2.1 Laws of thermodynamics in biological context

Thermodynamics in biology is all about energy flow and transformations. The laws of thermodynamics explain how energy moves through living systems, from photosynthesis to metabolism. They're key to understanding how organisms maintain order and function.

These laws govern everything from chemical reactions to ecosystem dynamics. We'll explore how energy is conserved, entropy increases, and how organisms use energy to stay alive. Understanding these principles is crucial for grasping how life works at a fundamental level.

Energy conservation in biological systems

First law of thermodynamics in biological systems

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• The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another
• In biological systems, the total energy of a system remains constant
• Energy transformations in biological systems follow the first law of thermodynamics
  • During photosynthesis, light energy is converted into chemical energy stored in glucose molecules
  • During cellular respiration, the chemical energy in glucose is converted into ATP, which is then used for various cellular processes

Energy types and balance in biological systems

• Energy in biological systems can be classified as either potential energy (stored energy) or kinetic energy (energy of motion)
  • Examples of potential energy include chemical bonds in molecules (glucose, ATP)
  • Examples of kinetic energy include the movement of molecules or organisms (muscle contraction, blood flow)
• The energy balance of an organism can be analyzed using the first law of thermodynamics
  • The energy input (from food or light) must equal the energy output (heat, work, and stored energy) for an organism to maintain homeostasis
• The efficiency of energy transformations in biological systems is limited by the second law of thermodynamics, as some energy is always lost as heat during these processes

Thermodynamics and biological processes

Second law of thermodynamics in biological systems

• The second law of thermodynamics states that the total entropy of an isolated system always increases over time
  • Entropy is a measure of the disorder or randomness of a system
• In biological systems, the second law of thermodynamics implies that spontaneous processes always lead to an increase in entropy
  • Biological processes, such as chemical reactions or the growth and development of organisms, are driven by an increase in entropy
• The second law of thermodynamics also dictates that energy transformations in biological systems are never 100% efficient
  • Some energy is always lost as heat during these processes, contributing to an increase in entropy

Maintaining order in living organisms

• The maintenance of highly ordered structures in living organisms, such as proteins and cellular organelles, requires a constant input of energy to counteract the natural tendency towards increasing entropy
• The second law of thermodynamics has implications for the evolution of biological systems
  • The increase in complexity and organization observed in living organisms over time is possible because the Earth is an open system that receives energy from the Sun, allowing for localized decreases in entropy

Entropy in biological systems

Entropy and molecular disorder

• Entropy is a measure of the disorder or randomness of a system
  • In thermodynamics, entropy is often described as the amount of thermal energy that is unavailable for useful work
• In biological systems, entropy can be thought of as the degree of disorder or randomness at the molecular level
  • A highly ordered protein structure has low entropy, while a denatured protein has high entropy
• The second law of thermodynamics dictates that the total entropy of an isolated system always increases over time
  • Biological processes, such as chemical reactions or the growth and development of organisms, are driven by an increase in entropy

Entropy, energy, and irreversibility in biological processes

• Living organisms are able to maintain a state of low entropy (high order) by constantly inputting energy from their surroundings
  • This energy is used to perform work, such as synthesizing complex molecules or maintaining concentration gradients across membranes
• The concept of entropy is closely related to the idea of irreversibility in biological processes
  • Many biological reactions, such as the hydrolysis of ATP or the diffusion of molecules across a membrane, are thermodynamically irreversible, meaning they proceed spontaneously in one direction and result in an increase in entropy
• The relationship between entropy and information content in biological systems has been explored in the field of bioinformatics
  • The information content of a DNA sequence can be quantified using entropy-based measures, which can provide insights into the complexity and organization of genomes

Enthalpy, Entropy, and Gibbs free energy

Defining enthalpy, entropy, and Gibbs free energy

• Enthalpy (H) is a measure of the total heat content of a system
  • In biological reactions, changes in enthalpy (ΔH) reflect the amount of heat absorbed or released during the reaction
• Entropy (S) is a measure of the disorder or randomness of a system
  • In biological reactions, changes in entropy (ΔS) reflect the change in the degree of disorder or randomness of the system
• Gibbs free energy (G) is a thermodynamic quantity that combines the effects of enthalpy and entropy to determine the spontaneity of a reaction
  • The change in Gibbs free energy (ΔG) for a reaction is given by the equation: $ΔG = ΔH - TΔS$, where T is the absolute temperature

Gibbs free energy and spontaneity of biological reactions

• For a biological reaction to occur spontaneously, the change in Gibbs free energy must be negative (ΔG < 0)
  • This means that the reaction must either release heat (exothermic, ΔH < 0) or result in an increase in entropy (ΔS > 0), or both
• Exergonic reactions are those that have a negative ΔG and release energy, such as the hydrolysis of ATP
• Endergonic reactions are those that have a positive ΔG and require an input of energy, such as the synthesis of glucose during photosynthesis
• The relationship between enthalpy, entropy, and Gibbs free energy can be used to predict the direction and spontaneity of biological reactions
  • The coupling of an exergonic reaction (such as ATP hydrolysis) with an endergonic reaction (such as the synthesis of a complex molecule) can drive the overall process forward
• The concept of Gibbs free energy is also important in understanding the role of enzymes in biological reactions
  • Enzymes lower the activation energy of a reaction, making it more likely to occur spontaneously by reducing the ΔG of the transition state