Google Scholar. Heibert, T. Ion channels in the chloroplast envelope membrane. Biochemistry 34, — Heldt, H. Alkalization of the chloroplast stroma caused by light dependent proton flux into the thylakoid space. Hochmal, A. Calcium-dependent regulation of photosynthesis. Redox chains in chloroplast envelope membranes: spectroscopic evidence for the presence of electron carriers, including iron—sulfurcenters. Katoh, A. Absence of light-induced proton extrusion in a cotA-less mutant of Synechocystis sp.
Kovacs-Bogdan, E. Protein import into chloroplasts: the Tic complex and its regulation. Kramer, D. Balancing the central roles of the thylakoid proton gradient.
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Kunz, H. Plastidial transporters KEA1, -2, and -3 are essential for chloroplast osmoregulation, integrity, and pH regulation in Arabidopsis. Marmagne, A. Martiniere, A. In vivo intracellular pH measurements in tobacco and Arabidopsis reveal an unexpected pH gradient in the endomembrane system. Plant Cell 25, — Mi, F. Miyaji, T. AtPHT4;4 is a chloroplast-localized ascorbate transporter in Arabidopsis. Muller, M. Decreased capacity for sodium export out of Arabidopsis chloroplasts impairs salt tolerance, photosynthesis and plant performance.
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EMBO J. Roth, C. Characterization of a protein of the plastid inner envelope having homology to animal inorganic phosphate, chloride and organic-anion transporters. Planta , — Sawada, Y. Arabidopsis bile acid:sodium symporter family protein 5 is involved in methionine-derived glucosinolate biosynthesis.
Scherer, S. Effect of monochromatic light on proton efflux of the blue-green alga Anabaena variabilis. Schumacher, K. Plant Biol. Shabala, S. Demidchik and F. Maathuis Heidelberg: Springer Berlin , 87— Shen, J. Organelle pH in the Arabidopsis endomembrane system. Plant 6, — Sheng, P. Albino midrib 1, encoding a putative potassium efflux antiporter, affects chloroplast development and drought tolerance in rice. Plant Cell Rep. Shingles, R. As a result of proton pumping into the thylakoid lumen, an alkaline stromal pH develops, which is required for full activation of pH-dependent Calvin Benson cycle enzymes.
This implies that a pH gradient between the cytosol pH 7 and the stroma pH 8 is established upon illumination. To maintain this pH gradient chloroplasts actively extrude protons. The established pH gradients are important to drive uptake of essential ions and solutes, but not many transporters involved have been identified to date.
In this mini review we summarize the current status in the field and the open questions that need to be addressed in order to understand how pH gradients are maintained, how this is interconnected with other transport processes and what this means for chloroplast function.
In plant cells, several compartments with different pH exist in parallel. While the apoplast and the vacuole maintain fairly acidic pH levels generally between pH 5 and 7 Grignon and Sentenac, ; Martiniere et al. The cytosol has to stay neutral pH 7. Only mitochondria, the chloroplast stroma and some peroxisomes offer alkaline reaction conditions with pH values equal to or higher than 8 Shen et al.
The pH gradients have to be actively maintained. Given the central role of mitochondria and chloroplasts for plant energy metabolism, these systems are urgently awaiting more targeted research activity.
This review focuses on the chloroplast, which harbors the pathway of arguably the most important biochemical process for life on earth, photosynthesis. Although the luminal pH was suggested to drop below pH 5 in some circumstances, recent studies show more modest values of 5.
First, the two-pore potassium channel TPK3 was confirmed by immunodetection to localize to the stroma lamellae Carraretto et al. Chloroplast with its electron transport chain and selected transporters in the thylakoid membrane.
Transport directions are shown for chloroplasts in the light. Both is indicated by question marks. The black dashed line indicates linear electron transport in the thylakoid membrane. Ratios and dimensions of the protein complexes involved in the electron transport chain are schematic and do not reflect physiological conditions.
Armbruster et al. At the same time, reduced luminal acidification in plants with impaired TPK3 function causes increased sensitivity to ambient light intensities, as the mechanisms to dissipate excess light energy cannot be activated Carraretto et al.
Genes with high sequence similarity to KEA3 exist in green algae, moss and higher plants, with one gene copy appearing in both Chlamydomonas and rice, and two gene copies in Physcomitrella Chanroj et al. The cyanobacterium Synechocystis sp. The authors therefore concluded that NhaS3 is involved in thylakoid lumen ion-homeostasis Tsunekawa et al. Ettinger et al. An alkaline stroma pH is a prerequisite for the full activation of pH-dependent Calvin Benson cycle enzymes Heldt et al.
At higher KCl concentrations, similar to the situation inside the cell, this membrane potential becomes very small Demmig and Gimmler, Selected transporters in the chloroplast envelope membrane. Transporter activities are shown for chloroplasts in the light.
However, no ATPases were identified in chloroplast proteomics studies of envelope membranes. In algae, the plastome-encoded, putative heme-binding, inner envelope protein YCF10 from Chlamydomonas is essential for enhancing carbon fixation and inorganic carbon uptake Rolland et al.
In ycf10 deficient mutants of Chlamydomonas the CO 2 or HCO 3 - uptake was compromised and cells failed to grow photoautotrophically Rolland et al. The cotA insertion mutant MA29 of the cyanobacterium Synechocystis sp. The chloroplast inner envelope is also reported to contain non-selective cation channel activities Pottosin et al.
However, up to today the molecular identity of these channels is unknown. Light induced shrinkage of chloroplasts was observed more than 40 years ago Nobel et al. A short version of the KEA2 protein comprising the antiport domain and the regulatory C-terminal KTN domain, but not the long N-terminal domain of unknown function Chanroj et al. KEA1 and KEA2 represent very close homologs, most likely due to a gene-duplication event and thus are expected to facilitate the same transport function.
Moreover, single T-DNA insertion kea1 or kea2 mutants do not show visible phenotypes indicating functional redundancy in Arabidopsis Kunz et al. Interestingly, this phenotype depends on the leaf developmental stage, i. Additionally, since the pmf across the thylakoid membrane depends not only on light but also on the stromal pH Hauser et al. This further emphasizes the linkage between plastid ion and pH homeostasis, and their importance for efficient photosynthesis. The rice chloroplast envelope KEA homolog is encoded by a single locus named AM1 , a lbino m idrib mutant1.
The am 1 mutant was isolated in an EMS screen and displays green- and white leaf variegations Sheng et al. Surprisingly, no photosynthetic alterations were found in am1 mutants. However, mutants did reveal altered chloroplast ultrastructure along with increased drought tolerance Sheng et al.
The authors attribute AM1 function in rice to chloroplast development and drought stress Sheng et al. Interestingly, the growth and photosynthesis defects in Arabidopsis kea1kea2 double mutant plants could be rescued partially in high NaCl concentrations. Salt or drought stress are known to increase cytoplasmic and vacuolar osmotic values due to accumulation of salt or synthesis of osmolytes. Possibly, this counteracts the increased osmotic values of the chloroplast stroma in absence of these KEA antiporters Kunz et al.
That difference gave rise to a natural proton gradient across the vent membranes that had the same polarity outside positive and a similar electrochemical potential about millivolts [mV] across the membrane as modern cells have. Russell has long maintained that natural proton gradients played a central role in powering the origin of life. There are, of course, big open questions — not least, how the gradients might have been tapped by the earliest cells, which certainly lacked such sophisticated protein machinery as the ATP synthase.
There are a few possible abiotic mechanisms, presently under scrutiny in Russell's lab and elsewhere. But thermodynamic arguments, remarkably, suggest that the only way life could have started at all is if it found a way to tap the proton gradients Lane et al. Net growth is not possible.
In the graph, energy is shown on the y-axis. A horizontal, dashed line shows the starting level of ATP. The graph is a bell-shaped curve starting at the dashed line, rising above it, and ending below it.
The rising portion of the curve shows that one ATP molecule is necessary for the activation energy to get the chemical reaction started. The production of only a single ATP molecule is counteracted by the energy usage of one ATP molecule, so there is no net gain in energy. Life hydrogenates carbon dioxide.
In other words, to convert carbon dioxide into organic molecules, life attaches hydrogen atoms to CO 2. There are only so many ways of doing this, and all life uses just five primary pathways.
All but one of these costs energy for example, the energy of the sun in photosynthesis. The exception is an ancient pathway called the acetyl-CoA pathway, in which hydrogen gas is reacted, via a few steps, with CO 2. This pathway is exothermic releasing energy that can be captured as ATP right through to pyruvate, one of the central molecules in cell metabolism. It's "a free lunch that you're paid to eat," in the words of Everett Shock.
All cells that use the acetyl-CoA pathway today depend on proton gradients. None of them can grow by fermentation — that is, by the chemistry of glycolysis. Why not? Because CO 2 is a stable molecule and does not react easily, even with hydrogen — even when thermodynamics says it should react.
CO 2 is a bit like oxygen in this respect: Once it starts to react, it's not easily stopped. But a fire needs a spark to get it going, and so, too, does CO 2. If there's no gain, there's no growth; no growth, no life. Figure 5: Why chemiosmosis solves the problem If a reaction doesn't release enough energy to generate 1 ATP, it can be repeated endlessly until it has pumped enough protons to generate 1 ATP.
Chemiosmosis allows cells to save loose change, so to speak. Seventeen protons are shown on one side of the membrane as a result of this proton pumping. The accumulation of protons drives ATP synthesis by ATP synthase, which is depicted on the right side of the diagram as a red circle and cylinder in the membrane. Grey boxes show that energy release by the electron transport chain on the right can be separated from ATP synthesis by ATP synthase on the left.
It's not quite true to say that the reaction of CO 2 with H 2 releases enough energy to make 1 ATP: it's actually enough to make 1. But of course there's no such thing as 1. But that doesn't happen with a gradient Figure 5. In principle, a reaction can be repeated over and over again, just to pump a proton over a membrane. When enough protons have accumulated, the proton motive force powers the formation of ATP. So a gradient allows cells to save up protons as "loose change", and that makes all the difference in the world — the difference between growth and no growth, life and no life.
Figure 6 Despite their power, protons have their share of problems, and these problems might explain why life got stuck in a rut for 2 billion years. All complex life on Earth today is composed of a certain type of complex cell, known as a eukaryotic cell. Generally much larger than bacteria or archaea, the eukaryotic cell contains a nucleus , and a much larger genome , and all kinds of specialized organelles little organs , such as mitochondria.
The strange thing is that eukaryotes have repeatedly given rise to large, complex, multicellular organisms like plants, animals, fungi, and algae — but prokaryotes show little or no tendency to evolve greater morphological complexity, despite their biochemical virtuosity. One possible answer relates to the control of proton gradients. All eukaryotic cells turn out to have mitochondria, or once had them and later lost them by reductive evolution back toward a prokaryotic state.
No mitochondria, no eukaryotes Figure 6. All mitochondria capable of oxidative phosphorylation have retained a tiny genome of their own, which appears to be necessary to maintain control over membrane potential Allen A membrane potential of mV across the 5-nanometer membrane gives a field strength of 30 million volts per meter — equivalent to a bolt of lightning. This huge electrochemical potential makes the mitochondrial membranes totally different from any other membrane system in the cell such as the endoplasmic reticulum which, according to Allen, is why mitochondrial genes are needed locally in cellular subregions.
In effect, by responding to local changes in electrochemical potential, they prevent the cell from electrocuting itself. No mitochondrial genome, no oxidative phosphorylation. It could be, then, that bacteria can't expand in cell and genome size because they can't physically associate the right set of genes with their energetic membranes.
If that's the case, the acquisition of mitochondria and the origin of complexity could be one and the same event. The question is, what kind of a cell acquired mitochondria in the first place? Most large-scale genomic studies suggest that the answer is an archaeon — that is, a prokaryotic cell that is in most respects like a bacterium.
That begs the question, how did mitochondria get inside an archaeon? The answer is a mystery but might go some way toward explaining why complex life derives from a single common ancestor, which arose just once in the 4 billion years of life on Earth. Peter Mitchell's demonstration that ATP synthesis is powered by proton gradients was one of the most counterintuitive discoveries in biology, and it took a long time to be accepted.
The precise mechanisms by which a proton gradient is formed and coupled to ATP synthesis chemiosmotic coupling is now known in atomic detail, but the broader question that drove Mitchell — why are proton gradients so central to life? Recent research suggests that proton gradients are strictly necessary to the origin of life and highlights the geological setting in which natural gradients form across membranes, in much the same way as they do in cells.
But the dependence of life on proton gradients might also have prevented the evolution of life beyond the prokaryotic level of complexity, until the unique chimeric origin of the eukaryotic cell overcame this obstacle. Allen, J. The function of genomes in bioenergetic organelles.
Efremov, R. Nature , — doi Lane, N. How did LUCA make a living? Chemiosmosis in the origin of life. Bioessays 32 , — doi The energetics of genome complexity. Nature , Martin, W. On the origin of biochemistry at an alkaline hydrothermal vent. Mitchell, P.
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