesis and respiration. Chlorella cells which have once 

 been used for the study of Hill reaction are permanently incapable of photo- 

 synthesis (at least in carbonate buffer — no experiments were made in 

 phosphate buffer). Exposure to quinone (1 hr. in 0.08% solution) in dark- 

 ness also destroys the capacity for photosynthesis. Respiration, too, is 

 strongly but not completely inhibited by incubation with quinone. 



It thus seems that despite unchanged appearance the term "intact 

 cells" should be used with caution in application to cells which have been 

 exposed to quinone. 



The Hill reaction in Chlorella cells with quinone as oxidant also is in- 

 hibited by ultraviolet irradiation (X = 253.6 m//). According to Holt, 

 Brooks and Arnold (1951) the kinetics of this inactivation is similar to that 

 observed in the study of photosynthesis or of the Hill reaction in chloroplast 

 preparations. The absolute rate of inactivation also seems to be the same 

 as in the case of photos3aithesis. 



Different Algae. The Hill reaction, with quinone or ferricyanide-ferric 

 oxalate mixture as oxidant, can be observed also with blue-green algae 

 Cylindrospermum (Clendenning and Ehrmantraut, unpublished). The 

 yield appeared to be considerably lower than with Chlorella. No experi- 

 ments have as yet been reported on the possibility of carrying out the 

 Hill reaction with live higher plants. 



Different Oxidants. From the oxidants tested so far, o-benzoquinone 

 appears to be the only effective one for whole cells; yet its photochem- 

 ical instability makes its use desirable only in light with X > 500 m^t, and its 

 poisonous properties preclude its use in any but very low concentrations. 



Because of these drawbacks, it would be useful to find another oxidant 

 capable of penetrating into live cells which would be more stable and less 

 poisonous. Clendenning and Ehrmantraut tried some of the dyestuffs 

 that gave good results with chloroplast suspensions, such as phenol indo- 

 phenol, but obtained no oxygen liberation with whole Chlorella cells; per- 

 haps these dyes could not penetrate through the cell membrane. No posi- 

 tive results were obtained also with chr ornate. 



Clendenning and Ehrmantraut (1951) reported oxygen evolution by 

 Chlorella cells from Hill's mixture in light; but Ehrmantraut and Rabino- 

 witch (1952) found that ferric oxalate underwent direct photolysis in cell- 

 free solution (in the white light used by Clendenning and Ehrmantraut) 

 at about the rate at which oxygen is liberated by cells in Hill's mixture. 

 This indicates that the latter reaction is, photosynthesis utilizing the carbon 



