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chemiosmotic mechanism of ATP synthesis

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Polyamines (PAs) are low molecular weight amines that occur in every living organism. The three main PAs (putrescine, spermidine, and spermine) are involved in several important biochemical processes covered in recent reviews. As rule of thumb, increase of the cellular titer of PAs in plants is related to cell growth and cell tolerance to abiotic and biotic stress. In the present contribution, we describe recent findings from plant bioenergetics that bring to light a previously unrecognized dynamic behavior of the PA pool. Traditionally, PAs are described by many authors as organic polycations, when in fact they are bases that can be found in a charged or uncharged form. Although uncharged forms represent less than 0.1% of the total pool, we propose that their physiological role could be crucial in chemiosmosis. This process describes the formation of a PA gradient across membranes within seconds and is difficult to be tested in vivo in plants due to the relatively small molecular weight of PAs and the speed of the process. We tested the hypothesis that PAs act as permeable buffers in intact leaves by using recent advances in vivo probing. We found that an increase of PAs increases the electric component (Δψ) and decreases the ΔpH component of the proton motive force. These findings reveal an important modulation of the energy production process and photoprotection of the chloroplast by PAs. We explain in detail the theory behind PA pumping and ion trapping in acidic compartments (such as the lumen in chloroplasts) and how this regulatory process could improve either the photochemical efficiency of the photosynthetic apparatus and increase the synthesis of ATP or fine tune antenna regulation and make the plant more tolerant to stress.

Photosystems I and II - Encyclopedia Britannica

Photosynthetic electron transport produces the reductant (NADPH) that drives several important metabolic processes, including the assimilation of carbon dioxide into carbohydrates. Photosynthetic electron transort also drives the movement of H+ from the chloroplast stroma into the thylakoid lumen, producing an electrochemical potential across the membrane that is used to phosphorylate ADP, yielding ATP.

Plant Energy Transformations-Photosynthesis - …

Polyamines in chemiosmosis in vivo: A cunning mechanism for the regulation of ATP synthesis during growth and stress.

As Mitchell predicted (1-3), the mitochondrial, bacterial, and chloroplast membranes that couple ATP synthesis to electron transport are poorly permeable to protons, except when proton-linked processes, such as ATP formation, occur at high rates. Proton transport was shown to be linked to electron transportin mitochondria (4), chloroplasts (5), and bacteria (6). The measurements of the magnitudes oficross these membranes turned out to be difficult, but in most instances, values approaching 20 kJ mol-1 (Dp ~ 200 mV) have been measured during steady state, rapid electron transport. As predicted by Mitchell, lipophilic weak acids (eg, 2,4-dinitrophenol) could collapse the A^H-by shuttling protons across the membrane. ATP synthesis is also inhibited by these reagents, which are termed "uncouplers" because they uncouple electron flow and ATP synthesis.

The essence of chemiosmotic coupling is that exergonic electron transport by the respiratory and photosynthetic electron chains is obligatorily linked to transmembrane proton flux, resulting in the generation ofThe flow of protons down their electrochemical potential provides the driving force for ATP synthesis. There is compelling evidence in support of these proposals.


05/12/2017 · Chemiosmosis is a process for ATP synthesis in this enzyme ATP synthase is used for phosphorylation of ADP to ATP

We expand on chemiosmosis once again by introducing natural amines that their existence in thylakoids is well established and their molecular role is getting better understood. Moreover the intermediate is not obligatory for ATP synthesis as one may assume. Thus, ATP synthesis can occur in vitro without PAs. However, the intermediate (i.e., PAs) increases the efficiency of ATP synthesis and allows regulation (, ).

New chemiosmosis may help to elucidate the role of PAs during stress. Below we consider only two cases (salt and osmotic stress) but the concept could be adopted with some modifications in other stress cases as well. “For example in Arabidopsis grown under high salt stress, photosynthesis would likely need to operate under conditions where the ionic strength inside the plastid is high. In this case, pmf storage would be heavily biased toward ΔpH formation (; ; ). Consequently, energy dissipation would be more easily and strongly induced at low and moderate light intensities, severely limiting the productivity and growth of the plant, even if water and CO2 were not limiting factors. Thus, the accumulation of Put observed in plants grown under high salt stress (; ) and particularly in Arabidopsis through adc2 induction () could serve to increase buffering solutes, rebalancing pmf toward Δψ and optimizing the regulation of energy transduction. In line with this view, blocking this up-regulation of Put during salt stress, e.g., in the adc-2-1 mutant of Arabidopsis, leads to increased sensitivity to salt stress, which is restored upon addition of Put (; ), whereas over-expressing adc increases tolerance to drought ()” (). Similarly, under osmotic stress the plant faces a decrease in relative water content. This decrease in water content is also evident in chloroplasts. It is well established that under conditions of water stress arginine decarboxylase (ADC) which is located in thylakoid membranes of chloroplasts () is significantly upregulated, i.e., 2–60-fold increase (). For recent works of ADC up-regulation upon stress see . This increase of Put titer is part of the protective response of the plant to osmotic stress. Artificial increase of Put titer in leaf disk 1 h before the stress significantly protects the photosynthetic apparatus (). In all cases data from leaf discs should be examined with caution. In addition, the role of other organelles such as the vacuole that contain most of the water reserves in the plant cell could be important. Hence, the titer of PAs in each compartment of the cell (e.g., chloroplasts, mitochondria, vacuole, nucleus) should be estimated both under physiological conditions and under stress. In this capacity, new protocols and methods should be used solving problems that derive from the properties of PAs (i.e., high pKs and rapid penetration of membrane barriers). PAs will accumulate in vivo in every cellular compartment/organelle that is more acidic than the surrounding microenviroment (the driving force is their high pK as explained before in the ion trapping) and will be depleted rapidly (within seconds) upon grinding of the tissue.

Define chemiosmotic: relating to or being a theory that seeks to explain the mechanism of ATP formation in oxidative phosphorylation by mitochondria …
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  • and hydrogen transfer by a chemi-osmotic type of mechanism"

    C C 3 pathway

  • The mechanism that generates ATP in a chloroplast

    See Calvin cycle

  • Chemiosmosis - Biology-Online Dictionary

    C 3 plant

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Light Reactions and Chemiosmosis by Megan Scalia on Prezi

Evidence for chemiosmosis includes experiments showing that ATP synthesis occurs in the absence of electron transport if a proton gradient is created in some other way.

Light Reactions and Chemiosmosis ..

The ATP synthase is a large protein complex consisting of about 24 protein subunits. It acts as a channel through which protons that are in the thylakoid lumen can escape back into the chloroplast stroma, driven by the electrochemical gradient. The passage of H+ through the ATP synthase provides energy for ATP synthesis. About 3 protons are passed for each ATP made. A simple sketch of the ATP synthase is shown below.

most ATP synthesis in cells occurs by chemiosmosis.

Thylakoid membranes isolated from chloroplasts can be made to synthesize ATP in the absence of photosynthetic electron transport in the following experiment. Thylakoids are isolated from chloroplasts and suspended in a buffer at pH 4. The lumen spaces of these thylakoids gradually become pH 4 as H+ moves into them from the surrounding solution. Some of the pH 4 thylakoids are then removed from the pH 4 buffer and placed in a second beaker, also at pH 4. This beaker has phosphate and ADP in it but the thylakoids do not make ATP. Another portion of the pH 4 thylakoids are placed in a beaker having ADP and phosphate but buffered to pH 8. This time ATP is synthesized. This experiment is carried out in darkness, proving that light is not directly required for ATP synthesis but that a proton gradient between the thylakoid lumen and the outside is required.

Chemiosmosis - an overview | ScienceDirect Topics

Uncouplers are called uncouplers because they "uncouple" electron transport from ATP synthesis. They inhibit ATP synthesis with affecting electron transport. In fact, electron transport accelerates because it is no longer pumping protons against a gradient. The fact that uncouplers can abolish ATP synthesis but have no negative effect on electron transport is more evidence in support of the chemiosmotic hypothesis.

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