Biological energy is produced by the breakdown of various organic compounds through fermentation and respiration*1, by respiration using inorganic compounds, and by photosynthesis using light energy. It is controlled by the high-energy phosphate bonds of ATP and other substances and by the concentration gradient of H+ for use by organisms in various activities. As outlined in Chapter 7, free energy change (ΔG) resulting from the hydrolysis of ATP’s terminal phosphate bond is expressed by Equation 8-1. Interestingly, ATP has two phosphate bonds, and only the terminal one is formed by the processes of respiration and photosynthesis discussed in this chapter; the inner bond is formed by other enzymes*2. The concentration gradient of H+ across the membrane can be expressed as the electrochemical potential of H+ (ΔμH+) (Equation 8-2). This is the same as the free energy change that occurs when H+ is transported across the membrane. Here, log10[H+in] and log10[H+out] can be converted to the pH inside and outside the membrane, and ΔΦ is the membrane potential. In other words, even membrane potential formed by the transport of ions other than H+ contributes to the high-energy state of H+.
Free energy change by ATP hydrolysis:
ΔG = ΔG˚’ + 2.3RTlog10[ADP][H3PO4] / [ATP]
Electrochemical potential of H+ across the membrane:
ΔμH+ = 2.3RTlog10 [H+in] / [H+out] + FΔΦ
where R is the gas constant, T is the absolute temperature, ΔG˚’ is the free energy change under standard conditions (pH7) and F is the Faraday constant.
Organisms have many enzymes that catalyze a range of chemical reactions in their cells. Cells working in normal conditions constantly incorporate nutrients and energy from outside and discard waste materials, thereby keeping the intracellular conditions constant (i.e., in a state of dynamic equilibrium). This state is achieved through a series of metabolic reactions that proceed almost constantly through the catalytic action of enzymes. If this is the case, do all chemical reactions catalyzed by enzymes in living cells reach dynamic equilibrium? The answer is no. If all such reactions reached this state, new reactions would not occur without the entry and exit of new materials, although existing conditions might be maintained. Such cells could no longer be called autonomously living cells.
In performing new reactions, actual organisms change equilibrium by putting energy into particular reactions. For this purpose, a high-energy state that deviates from equilibrium needs to be maintained. Such a state involves the concentration gradient of ATP and H+, and the mechanism of putting in energy as appropriate involves the activity regulation of enzymes that use the energy (e.g., the ATPase of myosins that is used for cell movement). The main function of organisms is to convert the energy produced by the fermentation of various materials, as well as by respiration and photosynthesis, into the standardized forms, namely ATP and concentration gradient of H+, and maintain it.
The organic compounds and energy that support the activities of almost all organisms on earth are produced and supplied through photosynthesis by plants and other organisms. All oxygen is also produced and released by this process. The mechanism of photosynthesis is completely different from that of fermentation and respiration in that it uses physical energy in the form of light, but the mechanism of ATP synthesis using the physical energy obtained from the concentration gradient of H+ created by electron transport is very similar among the three mechanisms. In processes other than photosynthesis, organic compound energy is supplied through autotrophic chemical synthesis by bacteria*3.
Fermentation and respiration: In energy metabolism, fermentation refers to reactions that do not accompany oxidation and reduction, while respiration refers to reactions that accompany oxidation by oxygen or other substances. Sulfate respiration and nitrate respiration, which respectively use SO42- and NO3- instead of oxygen, are also known.
ATP’s terminal high-energy phosphate bond is formed by ATP synthase. High-energy phosphate bonds in ADP are formed by adenylate kinase from AMP and ATP: AMP + ATP ⇄ 2ADP
The sulfur bacteria (Thiomicrospira spp.) found in organisms such as Calyptogena spp. that live in hydrothermal vents on the ocean floor (also known as black smokers) are an example of this.