Phosphorylation in isolated chloroplasts coupled to dichlorophenyldimethylurea-insensitive silicomolybdate reduction

1. The electron transport in isolated chloroplasts with silicomolybdate as electron acceptor has been reinvestigated. The silicomolybdate reduction has been directly measured as ΔA₇₅₀ or indirectly as O₂ evolution (in the presence or absence of ferricyanide).2. Silicomolybdate-dependent O₂ evolution is inhibited to a similar extent by 3-(3,4-dichlorophenyl) 1, 1-dimethylurea (DCMU) or dibromothymoquinone (DBMIB), indicating the existence of two different sites of silicomolybdate reduction: one before the DCMU block (i.e. at Photosystem II) and one after the DBMIB block (i.e. at Photosystem I).3. Silicomolybdate-dependent O₂ evolution is coupled to ATP synthesis with an $\text{ATP}\text{2}\text{e}{}^{\mathrm{?}}$ ratio of 1.0 to 1.1. The presence of ferricyanide inhibits this ATP synthesis ($\text{ATP}\text{2}\text{e}{}^{\mathrm{?}}$ ratio then is about 0.3).4. Silicomolybdate-dependent O₂ evolution is also coupled to ATP-synthesis in the presence of DCMU with an $\text{ATP}\text{2}\text{e}{}^{\mathrm{?}}$ ratio of 0.6–0.8 characteristic of Site II; in this case the electron transport itself is not affected by uncouplers or energy-transfer inbihitors.5. The data are interpreted as a further demonstration that the water-splitting reaction is responsible for the conservation of energy at Photosystem II.     

Photosynthetic control and photophosphorylation in photosystem II of isolated spinach chloroplasts

Two cycles of photosynthetic control have been observed in isolated spinach chloroplasts in the presence of lipophilic class III electron acceptors, which may accept electrons at PS II. $\text{ADP}\text{O}$ ratios of 0.8 to 0.9 were recorded;rates of oxygen evolution were stimulated by phosphorylating reagents and uncouplers. Addition of the plastoquinone antagonist DBMIB decreased photosynthetic control, oxygen evolution and photophosphorylation. We believe that there is a coupling site associated with PSII which can be rate limiting. Comparison of the $\text{P}\text{2e}$ ratios observed with class I and class III electron acceptors leads us to propose that more than 0.6 and possibly approaching one molecule of ATP can be formed for every pair of electrons transported from water to PSII acceptors.     

Preparation of O2-evolving Photosystem-II subchloroplasts from spinach

An O₂-evolving Photosystem II subchloroplast preparation was obtained from spinach chloroplasts, using low concentrations of digitonin and Triton X-100. The preparation showed an O₂ evolution activity equivalent to 20% of the uncoupled rate of fresh broken chloroplasts, but had no significant Photosystem-I-dependent O₂ uptake activity. The preparation showed a chlorophyll $\text{a}\text{b}$ ratio of 1.9 and a $\text{P-700}\text{chlorophyll}$ ratio of $\text{1}\text{2400}$. Absorption spectra at room temperature and fluorescence emission spectra of chlorophyll at 77 K suggested a significant decrease in Photosystem I antenna chlorophylls in the O₂-evolving Photosystem II preparation.     

Dual action of ionophore A23187 on intact chloroplasts

1. Ionophore A23187 induces uncoupling of potassium ferricyanide-dependent O₂ evolution by envelope-free chloroplasts and oxaloacetate-dependent O₂ evolution by intact chloroplasts. The half maximal concentration ($\text{C}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$) for stimulation of oxygen evolution in both cases is approximately 4 μM · 100 μg chlorophyll · ml^?1.2. Ionophore A23187 also induces inhibition of CO₂ and 3-phosphoglycerate-dependent O₂ evolution by intact chloroplasts in the presence of 3 mM MgCl₂. The half maximal concentrations ($\text{C}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$) for inhibition of O₂ evolution are 3 μM and 5 μM respectively · 100 μg^?1 chlorophyll · ml^?1.3. A very high concentration of ionophore A23187 (10 μM · 20 μg^?1 chlorophyll · ml^?1) plus 0.1 mM EDTA lowers the fluorescence yield of intact chloroplasts suspended in a cation-free medium in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, indicating loss of divalent cation from the diffuse double layers of the thylakoid membranes.4. These results are discussed in relation to ionophore A23187-induced divalent cation/proton exchange at both the thylakoid and the envelope membranes of intact chloroplasts.     

Superoxide reduction as a mechanism of ascorbate-stimulated oxygen uptake by isolated chloroplasts

Addition of 1m$\text{M}$ ascorbate to isolated chloroplasts with methyl viologen (MV) as electron acceptor trebled the rate of oxygen uptake and decreased the $\text{ADP}\text{O}$ ratio to a third of that with no ascorbate present. These effects of ascorbate were reversed by superoxide dismutase (SOD), which in the absence of ascorbate had little effect on O₂ uptake or $\text{ADP}\text{O}$ ratio. A chloroplast-associated SOD activity equivalent to 500 units/mg chlorophyll was detected. The effects of ascorbate and SOD on O₂ uptake were similar in both coupled and uncoupled chloroplasts. The results are consistent with the hypothesis that ascorbate stimulates O₂ uptake by reduction of superoxide, which is formed by autoxidation of the added electron acceptor (MV), and which dismutates in the absence of ascorbate. Ascorbate does not seem to stimulate O₂ uptake by replacing water as the photosystem II donor.     

Oxygen exchange associated with electron transport and photophosphorylation in spinach thylakoids

O₂ uptake in spinach thylakoids was composed of ferredoxin-dependent and -independent components. The ferredoxin-independent component was largely 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) insensitive (60%). Light-dependent O₂ uptake was stimulated 7-fold by 70 μM ferredoxin and both uptake and evolution (with O₂ as the only electron acceptor) responded almost linearly to ferredoxin up to 40 μM. NADP⁺ reduction, however, was saturated by less than 20 μM ferredoxin. The affinity of O₂ uptake for for O₂ was highly dependent on ferredoxin concentration, with $\text{K}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(O}{}_2\text{)}$ of less than 20 μM at 2 μM ferredoxin but greater than 60 μM O₂ with 25 μM ferredoxin. O₂ uptake could be suppressed up to 80% with saturating NADP⁺ and it approximated a competitive inhibitor of O₂ uptake with a K_i of 8–15 μM. Electron transport in these thylakoids supported high rates of photophosphorylation with NADP⁺ (600 μmol ATP/mg Chl per h) or O₂ (280 μmol/mg Chl per h) as electron acceptors, with $\text{ATP}\text{2}\text{e}$ ratios of 1.15–1.55. Variation in $\text{ATP}\text{2}\text{e}$ ratios with ferredoxin concentration and effects of antimycin A indicate that cyclic electron flow may also be occurring in this thylakoid system. Results are discussed with regard to photoreduction of O₂ as a potential source of ATP in vivo.     

Flash-induced photophosphorylation in chloroplasts with activated ATPase

Chloroplasts which were rapidly isolated from illuminated leaves showed activity of ATP hydrolysis at a level much higher than that of the dark control. Under the high-intensity illumination or under repetitive flash excitation, the activated chloroplasts synthesized more ATP than those with a low ATP hydrolysis activity. formed under repetitive flashes was smaller in the activated chloroplasts than in the inactive chloroplasts. The inhibition of ATP yield per flash by valinomycin or nigericin in the presence of K⁺ was stronger in the inactive chloroplasts than in the activated chloroplast. ATP synthesis in the activated chloroplasts seems to have a lower threshold.     

A membrane-bound alkaline inorganic pyrophosphatase in isolated spinach chloroplasts

An alkaline inorganic pyrophosphatase is found in association with isolated spinach chloroplast membranes. The enzyme is not removed from chloroplasts by repeated washings in an iso-osmotic medium. Suspension of the chloroplasts in hyper- or hypo-osmotic medium, however, results in the loss of pyrophosphatase activity in the chloroplasts. Fractionation of an isolated chloroplast suspension by differential centrifugation yields chloroplast fractions possessing high levels of alkaline pyrophosphatase activity but practically devoid of cytoplasmic acid pyrophosphatase.The alkaline pyrophosphatase exhibits a pH optimum of 8.2–8.5. In addition, there is an absolute requirement for Mg²⁺, with maximal rates of pyrophosphate hydrolysis occurring at $\text{Mg}{}^{\mathrm{2+}}\text{PP}{}_{\mathrm{i}}$ ratios greater than 2. From these findings the actual substrate for the enzyme is evidently Mg₂P₂O₇⁰ with pyrophosphate (P₂O₇^4?) acting as a potent inhibitor.The enzyme is inhibited by high concentrations of ATP (>3 mm), but increasing the concentration of Mg²⁺ effectively relieves this inhibition. At lower ATP concentrations, however, there is a stimulation of pyrophosphatase activity.The rate of hydrolysis of pyrophosphate by isolated chloroplasts is not affected by methylamine, 4′-deoxyphlorizin, and light. The possible role of this enzyme in photophosphorylation is discussed.     

Flash-induced absorption changes of the primary donor of photosystem II at 820 nm in chloroplasts inhibited by low pH or tris-treatment

A comparative study is made, at 15 °C, of flash-induced absorption changes around 820 nm (attributed to the primary donors of Photosystems I and II) and 705 nm (Photosystem I only), in normal chloroplasts and in chloroplasts where O₂ evolution was inhibited by low pH or by Tris-treatment.At pH 7.5, with untreated chloroplasts, the absorption changes around 820 nm are shown to be due to P-700 alone. Any contribution of the primary donor of Photosystem II should be in times shorter than 60 μs.When chloroplasts are inhibited at the donor side of Photosystem II by low pH, an additional absorption change at 820 nm appears with an amplitude which, at pH 4.0, is slightly higher than the signal due to oxidized P-700. This additional signal is attributed to the primary donor of Photosystem II. It decays ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ about 180 μs) mainly by back reaction with the primary acceptor and partly by reduction by another electron donor. Acid-washed chloroplasts resuspended at pH 7.5 still present the signal due to Photosystem II ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ about 120 μs). This shows that the acid inhibition of the first secondary donor of Photosystem II is irreversible.In Tris-treated chloroplasts, absorption changes at 820 nm due to the primary donor of Photosystem II are also observed, but to a lesser extent and only after some charge accumulation at the donor side. They decay with a half-time of 120 μs.     

Flexibility of coupling and stoichiometry of ATP formation in intact chloroplasts

Since coupling between phosphorylation and electron transport cannot be measured directly in intact chloroplasts capable of high rates of photosynthesis, attempts were made to determine $\text{ATP}\text{2}\text{e}$ ratios from the quantum requirements of glycerate and phosphoglycerate reduction and from the extent of oxidation of added NADH via the malate shuttle during reduction of phosphoglycerate in light. These different approaches gave similar results. The quantum requirement of glycerate reduction, which needs 2 molecules of ATP per molecule of NADPH oxidized was found to be pH-dependent. 9–11 quanta were required at pH 7.6, and only about 6 at pH 7.0. The quantum requirement of phosphoglycerate reduction, which consumes ATP and NADPH in a $\text{1}\text{1}$ ratio, was about 4 both at pH 7.6 and at 7.0. $\text{ATP}\text{2}\text{e}$ ratios calculated from the quantum requirements and the extent of phosphoglycerate accumulation during glycerate reduction were usually between 1.2 and 1.4, occasionally higher, but they never approached 2.Although the chloroplast envelope is impermeable to pyridine nucleotides, illuminated chloroplasts reduced added NAD via the malate shuttle in the absence of electron acceptors and also during the reduction of glycerate or CO₂. When phosphoglycerate was added as the substrate, reduction of pyridine-nucleotides was replaced by oxidation and hydrogen was shuttled into the chloroplasts to be used for phosphoglycerate reduction even under light which was rate-limiting for reduction. This indicated formation of more ATP than NADPH by the electron transport chain. From the rates of oxidation of external NADH and of phosphoglycerate reduction at very low light intensities $\text{ATP}\text{2}\text{e}$ ratios were calculated to be between 1.1 and 1.4.Fully coupled chloroplasts reduced oxaloacetate in the light at rates reaching 80 and in some instances 130 μmoles · mg^?1 chlorophyll · h^?1 even though ATP is not consumed in this reaction. The energy transfer inhibitor phlorizin did not significantly suppress this reduction at concentrations which completely inhibited photosynthesis. Uncouplers stimulated oxaloacetate reduction by factors ranging from 1.5 to more than 10. Chloroplasts showing little uncoupler-induced stimulation of oxaloacetate reduction were highly active in photoreducing CO₂. Measurements of light intensity dependence of quantum requirements for oxaloacetate reduction gave no indication for the existence of uncoupled or basal electron flow in intact chloroplasts. Rather reduction is brought about by loosely coupled electron transport. It is concluded that coupling of phosphorylation to electron transport in intact chloroplasts is flexible, not tight. Calculated $\text{ATP}\text{2}\text{e}$ ratios were obtained under conditions, where coupling should be expected to be optimal, i.e. at low phosphorylation potentials $\text{[}\text{ATP}\text{]}\text{[}\text{ADP}\text{]}\text{[}\text{P}{}_{\mathrm{<mtext>i</mtext>}}\text{]}$. Flexible coupling implies, that $\text{ATP}\text{2}\text{e}$ ratios should decrease with increasing phosphorylation potentials inside the chloroplasts.     

The ATP/2e ratio during photosynthesis in intact spinach chloroplasts.

Addition of ribose-5-phosphate to intact spinach chloroplasts in the absence of added P_i resulted in a conversion of part of the Benson-Calvin cycle into a linear sequence so that triose phosphate accumulated during CO₂ fixation stoichiometrically with the O₂ evolved (triose phosphate / O₂ ratio was 2.0). The fortunate consequence of this effect is that the $\text{ATP}\text{2e}$ ratio may be calculated from the 3-phosphoglycerate and triose phosphate accumulated and the O₂ evolved. In this way the $\text{ATP}\text{2e}$ ratio was shown to be 2.0, with cyclic or pseudocyclic phosphorylation contributing less than 9% to the total phosphorylation.     

Photosystem II energy coupling in chloroplasts with H2O2 as electron donor

NH₂OH-treated, non-water-splitting chloroplasts can oxidize H₂O₂ to O₂ through Photosystem II at substantial rates (100–250 μequiv · h^?1 · mg^?1 chlorophyll with 5 mM H₂O₂) using 2,5-dimethyl-p-benzoquinone as an electron acceptor in the presence of the plastoquinone antagonist dibromothymoquinone. This H₂O₂ → Photosystem II → dimethylquinone reaction supports phosphorylation with a $\text{P}\text{e}{}_2$ ratio of 0.25–0.35 and proton uptake with $\text{H}{}^{\mathrm{+}}\text{e}$ values of 0.67 (pH 8)–0.85 (pH 6). These are close to the $\text{P}\text{e}{}_2$ value of 0.3–0.38 and the $\text{H}{}^{\mathrm{+}}\text{e}$ values of 0.7–0.93 found in parallel experiments for the H₂O → Photosystem II → dimethylquinone reaction in untreated chloroplasts. Semi-quantitative data are also presented which show that the donor → Photosystem II → dibromothymoquinone (→O₂) reaction can support phosphorylation when the donor used is a proton-releasing reductant (benzidine, catechol) but not when it is a non-proton carrier (I^?, ferrocyanide).     

Thiol modulation of the chloroplast protonmotive ATPase and its effect on photophosphorylation

Thiol modulation of the chloroplast protonmotive ATPase (CF₀-CF₁) by preillumination of broken chloroplasts in the presence of dithiothreitol (or preillumination of intact chloroplasts in the absence of added thiols) had the following effects on photophosphorylation. (1) When assayed at pH 8 and saturating light, the initial rate of photophosphorylation was increased by 10–40%. There was an accompanying increase in the rate of coupled electron transport with no significant change in the overall $\text{P}\text{2}\text{e}$ ratio. (2) On lowering the pH of the assay medium to pH 7, the stimulatory effect of thiol modulation on photophosphorylation and coupled electron flow was enhanced. At pH 7, there was also a small increase in $\text{P}\text{2}\text{e}$ ratio. (3) Addition of a non-saturating amount of uncoupler to the assay medium enhanced the stimulatory effect of thiol modulation on photophosphorylation. In the presence of 1 mM NH₄Cl, there was only a small increase in coupled electron flow and a correspondingly larger increase in $\text{P}\text{2}\text{e}$ ratio. (4) Lowering the light intensity, or inhibiting electron transport, diminished the stimulatory effect of thiol modulation on photophosphorylation, coupled electron transport and $\text{P}\text{2}\text{e}$ ratio. (5) Under all the above conditions, the ΔpH maintained across the thylakoid membrane was lower after thiol modulation, even when photophosphorylation markedly increased in rate. (6) Thiol modulation of CF₀-CF₁ increased the observed Michaelis constant for ADP (K_m(ADP)) and the apparent maximum rate (V_app of photophosphorylation by the same factor, so that ratio $\text{V}{}_{\mathrm{<mtext>app</mtext>}}\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ was not altered. $\text{V}{}_{\mathrm{<mtext>app</mtext>}}\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ was also unaffected by changing the medium pH, but was significantly decreased upon addition of uncouplers to the medium. These results indicate that the observed rate of ATP synthesis catalysed by thiol demodulated chloroplasts is limited kinetically by the fraction (α) of enzyme molecules that are active during photophosphorylation. A model based on a dual pH optimum requirement for activation of CF₀-CF₁ is presented to explain the dependence of α on ΔpH. Thiol modulation of CF₀-CF₁ is proposed to stimulate photophosphorylation by causing the enzyme to become active over a lower range of ΔpH, thereby reducing the kinetic limitation on ATP synthesis imposed by the activation process.     

Active sodium extrusion reduces net efficiencies of oxidative phosphorylation in the strictly photoautotrophic cyanobacterium Anacystis nidulans

Efficiencies of oxidative phosphorylation ($\text{P}\text{O}$ ratios), intracellular high-energy phosphate pools (ATP and ADP) under aerobic and anaerobic dark conditions, and photosynthetic oxygen evolution measured with intact cells of Anacystis nidulans were found to be specifically depressed by NaCl, but not by KCl. A scheme is proposed which explains the deleterious effect of sodium on the energy metabolism of A. nidulans by competition for protons between ATP synthesis and active sodium extrusion.     

H2 metabolism in photosynthetic organisms. II. Light-dependent H2 evolution by preparations from Chlamydomonas, Scenedesmus and spinach.

Light-dependent H₂ evolution from dithiothreitol as electron donor was observed with cell-free preparations of anaerobically adapted $\text{Chlamydomonas reinhardii, Scenedesmus obliquus}$ and from spinach chloroplasts mixed with $\text{Chlamydomonas}$ hydrogenase. NADH substituted for dithiothreitol as electron donor only in the $\text{Chlarmydomonas}$ preparation. Dibromothymoquinone, an antagonist of plastoquinone, selectively inhibited H₂ photoevolution from NADH. These results are interpreted as indicating that 3-(3,4-dichlorophenyl)-1,1-dimethyl urea insensitive H₂ photoevolution by algae containing hydrogenase is due to the capability of NADH to reduce plastoquinone in the electron transport chain, and to evolve H₂ by a low redox potential carrier of photosystem I.     

Energy-coupling mechanisms under aerobic and anaerobic conditions in autotrophically grown Pseudomonas saccharophila

The cell-free preparations from autotrophieally grown Pseudomonas saccharophila catalyzed the process of electron transport from H₂ or various other organic electron donors to either O₂ or NO₃^? with concomitant ATP generation. The respective $\text{P}\text{O}$ ratios with H₂ and NADH were 0.63 and 0.73, the respective $\text{P}\text{NO}{}_3{}^{\mathrm{?}}$ ratios were 0.57 and 0.54. In contrast, the $\text{P}\text{O}$ and $\text{P}\text{NO}{}_3{}^{\mathrm{?}}$ ratios with succinate were 0.18 and 0.11, respectively. ATP formation coupled to the oxidation of ascorbate, in the absence or presence of added N,N,N′,N′-tetramethyl-p-phenylenediamine or cytochrome c, could not be detected. Various uncouplers inhibited phosphorylation with either O₂ or NO₃^? as terminal electron acceptors without affecting the oxidation of H₂ or other substrates. The NADH oxidation at the expense of O₂ or NO₃^? reduction as well as the associated phosphorylation were inhibited by rotenone and amytal. The aerobic and anaerobic H₂ oxidation and coupled ATP synthesis, on the other hand, was unaffected by the flavoprotein inhibitors as well as by the NADH trapping system. The NADH, H₂, and succinate-linked electron transport to O₂ or NO₃^? and the associated phosphorylations were sensitive, however, to antimycin A or 2-n-nonyl-4-hydroxyquino-line-N-oxide, and cyanide or azide. The data indicated that although the phosphorylation sites 1 and II were associated with NADH oxidation by O₂ or NO₃^?, the energy conservation coupled to H₂ oxidation under aerobic or anaerobic conditions appeared to involve site II only.     

Measurement of the intramitochondrial P/O ratio

Measurements of the initial rate of ATP synthesis and the initial rate of oxygen consumption in mitochondria in which transport of ADP, Pi and ATP were inhibited were used to obtain a value for the intramitochondrial $\text{P}\text{O}$ ratio. With succinate as substrate this method yielded a $\text{P}\text{O}$ ratio of 2.8 for the phosphorylation of intramitochondrial ADP.     

Photosynthesis and phosphorylation of light-harvesting chlorophyll a/b—protein in intact chloroplasts: Effects of uncouplers

Protein phosphorylation in isolated, intact pea chloroplasts was measured during the onset of CO₂-dependent O₂ evolution. Total incorporation of ³²P (from ³²P_i) into the light-harvesting chlorophyll a/b—protein was found to be less sensitive than O₂ evolution to inhibition by the uncouplers FCCP and NH₄C1 It is concluded that changes in the rate of ATP synthesis cannot affect protein phosphorylation without also affecting the rate of CO₂-fixation in this system. The ATP/ADP ratio is therefore unlikely to regulate photosynthetic protein phosphorylation under normal physiology conditions.     

Influence of adenosine phosphates and magnesium on photosynthesis in chloroplasts from peas, sedum, and spinach

¹⁴CO₂ photoassimilation in the presence of MgATP, MgADP, and MgAMP was investigated using intact chloroplasts from Sedum praealtum, a Crassulacean acid metabolism plant, and two C₃ plants: spinach and peas. Inasmuch as free ATP, ADP, AMP, and uncomplexed Mg²⁺ were present in the assays, their influence upon CO₂ assimilation was also examined. Free Mg²⁺ was inhibitory with all chloroplasts, as were ADP and AMP in chloroplasts from Sedum and peas. With Sedum chloroplasts in the presence of ADP, the time course of assimilation was linear. However, with pea chloroplasts, ADP inhibition became progressively more severe, resulting in a curved time course. ATP stimulated assimilation only in pea chloroplasts. MgATP and MgADP stimulated assimilation in all chloroplasts. ADP inhibition of CO₂ assimilation was maximal at optimum orthophosphate concentrations in Sedum chloroplasts, while MgATP stimulation was maximal at optimum or below optimum concentrations of orthophosphate. MgATP stimulation in peas and Sedum and ADP inhibition in Sedum were not sensitive to the addition of glycerate 3-phosphate (PGA).

PGA-supported O₂ evolution by pea chloroplasts was not inhibited immediately by ADP; the rate of O₂ evolution slowed as time passed, corresponding to the effect of ADP on CO₂ assimilation, and indicating that glycerate 3-phosphate kinase was a site of inhibition. Likewise, upon the addition of AMP, inhibition of PGA-dependent O₂ evolution became more severe with time. This did not mirror CO₂ assimilation, which was inhibited immediately by AMP. In Sedum chloroplasts, PGA-dependent O₂ evolution was not inhibited by ADP and AMP. In chloroplasts from peas and Sedum, the magnitude of MgADP and MgATP stimulation of PGA-dependent O₂ evolution was not much larger than that given by ATP, and it was much smaller than MgATP stimulation of CO₂ assimilation. Analysis of stromal metabolite levels by anion exchange chromatography indicated that ribulose 1,5-bisphosphate carboxylase was inhibited by ADP and stimulated by MgADP in Sedum chloroplasts.

The appearance of label in the medium was measured when [U-¹⁴C] ADP-loaded Sedum chloroplasts were challenged with ATP, ADP, or AMP and their Mg²⁺ complexes. The rate of back exchange was stimulated by the presence of Mg²⁺. This suggests that ATP, ADP, and AMP penetrate the chloroplast slower than their Mg²⁺ complexes. A portion of the CO₂ assimilation and O₂ evolution data could be explained by differential penetration rates, and other proposals were made to explain the remainder of the observations.