Metabolic reactions can be split into two categories:
- Catabolism: it encompasses chemical reactions in which complex molecules are broken down into simpler components, releasing the energy trapped in the chemical bonds in the process.
- Anabolism: it includes metabolic pathways that result in the synthesis of molecules from their smaller building blocks. In this case, energy needs to be supplied.
Understandably, the Gibb’s free energy and the substrates available to organisms will determine which metabolisms are viable. Different chemical reactions will yield different energy outcomes that will power the cells. For example, the oxidation of glucose in the presence of oxygen releases a vast amount of energy, which will result in aerobic heterotrophic organisms having very low doubling times (the time that it takes a microbial culture to double the number of members). Other reactions are not as efficient, as we can see in the redox tower,
E is the electric potential of the reaction, which is defined as the energy per unit charge available from the oxidations/reduction reaction. The relationship between the electric potential (E) and the Gibb’s Free Energy is very straightforward and it is captured by the equation:
Where n is the number of electrons transferred and E is F is the Faraday constant 96,485J/(V⋅mol).
Gibb’s Free Energy and Cellular Growth
Chemical reactions have been extensively characterized thermodynamically in order to build tables that contain values such as enthalpy, entropy, and Gibb’s free energy. However, cells, due to their intrinsic complexity, have fallen behind. Nevertheless, efforts have been made to try to characterize the properties of microbial biomass.
In this pivotal work, we can see different ∆G values for different microbial species. The immediate conclusion for this work is that, for cells to replicate themselves, they would need to allocate energy to synthesize new biomass. The energy required for such a process should then derive from the redox reactions we mentioned before. The yield of these reactions will then determine how much energy is available and if it is viable for the cell to replicate. Recent studies have aimed to try to find the relation between Gibb’s free energy and the microbial growth of microorganisms, highlighting the importance of thermodynamics in life.
Why Unravelling Biothermodynamics Matters
Despite being a very complex tapestry of interconnected chemical reactions contained in cells, life is governed by the same fundamental principles as fundamental chemistry. In its fight for survival, laws such as that energy is neither created nor destroyed and that the universe tends towards entropy lay the ground rules for life. Gibb’s free energy emerges as a critical player, determining the spontaneity of these chemical reactions. Organisms, as tiny biological machines, have the ability to harness the energy released by spontaneous chemical reactions to drive their metabolism.
This is why it is fundamental that we unravel biothermodynamics. The ramifications of this knowledge are vast. It will help us understand how much organisms can grow. It will provide us with tools to optimize our processes, redirecting the energy allocation of cells into products that we could be interested in. But it can also provide information about in which kind of setting organisms can grow, opening the gates for fields as exotic as astrobiology. Tools such as adiabatic calorimetry and simulation can be of unmeasurable help for this.
Watch our video on biothermodynamics here: