Narrow concentration intervals were used, covering 10−6– 10−4M desaspidin. The interaction with glycolysis involves three steps, the inhibitor constants (Ki:s) being in turn 2.7 × 10−5M, 1.3 × 10−4M, and high. About 18% of total glycolysis is inhibited in each of the two first steps, and 65% left for the third reaction. After compensation for glycolysis, oxidative phosphorylation may show a sudden jump to about 10% inhibition at 1.5 × 10−5M desaspidin, the possible Ki of the reaction starting here being very high. Correcting for glycolysis, desaspidin affects total Photophosphorylation in two steps, with the Ki values of 7.8 × 10−5M and 4.6 × 10−4M respectively. Inhibition in the first step is about 27% of the total photophosphorylation. By applying 10−6M DCMU[/3‐(3, 4‐dichlorophenyl)‐l, l‐dimethy lurea], one can abolish non‐cyclic photophosphorylation. Desaspidin then reacts in a single step with a Ki of 1.4 × 10−4M. At 5 × 10−5M DCMU, also the pseudocyclic photophosphorylation is abolished. The remaining, true cyclic photophosphorylation has a single Ki of 2.3 × 10−5M for desaspidin. Under non‐cyclic conditions, the true cyclic process contributes about 25% to total Photophosphorylation. Under pseudocyclic conditions, no cyclic photophosphorylation occurs. Under true cyclic conditions, the non‐cyclic and pseudocyclic processes are inoperative. This indicates a regulative system, so that either (1) the (non‐cyclic + true cyclic), (2) only the pseudocyclic, or (3) only the true cyclic systems can be traced, dependent on the level of DCMU applied. There are two sites for non‐cyclic Photophosphorylation, one of them common to the pseudocyclic pathway. Cyclic photophosphorylation has a third site, different from the other two.

The ATP levels in photophosphoryiation, glycolysisand oxidative phosphorylation, in the unicellalar green alga Scenedesmus obtttsiusculus, were titrated with narrow concentration intei-vals of desaspidin in the presence of different concentrations of 3-(3,4dicliloropheny])-l,l-dimethylurea (DCMU),, which allows the differentiation between nonHjyclic,, pseudocyclic and true cyclic photophosphorylation. The data on photophosphorylative ATP levels were compared with earlier data on total binding of phosphate. In the true cyclic process, both parameters are equally sensitive towards desaspidin. Under pseudocyclic conditions and in non-cyclic photophosphorylation, the level of ATP is more sensitive towards desaspidin than is total binding of phosphate. This suggests a structural difference between the cyclic and the two non-cyclic (one of which is also pseudocyclic) sites. The noncyclic ATP level is more sensitive towards desaspidin than is pseudocyclic. This may be connected with the higher AT1P level under pseudocyclic as compared to non-cyclic conditions.

The dependence of in vivo photophosphorylation on Hght intensity was studied in the unicellular green alga Scenedesmus obtusiusculus. By selective use of the inhibitor DCMU, phosphorylation in (I) the complete system, (II) the pseudocyclic system alone, and (HI) the true cyclic system alone, were followed. When the total binding of phosphate was studied, all reaction types became light saturated in about the same manner. The effect of DCMU on the level of ATP varied according to light intensity. As for the specific systems of photophosphorylation, the following ATP data were found: (I) In the complete system the level of ATP decreases with light intensity. (II) Under pseudocyclic conditions light first increases and then decreases the ATP level. Under the atmospheric conditions used (i.e. CO^-free nitrogen) this indicates a regulation between photopbosphorylation and glycolysis, for which possible explanations are discussed. (ID) In the true cyclic conditions light has little effect on the ATP level. The possibility is indicated that there is a structural difference between the non-cyclic (site 1) and the pseudocyclic (site 2) sites of photophosphorylation on tbe one hand and the true cyclic site (3) on the other.