DG in biology refers to the change in Gibbs free energy (ΔG), representing the amount of energy available to do work in a biological reaction at constant temperature and pressure.
The Gibbs free energy (G), also known as the Gibbs function, Gibbs energy, or free enthalpy, combines enthalpy and entropy to determine the spontaneity of a chemical reaction or process. The change in Gibbs free energy (ΔG) predicts whether a reaction will occur spontaneously (without needing additional energy input) under specific conditions.
Here's a breakdown of key aspects related to ΔG in biology:
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Definition: ΔG = ΔH - TΔS
- ΔG: Change in Gibbs free energy
- ΔH: Change in enthalpy (heat content)
- T: Absolute temperature (in Kelvin)
- ΔS: Change in entropy (disorder)
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Spontaneity of Reactions:
- ΔG < 0 (Negative): The reaction is spontaneous (exergonic) and releases energy. This means the reaction can occur without the input of energy.
- ΔG > 0 (Positive): The reaction is non-spontaneous (endergonic) and requires energy input to proceed.
- ΔG = 0: The reaction is at equilibrium.
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Biological Significance:
- Coupled Reactions: Cells often couple exergonic reactions (like ATP hydrolysis) with endergonic reactions to drive energetically unfavorable processes. For example, the breakdown of ATP (ΔG < 0) can provide the energy needed for muscle contraction (ΔG > 0).
- Metabolic Pathways: ΔG is crucial for understanding the direction and regulation of metabolic pathways. Enzymes catalyze reactions by lowering the activation energy, but they don't change the overall ΔG of the reaction.
- Enzyme Function: Enzymes facilitate reactions but do not alter the overall change in Gibbs Free Energy. They lower the activation energy (the energy required to start the reaction) but the difference between the free energy of reactants and products remains the same.
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Examples:
- Cellular Respiration: The breakdown of glucose to produce ATP is an exergonic process (ΔG < 0), releasing energy that the cell can use.
- Photosynthesis: The synthesis of glucose from carbon dioxide and water is an endergonic process (ΔG > 0), requiring energy input from sunlight.
| Property | Definition | Sign Convention | Implications |
|---|---|---|---|
| Gibbs Free Energy (G) | Energy available to do work in a system at constant T and P | N/A | Represents the potential energy of a system. A lower G indicates a more stable system. |
| Change in G (ΔG) | Difference in Gibbs free energy between products and reactants | Negative | Indicates a spontaneous reaction (exergonic). The reaction releases energy and proceeds without external energy input. |
| Change in G (ΔG) | Difference in Gibbs free energy between products and reactants | Positive | Indicates a non-spontaneous reaction (endergonic). The reaction requires energy input to proceed. |
| Change in G (ΔG) | Difference in Gibbs free energy between products and reactants | Zero | Indicates that the reaction is at equilibrium. The rates of the forward and reverse reactions are equal, and there is no net change in the concentrations of reactants and products. |
In summary, DG biology is the application of thermodynamics, specifically the concept of Gibbs free energy, to understand and predict the spontaneity and energy requirements of biological reactions and processes. It's fundamental to understanding how cells manage energy and carry out life processes.
