Thermodynamics

Introduction to Thermodynamics in Biological Systems

  • Biology is defined as the process of carefully regulated making-and-breaking of chemical bonds.

  • The field of biology is inherently connected to the laws of chemistry.

  • The focus of discussion is on the laws of thermodynamics, particularly their application in biological systems.

Overview of ATP

  • Adenosine Triphosphate (ATP) is a critical molecule in biological systems.

    • Structure: Comprises adenine (a sugar) attached to three phosphates.

    • Characteristics: Has a strong negative charge due to the phosphates.

Laws of Thermodynamics

  • Thermodynamics deals with energy and its transformations within the universe.

    • Important distinction: A single organism or cell is regarded as a closed system that exists within the larger universe.

First Law of Thermodynamics

  • Statement: Energy cannot be created or destroyed; it can only be converted from one form to another.

    • Example: Light energy from the sun converts into:

    • Chemical energy (e.g., through photosynthesis in plants).

    • Subsequently, this energy can transform into:

      • Heat energy.

      • Mechanical energy.

      • Electrical energy.

Second Law of Thermodynamics

  • Statement: Spontaneous reactions progress from reactants that possess high energy to products that exhibit lower energy.

    • Concept of Spontaneity: Occurs without the necessity for additional energy input.

    • Significance for biologists: Determines whether cellular reactions need extra energy.

Entropy

  • Definition: Entropy is often described as a measure of disorder or randomness. More precisely, it represents the amount of energy in a system that is no longer available to perform work.

    • Example: After exercising, the body releases heat, which indicates that the energy can no longer be harnessed for doing mechanical work.

  • Key Insight: The entropy of the universe is continually increasing, which means a cell's internal entropy can decrease as long as the overall universe's entropy increases.

Gibb's Free Energy

  • Definition: Gibb's free energy (ΔG) combines concepts from both first and second laws of thermodynamics.

    • It is influenced by:

    • Change in enthalpy (ΔH)

    • Absolute temperature (T in Kelvin)

    • Change in entropy (ΔS)

  • Spontaneity of Reactions:

    • Reactions with a negative ΔG (ΔG < 0) are classified as exergonic.

    • These reactions release energy to the surroundings, making them favorable for the universe.

    • Reactions with a positive ΔG (ΔG > 0) are classified as endergonic.

    • Such reactions require energy to be supplied from the universe to proceed.

Energy Storage and Usage in Biological Systems

  • Energy in Biological Context: Often, energy is spoken of in terms of ATP in biological systems.

  • Important Clarification: Although biologists may say a reaction "uses energy", they are referring to the potential energy stored in chemical bonds of molecules like ATP.

  • Important Principle: Energy must always be tracked from its source to its final use in any biological reaction.

Pathway of Energy from the Sun to Cellular Processes

  • Source of Earth's Energy: The sun through chemical fusion, producing heat and light.

    • Plants utilize this energy form through photosynthesis, converting it into chemical energy (sugars).

  • Examples of Energy Transformation:

    • Consuming fructose (like in strawberries) translates sugars to energy through glycolysis and cellular respiration, yielding ATP.

Structure and Energy Dynamics of ATP

  • Stability of ATP: ATP is a relatively stable molecule, yet exhibits unique energy properties due to its structure:

    • Contains four highly negatively charged groups that repel each other but are held together by covalent bonds.

    • Thus, ATP is categorized as a high-energy molecule.

  • Hydrolysis of ATP:

    • Breaking down ATP results in ADP and inorganic phosphate.

    • Outcomes of ATP breakdown:

    • Reduces charge repulsion, resulting in more stable, lower-energy molecules.

    • Is considered exergonic due to the increase in entropy from one molecule splitting into two.

Work Utilization of ATP

  • Cells can harness energy from ATP breakdown to execute work such as:

    • Pumping ions across cellular membranes (converting chemical energy to electrical energy).

    • Contracting muscle fibers (converting chemical energy to mechanical energy).

    • Transferring phosphate groups to other molecules (converting chemical energy to facilitate other chemical transformations).

Reaction Coupling

  • Concept of Coupling Reactions: Linking unfavorable reactions with favorable reactions using ATP energy.

    • Example: Converting glucose to glucose-6-phosphate is an endergonic reaction that does not occur spontaneously. However:

    • An enzyme facilitates this conversion while simultaneously breaking down ATP, coupling the energetics.

    • As a net result, the overall free energy change (ΔG) remains negative, allowing the reaction to proceed.

Equilibrium Considerations in Cells

  • Reactions at Equilibrium: Chemists study reactions at equilibrium, but cellular equilibrium implies metabolic activity cannot continue:

    • Important to recognize that a cell in equilibrium is functionally dead as it can no longer perform work.

    • Maintaining concentration gradients is critical for the functioning of cells and metabolism.

    • Discussion to be expanded in the upcoming class session.