Bioenergetics: Free Energy, ATP Hydrolysis, and Metabolic Pathways
Cellular Bioenergetics and Thermodynamics
Physical systems of the universe are governed by the laws of thermodynamics, tending always towards greater disorder (entropy).
The cell utilizes energy from its environment to perform vital functions, releasing equivalent amounts as heat or other forms of energy that contribute to the increased disorder of the universe.
Free Energy and the Laws of Thermodynamics
Free Energy: It is a form of energy capable of doing useful work under constant conditions of temperature and pressure.
Gibbs Free Energy (ΔG)
The Gibbs Free Energy equation indicates the amount of useful energy that the system (the cell) exchanges with the environment:
ΔG = ΔH – T · ΔS
- ΔG = Free Energy
- ΔS = Entropy (degree of disorder)
- ΔH = Enthalpy
Based on the value of ΔG, reactions are classified:
- ΔG < 0: Spontaneous reaction, energetically favorable. The reaction products contain less energy than the reactants. These are called exergonic reactions.
- ΔG > 0: Non-spontaneous reaction, energetically unfavorable. The products have more energy than the reactants. This is an endergonic reaction.
- ΔG = 0: The system is in equilibrium. The reaction proceeds in both directions. No energy is absorbed or released, and no work is done.
Energy Coupling in Metabolic Reactions
Energy coupling is the property where the energy released in a thermodynamically favorable (exergonic) reaction can be used to drive other reactions that are energetically unfavorable (endergonic).
The energy required for endergonic processes within the cell is typically obtained from another highly exergonic reaction, which is the hydrolysis of ATP (Adenosine Triphosphate).
In general, for every endergonic reaction that occurs in the cell, there is another exergonic reaction that serves as the energy source.
ATP stores large amounts of energy for a short time, as it cannot be accumulated indefinitely. The process involving the transfer of a phosphate group from ATP to another compound is called dephosphorylation.
Fundamentals of Metabolism
Defining Metabolic Pathways
A metabolic pathway is a process consisting of a chain of successive enzymatic reactions. Each of the substances involved is a metabolite.
Metabolism is broadly categorized into three types:
- Catabolism: The oxidative degradation of complex molecules, which produces energy.
- Anabolism: The synthesis of complex molecules, which requires energy and is made possible by catabolism.
- Amphibolism: Refers to metabolic processes in which metabolites are oxidized and large amounts of energy are stored, which is subsequently used in anabolism.
Anabolic processes (which are strongly endergonic) require an external energy source, as the energy cannot originate solely from the life form itself; its source comes from the environment.
Catabolic and amphibolic processes demonstrate the release of free energy.
Key Molecules in Metabolism
The following molecules are centrally involved in metabolic processes:
- Metabolites: Molecules entering the different routes of metabolism. They may be derived from catabolism or anabolism. The most important example is glucose.
- Nucleotides (Cofactors): These molecules (such as NAD+, NADP+, FAD, etc.) function to allow the oxidation or reduction of metabolites.
- High-Energy Molecules: Molecules like GTP, ATP, and Coenzyme A (which can also act as a metabolite) store and act as energy providers depending on need.
- Environmental Molecules: Molecules such as oxygen, water, and carbon dioxide (which also function as metabolites).
Performance and Energy Balance of Metabolism
The cell obtains energy primarily from oxidative degradation.
Chemical energy is the only form that living beings can utilize. This utilization is achieved through energy coupling or by storing energy in the form of high-energy bonds.
A parameter that measures the amount of energy exchanged is the energy balance. If the pathway is strictly anabolic, the balance will be negative; if catabolic, it will be positive.