Energy, Thermodynamics, and Metabolism in Biological Systems

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Energy in Chemical Reactions

Forms of Energy in Biological Systems

Energy is fundamental to all chemical reactions, including those in living organisms. The First Law of Thermodynamics states that energy cannot be created or destroyed, only transferred or transformed.

Kinetic Energy: The energy of motion. Atoms and molecules possess kinetic energy due to their movement. More movement means greater kinetic energy.
- Kinetic energy can be transferred from one object to another as heat (thermal energy) or light (radiation).
Potential Energy: The energy stored in bonds between atoms of a molecule. This energy is available for release during chemical reactions (when bonds are broken).
- Small, stable molecules have low potential energy. Breaking these molecules releases a small amount of energy.
- Large, unstable molecules have high potential energy. Breaking these molecules releases a large amount of energy.

Energy Changes in Chemical Reactions

Chemical reactions involve the making and breaking of bonds, which requires or releases energy.

Breaking molecular bonds requires energy input.
Forming molecular bonds releases energy.
Example: Hydrogen gas (H2) and oxygen gas (O2) are stable molecules; breaking their bonds requires energy. Forming water (H2O) from these gases releases energy.

Metabolism

Definition and Pathways

Metabolism refers to all chemical reactions in a biological organism. These reactions are classified as either catabolic or anabolic pathways.

Catabolic Pathways: Break down molecules and release energy (exergonic reactions).
- Energy is required to break the molecular bonds of reactants, but more energy is released in the formation of products.
- Products are more stable than reactants.
Anabolic Pathways: Build complex molecules from simpler ones and require energy input (endergonic reactions).
- It takes energy to break the molecular bonds of reactants, and less energy is released when new products are formed.
- Products are less stable than reactants; overall, energy must be put into the reaction for it to occur.

The Second Law of Thermodynamics and Entropy

Entropy and Energy Transformations

The Second Law of Thermodynamics states that when energy is transferred or transformed, there is a net increase in the entropy (disorder) of the universe.

During a chemical reaction, some potential energy is lost as heat or light, increasing entropy.
Some molecular bonds become unstable, further increasing disorder.

Defining Entropy

Entropy is a measure of randomness or disorder.
Entropy increases as matter moves from solid → liquid → gas (higher temperature, more disordered states).
The system is the organism or object being studied (e.g., a human body, plant, or cell).
The surroundings are everything else outside the system (e.g., air, water, other organisms).
The universe includes both the system and its surroundings.

If the entropy of a system decreases, the entropy of the surroundings must increase to a greater extent, ensuring a net increase in the universe's entropy.

Example: After exercise, your body cools by sweating and evaporation, which increases the entropy of your surroundings more than the decrease in your body's entropy.

Spontaneous and Non-Spontaneous Reactions

Spontaneous Reactions

Spontaneous reactions release energy (exergonic) and increase the entropy of the system.
Reactants start at a high Gibbs free energy state; products are more stable and at a lower Gibbs free energy state.
The change in Gibbs free energy () for a spontaneous reaction is negative.

Non-Spontaneous Reactions

Non-spontaneous reactions require energy input (endergonic) and decrease the entropy of the system.
Reactants start at a low Gibbs free energy state; products are less stable and at a higher Gibbs free energy state.
The change in Gibbs free energy () for a non-spontaneous reaction is positive.

Gibbs Free Energy Equation

The change in Gibbs free energy is calculated as:

Where is the change in enthalpy (heat content), is temperature in Kelvin, and is the change in entropy.

ATP and Coupled Reactions

ATP Hydrolysis and Energy Coupling

Organisms metabolize to drive essential chemical reactions. Some reactions are non-spontaneous and require energy input, which is often supplied by ATP hydrolysis.

When ATP is hydrolyzed to ADP, it releases energy (about 7.3 kcal/mol).
This energy can be used to drive non-spontaneous reactions, making them spontaneous when coupled.

Is the reaction spontaneous?	ATP coupling needed?
Energy required ≤ 7.3 kcal/mol	No
Energy required > 7.3 kcal/mol	Yes (hydrolysis of more than one ATP molecule needed)

Enzymes and Metabolic Reactions

Role of Enzymes

Enzymes are biological catalysts that increase the rate of metabolic reactions by lowering the activation energy required.

Enzymes facilitate the breaking and forming of chemical bonds.
They do not change the overall energy released or required by the reaction.
Enzymes are specific to their substrates and require optimal pH and temperature to function.

Enzyme Inhibition and Cofactors

Cofactors: Non-protein molecules (often metal ions or organic molecules) required for enzyme activity.
Competitive Inhibitors: Bind to the active site, blocking substrate binding.
Noncompetitive Inhibitors: Bind elsewhere, changing the enzyme's shape and preventing substrate binding.

Summary Table: Spontaneous vs. Non-Spontaneous Reactions

Type of Reaction	Energy Change	Entropy Change		Example
Spontaneous (Exergonic)	Releases energy	Increases	Negative	Cellular respiration
Non-Spontaneous (Endergonic)	Requires energy input	Decreases	Positive	Protein synthesis

Key Terms

Thermodynamics: The study of energy transformations.
Entropy (S): A measure of disorder or randomness.
Gibbs Free Energy (G): The energy available to do work in a system.
ATP (Adenosine Triphosphate): The primary energy carrier in cells.
Enzyme: A protein that catalyzes chemical reactions.