Proton motive force in chloroplasts produces energy and regulates photoprotection. Thus light-driven transthylakoid pmf plays several essential roles in plant physiology (). More particularly both the ΔpH and Δψ components of pmf contribute to ATP synthesis at the CFO–CF1 ATP synthase, in a thermodynamically equivalent fashion (). In addition, the ΔpH component of pmf is a key signal for initiating photoprotection. This photoprotection mechanism the so-called energization quenching (qE), is a process that harmlessly dissipates the excess absorbed light energy as heat (; ; ). Acidification of the lumen also controls photosynthetic electron transfer by slowing the rate of plastoquinol oxidation at the cytochrome b6f complex (; ), preventing the accumulation of highly reducing species within photosystem I ().
According to , Mitchell also tried to incorporate data from amines () under similar experimental conditions to ours () and broke the rules of chemiosmosis, expanding his theory. By that time it was not clear that amines could be used by nature in chemiosmosis, and were used as a tool to study phosphorylation. Even 10 years later Slater reviewed the numerous studies on the nature of the intermediate between the redox reaction and ATP synthesis concluding that the matter was still open (). In light of recent data, biogenic amines (i.e., PAs) seem to play the role of an intermediate in vivo, this matter is currently being better understood.
He linked this process to osmosis, the diffusion of water across a membrane, which is why it is called chemiosmosis. ATP synthase is the enzyme that makes ATP by chemiosmosis. It allows protons to pass through the membrane using the k...
Chemiosmosisin Mitochondria. Two Types of Phosphorylation. Anaerobic Respiration. Alcohol Fermentation. Lactic Acid Fermentation. Sample Questions and Answers. Photosynthesis. Review. Noncyclic Photophosphorylation. Cyclic Photophos...
A major theme in my career has been photophosphorylation; especially contributions to the early work on chemiosmosis, and later involvement in CF1 activation and function. A second theme has been interest inchloroplast biogenesis, wi...
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.
Organisms need ATP for many cellular processes such as translation, metabolite production, proliferation and stress response. Most ATP (95%) is produced by chemiosmosis (i.e., the movement of ions across a selectively permeable membrane, down their electrochemical gradient), therefore this synthesis is the most important process for cell physiology (, ). Not surprisingly, partial or full inhibition of chemiosmosis leads to disease or death in animals and plants. Hence, any factor (protein or solute) that increases or more generally speaking, modulates ATP synthesis is of exceptional biological significance. In this contribution, we will discuss the role of polyamines (PAs) in chemiosmotic ATP synthesis based on findings from plant bioenergetics. The chemiosmotic hypothesis states that ATP synthesis in respiring cells comes from the electrochemical gradient across membranes such as the inner membranes of mitochondria and chloroplasts (). In other words, energization of a single membrane simultaneously and continuously powers many ATP synthases. Usually in biochemistry, an enzyme converts a substrate into a product, but in chemiosmosis the situation is slightly more complex. Hence, for the purpose of this review it is important to clarify basic features of the chemiosmotic mechanism before the role of PAs is described. The chemiosmotic mechanism in plants, animals and microbes has three conserved features: (i) an electron transport chain that supports vectorial release of protons (proton producers), (ii) a coupling membrane or “energized” membrane (cristae membrane in mitochondria, thylakoid membrane in chloroplasts, and plasma membrane in bacteria), (iii) transmembrane proton motive ATPases that are vectorially embedded in the membrane (proton consumers). The following scheme (Figure ) illustrates the sequence of events in classical chemiosmosis.
A chemiosmotic unit (a membrane that houses many proton producers and many ATPases) functions as a battery and as long as it is charged phosphorylates ADP. This battery can be seen as a huge enzymatic complex that uses an electrochemical gradient also called proton motive force (pmf) to produce ATP. Pmf is a combination of two gradients across the membrane: a concentration proton gradient (ΔpH) and an electrical gradient (Δψ). In simpler terms, electron carriers and related enzymes in the membrane produce protons that are released on one side of the membrane and decrease the pH of this compartment (e.g., lumen of thylakoids). Consequently, protons will diffuse from an area of high proton concentration (lumen) to an area of lower proton concentration (stroma). The main efflux path for protons is the ATPase, which in turn uses protons’ free energy to phosphorylate ADP. Important factors for the amplitude of pmf are the proton release rate, the conductivity of the ATPase to protons and the ionic strength. In plants, pmf is established both in mitochondria and chloroplasts. Next, we will describe why pmf in chloroplasts has a more complex and important role than in mitochondria. Noteworthily, in plant science data derive both from in vitro and in vivo measurements. In other disciplines, most data in particular for ATPases come from in vitro experiments. Thus chemiosmosis in vivo is better understood and described in plants.
The function of the ETC in photosynthesis in photosystem II and photosystem I is to produce NADPH and ATP “fuel” for glucose synthesis in. It's to TRANSPORT ELECTRONS. Hence the name. What's up with the caps? Photosynthesis is a fascinating process, by which energy from.
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 (, ).