Free energy, enthalpy, and entropy are key players in biological processes. They determine how energy flows and changes in living systems, influencing everything from chemical reactions to cellular organization.

Understanding these concepts is crucial for grasping how life maintains order in a universe tending towards disorder. We'll explore how these thermodynamic principles shape biological functions and drive the complex machinery of life.

Free energy, enthalpy, and entropy

Defining thermodynamic quantities in biological systems

• Free energy is the energy in a system available to perform work
  • In biological systems, free energy is often found in high-energy phosphate bonds (ATP)
• Enthalpy is the total heat content of a system
  • In biological systems, enthalpy changes are often associated with chemical reactions involving the breaking and forming of chemical bonds
• Entropy is a measure of the disorder or randomness of a system
  • In biological systems, entropy tends to increase over time as ordered structures break down and energy is dissipated
• The second law of thermodynamics states that the total entropy of an isolated system always increases over time, which has important implications for the organization and function of living systems

Relating thermodynamic quantities

• The Gibbs free energy equation relates changes in free energy to changes in enthalpy and entropy: $ΔG = ΔH - TΔS$
  • $ΔG$ is the change in free energy
  • $ΔH$ is the change in enthalpy
  • $T$ is the absolute temperature
  • $ΔS$ is the change in entropy
• The relationship between the equilibrium constant ($K$) and the standard free energy change ($ΔG°$) is given by the equation: $ΔG° = -RT \ln K$
  • $R$ is the gas constant
  • $T$ is the absolute temperature
  • This equation can be used to calculate $ΔG°$ from experimentally determined equilibrium constants

Changes in thermodynamic quantities

Calculating changes in free energy, enthalpy, and entropy

• Changes in free energy ($ΔG$) can be calculated using the Gibbs free energy equation: $ΔG = ΔH - TΔS$
• The change in enthalpy ($ΔH$) can be determined by measuring the heat absorbed or released during a reaction at constant pressure
  • In biological systems, enthalpy changes often result from the breaking and forming of chemical bonds
• The change in entropy ($ΔS$) can be calculated using the equation: $ΔS = q/T$
  • $q$ is the heat absorbed or released by the system
  • $T$ is the absolute temperature
  • Entropy changes are often associated with changes in the degree of disorder or randomness in a system

Standard conditions and equilibrium constants

• Standard free energy changes ($ΔG°$) can be calculated using standard enthalpy changes ($ΔH°$) and standard entropy changes ($ΔS°$) under standard conditions
  • Standard conditions are defined as 1 atm pressure, 298 K temperature, and 1 M concentrations of reactants and products
• The relationship between the equilibrium constant ($K$) and the standard free energy change ($ΔG°$) is given by the equation: $ΔG° = -RT \ln K$
  • This equation can be used to calculate $ΔG°$ from experimentally determined equilibrium constants

Spontaneity of biological processes

Role of free energy in determining spontaneity

• The spontaneity of a biological process is determined by the change in free energy ($ΔG$) associated with that process
  • A negative $ΔG$ indicates a spontaneous process, while a positive $ΔG$ indicates a non-spontaneous process
• Exergonic reactions release free energy (negative $ΔG$) and are spontaneous
  • Many catabolic reactions, such as the breakdown of glucose during cellular respiration, are exergonic
• Endergonic reactions require an input of free energy (positive $ΔG$) and are non-spontaneous
  • Many anabolic reactions, such as the synthesis of complex molecules (proteins, nucleic acids), are endergonic

Coupled reactions and concentration dependence

• Coupled reactions involve linking an endergonic reaction with an exergonic reaction
  • The free energy released by the exergonic reaction drives the endergonic reaction
  • ATP hydrolysis is often coupled to endergonic reactions to make them spontaneous
• The free energy change of a reaction depends on the concentrations of reactants and products
  • As a reaction proceeds, the concentrations of reactants decrease and the concentrations of products increase, affecting the spontaneity of the reaction

Entropy and organization of life

Maintaining organization in living systems

• Living systems are highly organized and maintain a state of low entropy compared to their surroundings
  • This organization is maintained through the constant input of free energy and the dissipation of entropy into the environment
• The second law of thermodynamics states that the total entropy of an isolated system always increases over time
  • Living systems are open systems that exchange matter and energy with their surroundings, allowing them to maintain a state of low entropy
• The organization of living systems is maintained through the coupling of endergonic processes (synthesis of complex molecules) with exergonic processes (breakdown of nutrients)
  • This coupling allows living systems to use free energy to create and maintain ordered structures

Cellular structures and information content

• The formation of membranes and compartments within cells helps to maintain the organization of living systems
  • Membranes separate regions of different composition and allow for the selective transport of molecules
• The information content of living systems, stored in the form of genetic material (DNA and RNA), represents a highly ordered state with low entropy
  • The replication and transmission of this genetic information across generations helps to maintain the organization of living systems over time

Evolution and the second law of thermodynamics

• The process of evolution by natural selection can be seen as a means by which living systems maintain their organization in the face of the constant increase in entropy required by the second law of thermodynamics
  • Through the differential survival and reproduction of individuals with favorable traits, living systems adapt to changing environments
  • This adaptation allows living systems to maintain their organization over evolutionary time scales