4.3Fundamentals of the Flow of Energy and Substances
When discussing metabolism, it is necessary to consider two aspects. One is the substance that is actually being converted, and the other is the balance sheet of energy. The driving force for biochemical reactions is not internal energy or enthalpy (see Column the bottom) but free energy change (ΔG). According to thermodynamics, the driving force for a reaction at a given pressure is the decrease in enthalpy (ΔH) and the increase in entropy (ΔS). However, because even these are in conflict, whether a reaction will proceed spontaneously is determined on the basis of ΔG (ΔG = ΔH − TΔS). This ΔG represents the balance between the decrease in enthalpy and the increase in entropy (see Column the bottom). The reaction proceeds if ΔG is negative.
The mechanism responsible for the generation of ATP and NAD(P)H in an organism will be discussed in detail in Chapter 16, but a general overview is presented here to explain the flow of energy and substances that constitute living organisms worldwide (see Fig. 4-2). Energy enters the bodies of living organisms in the form of solar energy*3. Light energy is converted to electrical energy by photosynthesis that is used to convert ADP to ATP and synthesize oxygen, an oxidizing agent, and NADPH, a reducing agent. The latter process may be considered as electrolysis of water. NADPH is used to reduce carbon dioxide, which is how organic compounds such as carbohydrates are produced within organisms. Diurnal plants perform the steps leading to this point. Animals and bacteria (as well as nocturnal plants) derive reducing power in the form of NADH from ingested organic compounds and carbohydrates and expel carbon in the form of carbon dioxide. ATP is synthesized by the redox reaction of NADH and oxygen, and this process can be equated to the creation of a fuel cell. Thus, use of energy derived from sunlight to convert substances through redox reactions is the central focus of metabolism (see Fig. 4-3). Metabolism is a process responsible for the formation of complicated cellular structures, and ATP and NAD(P)H generated in central metabolism are used for this formation. ATP is generated not only during metabolism but also when used as an energy source for the movement of cells.
Fig. 4-2 Material and Energy Flow in Life
Solid lines indicate material changes and dotted lines indicate movement. The portions illustrating the electric circuit indicate that biological reactions are considered as battery reactions and not electrons that cycle around mitochondria and chloroplasts. In the two photosystems, two photons having a total energy corresponding to approximately 3.6 V are absorbed. The output is approximately 1.8 V, minus the lost portion. Of this, approximately 1.1 V is used to synthesize O2 and NADPH and the remaining is used to produce ATP. Respiration can be seen as a cycle driven by a battery of approximately 1.1 V, which produces ATP. Refer to Chapter 16 for more details on this mechanism.
All substances undergo a series of metabolic processes, but it is important to note that energy is continuously consumed in these processes. The amount of energy consumed by metabolic processes that recycle substances (converted into heat) is far more greater than that required for creating an organism or for movement, and hence, a large amount of entropy is released during these processes. The physicist Erwin Schrödinger claimed in his book “What is Life?” (1944) that a decrease in entropy accompanies self-assembly, which is the essence of life, and organisms incorporate this decrease in entropy from the “negative entropy” present in nutrition.
*3 Benthic (deep sea) microorganisms produce bioenergy using deep-sea redox materials, but quantitatively, solar energy is more prevalent.
Laws of Thermodynamics: Free Energy Conversion and Equilibrium Constant
Let us put together the basic laws of thermodynamics.
Enthalpy: H = U + PV
Gibbs free energy: G = H − TS
Note that U is internal energy, P is pressure, V is volume, T is temperature, and S is entropy.
In an exothermic reaction, internal energy (enthalpy at a given pressure) decreases, while in an endothermic reaction, internal energy (enthalpy) increases. However, for a reaction to proceed spontaneously, free energy has to decrease. Thus, even for an endothermic reaction to proceed spontaneously, a large increase in entropy is necessary.
Biochemical reactions generally occur at 1 atm (1.013 × 105 Pa and 1 × 105 Pa in the new standard, where Pa means Pascals) and a set temperature (approximately 298 K). These conditions are often defined as standard conditions, but the definition has several meanings). Thus, free energy change (ΔG) is given as follows:
ΔG°′ = ΔH°′ − TΔS°′
Here “°” on the upper right represents standard conditions (1 molar concentration, 25°C) and “′” indicates that the reaction occurs at a pH of 7.0.
If ΔG°′ of the reaction is negative, then the reaction equilibrium shifts toward the positive reaction. This is given by the following equation:
ΔG = ΔG′ + RTlnKeq′
Note that Keq′ is the equilibrium constant and R is the gas constant. Many biochemical reactions proceed as reversible reactions.
An actual reaction is represented without equilibrium as follows:
|ΔG′ = ΔG°′ +||products|