Thermodynamics in Biochemistry

Open vs. Closed Systems

Biochemical reactions occur in systems that can be classified as open or closed. Understanding the distinction is essential for analyzing energy flow.

• Open system: Can exchange both energy and matter with its surroundings (e.g., living cells).

• Closed system: Exchanges energy but not matter with its surroundings.

Free Energy, Enthalpy, and Entropy

The spontaneity and favorability of biochemical reactions are determined by changes in free energy (), enthalpy (), and entropy (). The relationship is given by the Gibbs free energy equation:

• Gibbs Free Energy Equation:

• Enthalpy (): Heat content of a system; reflects the energy absorbed or released.

• Entropy (): Measure of disorder or randomness; higher entropy favors spontaneity.

• Temperature (): Absolute temperature in Kelvin.

First and Second Laws of Thermodynamics

Thermodynamics governs energy changes in biochemical systems:

• First Law: Energy cannot be created or destroyed; energy lost by the system is gained by the surroundings.

• Second Law: The entropy of an isolated system tends to increase; reactions are favorable if they increase overall disorder.

Types of Biochemical Processes

Biochemical reactions can be classified based on their enthalpy and entropy changes:

• Enthalpy-driven processes: is negative (exothermic), is typically negative or small.

• Entropy-driven processes: is positive (increased disorder), may be positive or negative.

• Combined enthalpy and entropy-driven processes: Both and contribute to favorability.

Examples: Protein folding (often enthalpy-driven), micelle formation (entropy-driven).

Free Energy Change and Reaction Favorability

The sign and magnitude of determine whether a reaction is spontaneous, at equilibrium, or non-spontaneous:

Coupled Reactions in Biochemistry

Unfavorable reactions () can proceed in cells by coupling them to favorable reactions (such as ATP hydrolysis). This is a key strategy in metabolism.

• Coupled reaction: The energy released from a favorable reaction is used to drive an unfavorable one.

• Example: Synthesis of glutamine from glutamate and ammonia is coupled to ATP hydrolysis.

ATP as a Universal Energy Coupler

ATP (adenosine triphosphate) hydrolysis is a central process in cellular energy transfer. The breakdown of ATP releases energy that can be used to drive many cellular reactions.

• ATP hydrolysis:

• Role: Couples energy-releasing and energy-consuming reactions.

• Application: Muscle contraction, active transport, biosynthesis.

Substrate-Level Phosphorylation and Phosphate Group Transfer

Some reactions involve the transfer of phosphate groups from high-energy substrates to ADP, forming ATP.

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated intermediate.

• High-energy phosphate compounds: Molecules with unstable phosphate bonds (e.g., phosphoenolpyruvate, creatine phosphate).

• Thermodynamic basis: Hydrolysis of these compounds releases energy that can be "coupled" to drive unfavorable reactions.

Example: Phosphoenolpyruvate hydrolysis in glycolysis.

Summary Table: Types of Biochemical Reactions

Additional info:

• Substrate-level phosphorylation is a key mechanism in glycolysis and the citric acid cycle.

• ATP hydrolysis is often used to "drive" reactions that would otherwise be thermodynamically unfavorable.

• Phosphate group transfer potential is determined by the stability of products after hydrolysis.