Total difference in standard Gibbs energy of formation for respiratory response (energy gained)

　The respiration reaction is the oxidative decomposition of organic matter (reducing agent) with an oxidizing agent (such as oxygen). In a respiration reaction, carbon dioxide and water are produced after the reaction. Write down the reaction equation and calculate the total difference in the standard Gibbs energy of formation of the substances before and after the reaction. The total difference is the energy obtained from the respiration reaction.

Oxygen respiration

　Eukaryotes use oxygen to oxidize organic matter to obtain energy (oxygen respiration). In the column below, we have assumed the simplest organic substance, formaldehyde, and described the reaction of oxidation of organic matter by respiration. Under each substance, we note 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 in its original form (left side) (⊿∑G_f⁰) is the energy that the organism can obtain. The reason for the negative energy difference is that, from the reaction system's point of view, the energy is lost and the organism can obtain it.

*Before the reaction is called the original form, and after the reaction is called the product form.

　As in this example, the standard Gibbs energy of formation for each substance is written 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. It can be downloaded in one previous course.

　*The standard Gibbs energy of formation for 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 the standard Gibbs energy of formation for O₂(aq) as shown above. Also, if the organism exhales CO₂ in water, CO₂(aq) should be used, but if it exhales into the gas phase, the CO₂(gas) value should be used. Even when plants in water use CO₂ in photosynthesis, or whether it is H₂CO₃ or HCO₃^-, the energy used in the calculation will change. In this course, I have dealt with that in an appropriate manner.

Nitrate respiration

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

　Although it appears that slightly less energy is gained from nitrate respiration than from oxygen respiration, nitrate reduction actually occurs in two stages. Nitrate-reducing bacteria exhale NO₂^- through NO₃^- respiration and then nitrite-reducing bacteria exhale N₂ through NO₂^- respiration; splitting it into two stages (since the energy gained is split in half) makes it much less efficient than oxygen respiration.

Manganese respiration and iron respiration

　Some organisms (prokaryotes) have this specific form of respiration. There are also iron-breathing bacteria that use iron hydroxide as an oxidant, since iron is more common.

Sulphate respiration

　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.