2.3 Equilibrium and non-equilibrium processes

Equilibrium and non-equilibrium processes are key concepts in thermodynamics. In biological systems, equilibrium processes involve balanced states, while non-equilibrium processes drive life's dynamic functions. Understanding these processes is crucial for grasping how organisms maintain order and function.

Thermodynamics in biological systems encompasses both equilibrium and non-equilibrium processes. The laws of thermodynamics govern energy transformations, while concepts like steady state and open systems help explain how organisms maintain stability. These principles are essential for understanding life's complex energy dynamics.

Equilibrium vs Non-equilibrium Processes

Characteristics and Differences

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• Equilibrium processes occur when a system is in a state of balance, with no net change in the concentrations of reactants and products over time. Non-equilibrium processes are characterized by a net flow of energy or matter, resulting in changes in the concentrations of reactants and products
• Equilibrium processes are often associated with closed systems, where there is no exchange of matter or energy with the surroundings (isolated systems). Non-equilibrium processes are more common in living organisms, which are open systems that constantly exchange matter and energy with their environment

Examples in Biological Systems

• Equilibrium processes in biological systems include:
  • Binding of oxygen to hemoglobin in the blood
  • Dissociation of weak acids and bases in aqueous solutions (carbonic acid, bicarbonate buffer)
• Non-equilibrium processes in biological systems include:
  • Active transport of ions across cell membranes (sodium-potassium pump)
  • Synthesis of ATP through oxidative phosphorylation
  • Propagation of nerve impulses

Thermodynamic Laws and Energy

• The laws of thermodynamics govern both equilibrium and non-equilibrium processes in biological systems
  • First law of thermodynamics states that energy cannot be created or destroyed
  • Second law of thermodynamics states that the entropy of the universe always increases
• Non-equilibrium processes in living organisms are driven by the input of free energy, which is used to maintain the highly ordered structures and functions of life

Factors Influencing Equilibrium

Equilibrium Constant and Gibbs Free Energy

• The equilibrium state of a biological reaction is determined by the relative concentrations of reactants and products, as well as the equilibrium constant (K) of the reaction
• The equilibrium constant is a measure of the ratio of the concentrations of products to reactants at equilibrium and is determined by the Gibbs free energy change (ΔG) of the reaction

Temperature and pH Effects

• Temperature affects the equilibrium state of biological reactions by influencing the rates of the forward and reverse reactions
  • Increasing the temperature shifts the equilibrium in the direction of the endothermic reaction
  • Decreasing the temperature shifts the equilibrium in the direction of the exothermic reaction
• pH can influence the equilibrium state of biological reactions, particularly those involving weak acids and bases
  • Changes in pH can alter the ionization state of the reactants and products, shifting the equilibrium in favor of the side with the lower concentration of ions

Enzyme Catalysis and Le Chatelier's Principle

• The presence of enzymes can affect the equilibrium state of biological reactions by lowering the activation energy and increasing the rate of the reaction. However, enzymes do not alter the equilibrium constant or the overall ΔG of the reaction
• The concentration of reactants and products can be altered by the addition or removal of substances from the system, which can shift the equilibrium in accordance with Le Chatelier's principle
  • This principle states that a system at equilibrium will respond to a disturbance by shifting the equilibrium in the direction that minimizes the effect of the disturbance

Steady State in Biological Systems

Definition and Importance

• A steady state is a condition in which a system maintains a constant state over time, despite the continuous flow of energy or matter through the system. In a steady state, the rates of input and output are equal, resulting in no net change in the system's composition or properties
• Steady states are crucial for the maintenance of non-equilibrium processes in biological systems, as they allow organisms to maintain a stable internal environment (homeostasis) in the face of constantly changing external conditions

Open Systems and Feedback Mechanisms

• The concept of steady state is closely related to the idea of open systems, which exchange matter and energy with their surroundings. Living organisms are open systems that maintain a steady state by continuously importing nutrients and exporting waste products
• Steady states in biological systems are maintained by a complex network of feedback mechanisms that regulate the rates of input and output
  • Feedback mechanisms can be positive (amplifying changes) or negative (counteracting changes)
  • Feedback mechanisms are essential for the precise control of non-equilibrium processes

Examples of Steady States

• Maintenance of constant body temperature in endothermic animals (thermoregulation)
• Regulation of blood glucose levels by insulin and glucagon (glucose homeostasis)
• Balance between the synthesis and degradation of proteins in cells (protein turnover)

Thermodynamics in Biological Systems

Laws of Thermodynamics

• The laws of thermodynamics provide a framework for understanding the energy transformations that occur in biological systems
  • First law of thermodynamics states that energy cannot be created or destroyed
  • Second law of thermodynamics states that the entropy of the universe always increases

Equilibrium Thermodynamics

• Equilibrium thermodynamics can be applied to biological systems to determine the direction and extent of chemical reactions
  • The Gibbs free energy change (ΔG) of a reaction determines whether the reaction is spontaneous (ΔG < 0) or non-spontaneous (ΔG > 0) under standard conditions
  • The equilibrium constant (K) of a reaction is related to the ΔG by the equation $ΔG = -RT \ln K$, where R is the gas constant and T is the absolute temperature

Non-equilibrium Thermodynamics

• Non-equilibrium thermodynamics is particularly relevant to biological systems, as living organisms are open systems that constantly exchange matter and energy with their surroundings
  • The maintenance of non-equilibrium states in biological systems requires the continuous input of free energy, which is used to perform work and maintain the highly ordered structures and functions of life
• The concept of entropy is central to non-equilibrium thermodynamics in biological systems
  • Living organisms maintain a low-entropy state by continuously exporting entropy to their surroundings in the form of heat and waste products
  • This process requires the input of free energy, which is obtained through the breakdown of nutrient molecules such as glucose

Applications and Limitations

• The principles of non-equilibrium thermodynamics can be applied to understand the energy transformations that occur in biological processes such as:
  • Metabolism (glycolysis, citric acid cycle)
  • Photosynthesis (light-dependent and light-independent reactions)
  • Muscle contraction (sliding filament mechanism)
• These processes involve the coupling of exergonic (energy-releasing) and endergonic (energy-requiring) reactions, which allows living organisms to perform work and maintain their highly ordered structures and functions
• The efficiency of energy transformations in biological systems is limited by the second law of thermodynamics, which states that some energy is always lost as heat during any spontaneous process
  • The efficiency of biological processes such as ATP synthesis and muscle contraction is typically around 30-40%, reflecting the inherent limitations imposed by the second law