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The light reactions, if seen as a whole, are basically the oxidation of water to provide energy (ATP) to fuel the Calvin Cycle. The photons provide the activation energy for the same.

How then can we say that during chemo-synthesis bacteria get ATP in the first step by oxidizing inorganic substances like $H_2S$ or $CH_4$ because ultimately, it is similar to the light reactions and will require some activation energy?

Where does this energy come from?

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These chemo-synthetic bacteria gets energy from oxidation of inorganic compounds. Following table from Biology of Prokaryotes shows main reactions used by different species,

enter image description here

This oxidation happens only if electron acceptor is available. These all reactions have negative Gibbs Free Energy, hence they are favorable reactions and happens spontaneously. Generally oxygen is preferred electron acceptor but in case of lack of oxygen nitrate, sulfate, $HCO^{3-}$ etc are used.

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  • $\begingroup$ Could you please list the Gibbs free energy for a normal photosynthetic light reaction? $\endgroup$ – Chitinous Exoskeleton Nov 13 '15 at 13:44
  • $\begingroup$ Check this. It has all numbers and calculations you need. $\endgroup$ – Dexter Nov 13 '15 at 13:49

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