PubMed Health⌕ Search

Biomedical subjects

J Neumann

Publications and source records attributed to J Neumann.

At least 325 records · Page 18Linked to original sources
PubMed1971

Reversible pH Changes in Cells of Chlamydomonas reinhardi Resulting from CO(2) Fixation in the Light and Its Evolution in the Dark.

Illumination of a suspension of Chlamydomonas reinhardi causes an increase in the pH of the medium which is reversed in the dark. This pH change is a manifestation of CO(2) uptake in the light and its evolution in the dark. Simultaneous measurements of pH changes and oxygen evolution reveal that the photosynthetic coefficient approaches one.Intact cells of F-60, a mutant strain of C. reinhardi that lacks an active phosphoribulokinase, do not exhibit the light-dependent pH increase or oxygen evolution. However, chloroplast fragments prepared from the cells of the mutant strain exhibit a normal "proton pump" activity.The light-dependent pH increase shown by intact cells can be inhibited by KCN, by uncouplers of photosynthetic phosphorylation, and by Dio-9. It is markedly increased upon the addition of potassium bicarbonate, and all inhibitors tested inhibit the pH increase in both the presence and absence of potassium bicarbonate.The results of the present work negate the conclusion of other workers that the light-dependent pH changes in intact cells of C. reinhardi (and probably in other algae as well) are due to the operation of the "proton pump."

Journal Article↗
PubMed1970

The relation of the 515 nanometers absorbance change to adenosine triphosphate formation in chloroplasts and digitonin subchloroplast particles.

The flash-induced absorbance changes at 515 nanometers has been studied in chloroplasts and in digitonin subchloroplast particles of lettuce. The effect of various conditions and uncouplers was tested on the decay kinetics of this absorbance change and on ATP formation in the presence of phenazine methosulphate, either by continuous or flash illumination. It has been found that in chloroplasts, carbonyl cyanide m-chloromethoxyphenylhydrazone and nigericin in the presence of K(+) accelerate the decay of the 515 change and inhibit ATP formation. However, under a variety of conditions the rate of decay of the 515 absorbance change was found to be unrelated to ATP formation. Preillumination, addition of valinomycin in the presence of K(+), addition of Na(+), or divalent cations accelerate the decay of the 515 absorbance change markedly but have no effect on ATP formation. Addition of phosphorylation reagents has no effect on the decay rate beyond that obtained by Mg(2+) and inorganic phosphate. NH(4)Cl, and to some extent atebrin, while inhibiting ATP formation, do not affect the decay of the 515 absorbance change.In digitonin subchloroplast particles the decay kinetics of the absorbance change resemble that of chloroplasts, but the magnitude of the change is smaller. The pH change in this preparation is reduced much more than the 515 absorbance change.According to the chemiosmotic hypothesis, the sum of DeltaE(membrane potential) and DeltapH is the driving force for ATP formation. The lack of an increase in DeltaE in digitonin subchloroplast particles, which are practically devoid of DeltapH and have a normal ATP-forming activity, is inconsistent with the chemiosmotic hypothesis.

Journal Article↗