Cellular Metabolism: Energy, Reactions, and Processes
Free Energy
Free energy is the thermodynamic quantity used to study biological processes and predict whether they are energetically favorable. It represents the energy capable of performing useful work under constant temperature and pressure. The sign of ΔG indicates the reaction’s behavior:
- ΔG < 0: The reaction is exergonic (energetically favorable), releasing free energy.
- ΔG > 0: The reaction is endergonic (energetically unfavorable), requiring free energy absorption.
- ΔG = 0: The system is in equilibrium, with reactions proceeding equally in both directions.
Energy Coupling
Energy released from an exergonic reaction can drive endergonic reactions. This is energy coupling. Cells often use ATP hydrolysis, an exergonic reaction, to power endergonic processes, facilitated by specific enzymes. The overall process remains energetically favorable.
Metabolism Overview
Metabolism encompasses all chemical transformations and energy processes in living organisms. Metabolic pathways are chains of enzymatic reactions. Catabolism involves oxidative degradation, producing energy. Anabolism is the energy-requiring synthesis of molecules, powered by catabolism. Amphibolic pathways can be both catabolic and anabolic. Pre-existing biomolecules are constantly challenged, metabolites are oxidized, storing energy for anabolism. The primary energy source comes from the environment.
Molecules in Metabolism
- Metabolites: Molecules entering metabolic pathways (e.g., glucose, fatty acids).
- Nucleotides: Molecules enabling metabolite oxidation or reduction.
- High-Energy Link Molecules: Molecules with energy-rich phosphate bonds (e.g., GTP, ATP), storing and releasing chemical energy.
- Environmental Molecules: Molecules at the beginning or end of metabolic processes, from or transferred to the cellular environment.
Metabolic Yield and Energy Balance
The energy released in an exergonic process depends on the energy difference between initial and final states. Chemical energy is usable by living beings through direct coupling or storage in high-energy bonds. Energy balance measures the high-energy link molecules produced per oxidized metabolite.
- Positive Energy Balance: ATP molecules formed per oxidized metabolite (catabolic pathways).
- Negative Energy Balance: ATP molecules hydrolyzed for biosynthesis (anabolic pathways).
Energy efficiency is the percentage of stored energy versus total energy released, as some energy is lost as heat.
Redox Reactions
Energy-releasing molecular transformations in catabolism are oxidation reactions, involving electron transfer. Oxidation is accompanied by hydrogen atom loss. Electron transfer follows a precise order determined by the reduction potential of each redox couple (two molecular species involved in an oxidation-reduction reaction).
Aerobic Catabolism
Aerobic catabolism involves pathways producing ATP. Glucose and fatty acids are degraded via glycolysis and beta-oxidation, respectively. Proteins are broken down into amino acids. All enter the Krebs cycle and respiratory chain, ultimately generating ATP. Some energy is dissipated as heat.