Energy that can be acquired by the organism's respiration reaction

　Now you are asked to calculate the "total difference in standard Gibbs energy before and after the chemical reaction". This is a long word, and it is 100% up to you again. Let's get used to it by solving the example questions.

　First, let's consider the standard Gibbs energy of formation for carbon dioxide (CO₂), a familiar compound in biological metabolism: the elements that make up CO₂ are carbon (C) and oxygen (O). The stable states of each element on its own are carbon (C) and oxygen molecules (O₂), and the Gibbs energy required to produce one mole of CO₂ from C and O₂ is -394.4 (kJ/mol) as the standard Gibbs energy of formation of CO₂ (gas). This is reported as an experimental value and can be found in the Chemistry Handbook and other publications.

　Organisms use oxygen to oxidize organic matter to obtain energy (respiration). The figure below shows the reaction of oxidation of organic matter by respiration, assuming formaldehyde as the simplest organic matter. Below each substance is the standard Gibbs energy of formation for each substance. The total energy of the chemical reaction in its product form (right side) minus the total energy of the original form (left side) (⊿∑G_f⁰) is the energy that the organism can acquire. The reason why the energy difference has a negative value is because, from the point of view of the reaction system, the organism is losing energy, which it can gain.

※　The standard Gibbs energy of formation of O₂ in its standard state is 0 (kJ/mol), but dissolving it in water produces energy (16.3 kJ/mol). Assuming respiration in water, we used O₂(aq) as above. Also, the energy used in the calculation changes depending on whether the organisms in the water take in CO₂ as CO₂(aq) or as H₂CO₃ or HCO₃^-. In this course, I have dealt with this issue in an appropriate manner. Please forgive me.

　As in this example, the standard Gibbs energy of formation for each substance is noted below the reaction equation, and the total difference (⊿∑G_f⁰) is calculated before and after the reaction (left side: original form, right side: product form). The standard Gibbs energy of formation (G_f⁰) for each substance is an important thermodynamic constant and is summarized in the table below.

　Next, in the absence of oxygen, organisms appear that use nitric acid as an oxidant to respire. They are known as nitrate-reducing bacteria.

　Although nitrate respiration appears to provide slightly less energy than oxygen respiration, nitrate reduction actually occurs in two stages. Nitrate-reducing bacteria respire NO₃^- and exhale NO₂^-, then nitrite-reducing bacteria respire NO₂^- and exhale N₂. If divided into two stages, it would be much less efficient than oxygen respiration.

　Some organisms (prokaryotes) have this specific form of respiration. Some organisms respire iron hydroxide as an oxidant.

　Oxygen and even nitric acid are gone, and without oxidized minerals such as manganese oxide and iron hydroxide, organisms that use sulfuric acid as an oxidizing agent will show their faces.

　Of the dissolved sulfur compounds in the aqueous environment, sulfuric acid is the most energy-intensive to synthesize from its constituent elements (S and O). Stripping oxygen from such a stable compound requires a great deal of energy, and the energy available from organic matter is used to do so. This reduces the energy available to the organism, which is quite inefficient. It is precisely because of this harsh situation that bacteria with the special ability to reduce sulfuric acid dominate here and there.

　When sulfuric acid reduction occurs, hydrogen sulfide (H₂S) is produced as a reaction product. Hydrogen sulfide is what gives off the smell of rotting in a ditch river (often described as the smell of rotten eggs).　Sulfuric acid reduction occurs in places where large amounts of organic matter accumulate for long periods of time. However, the sulfuric acid ions that are the source of the sulfuric acid must be present.