Remarkably, subunits of cornĬhloroplast ATP synthase are more than 60% identical in amino acid sequence with those of human ATP The subunits contain theĬatalytic sites, similarto the F1 subunit of mitochondrial ATP synthase. II, and III correspond to subunits a, b, and c, respectively, of the mitochondrial F0 subunit, and subunitĬF1, the site of ATP synthesis, has a subunit composition 3 3. (17 kd), II (16), III (8 kd), and IV (27 kd) having an estimated stoichiometry of 1:2:12:1. It consists of four different polypeptide chains known as I CF0 conducts protonsĪcross the thylakoid membrane, whereas CF1 catalyzes the formation of ATP from ADP and Pi.ĬF0 is embedded in the thylakoid membrane. Synthase closely resembles the F1-F0 complex of mitochondria (Section 18.4). The proton-motive force generated by the light reactions is converted into ATP by the ATP synthase ofĬhloroplasts, also called the CF 1 -CF 0 complex (C stands for chloroplast and F for factor). The ATP Synthase of Chloroplasts Closely Resembles Those of Mitochondriaand Membrane corresponds to a proton-motive force of 0 V or a G of -4 kcal mol-1 (-20 kJ mol-1). A pH gradient of 3 units across the thylakoid Maintained and no membrane potential is generated. Induced transfer of H+ into the thylakoid space is accompanied by the transfer of either Cl- in the sameĭirection or Mg2+ (1 Mg2+ per 2 H+) in the opposite direction. Reason for this difference is that the thylakoid membrane is quitepermeable to Cl- and Mg2+. PH gradient, whereas, in mitochondria, the contribution from the membrane potential is larger. In chloroplasts, nearly all of p arises from the Inherent in the proton gradient, called the proton-motive force ( p), is described as the sum of twoĬomponents: a charge gradient and a chemical gradient. Light-induced transmembrane proton gradient is about 3 pH units. The thylakoid space becomes markedly acidic, with the pH approaching 4. Such a gradient can be maintainedīecause the thylakoid membrane is essentially Lumen or taken up from the stroma, generating a proton gradient. At various stages in this process, protons are released into the thylakoid We have seen how light induces electron transfer through photosystems II and The principles by which ATP synthesis takes place in chloroplasts are nearly identical with those for Unequivocally support the hypothesis put forth by Peter Mitchell that ATP synthesis is driven by proton. This incisive experiment was one of the first to A burst of ATP synthesis then accompanied the disappearanceof the pH gradientĪcross the thylakoid membrane (Figure 19). The pH of the stroma suddenly increased to 8, whereas the pH of the thylakoid To create this transient pH gradient, he soakedĬhloroplasts in a pH 4 buffer for several hours and then rapidly mixed them with a pH 8 bufferĬontaining ADP and Pi. Gradient is imposed across the thylakoid membrane. In 1966, André Jagendorf showed that chloroplasts synthesize ATP in the dark when an artificial pH This counterpoint is intended to set the record straight.ĪTP acetyl CoA pathway alkaline hydrothermal vent chemiosmotic coupling origin of life pH gradient.A Proton Gradient Across the Thylakoid Membrane Drives ATP Synthesis Unfortunately, Jackson mainly criticized his own interpretations of the theory, not what the literature says. Recently, however, a paper by Baz Jackson criticized the hypothesis, concluding that natural pH gradients were unlikely to have played any role in the origin of life. Their congruence with the physiology of anaerobic autotrophs that use the acetyl CoA pathway to fix CO 2 gives the alkaline vent model broad appeal to biologists. Alkaline hydrothermal vents harbour pH gradients of similar polarity and magnitude to those employed by modern cells, one of many properties that make them attractive models for life's origin. As argued previously in these pages, such deep conservation suggests that ion gradients arose early in evolution, and might have played a role in the origin of life. Chemiosmotic coupling - the harnessing of electrochemical ion gradients across membranes to drive metabolism - is as universally conserved as the genetic code.
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