Metabolism and the Laws of Thermodynamics

Overview of Metabolism

Metabolism refers to the totality of an organism’s chemical reactions. These reactions are organized into metabolic pathways, where a specific molecule is altered in a series of steps to produce a final product. Each step is catalyzed by a specific enzyme.

• Catabolic pathways: Break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolic pathways: Build complex molecules from simpler ones, consuming energy (e.g., protein synthesis).

• Metabolic pathway example: A starting molecule (A) is converted to product (D) through intermediates (B, C) by enzymes 1, 2, and 3.

Forms of Energy in Biological Systems

Types of Energy

Energy is the capacity to cause change. In biological systems, energy exists in various forms:

• Kinetic energy: Energy of motion.

• Thermal energy (Heat): Kinetic energy associated with the random movement of atoms or molecules.

• Light energy: Energy in the form of electromagnetic radiation, used in photosynthesis.

• Potential energy: Stored energy due to position or structure.

• Chemical energy: Potential energy available for release in a chemical reaction (e.g., energy stored in glucose).

Energy can be converted from one form to another, such as chemical energy in food being converted to kinetic energy for movement.

The Laws of Thermodynamics

Thermodynamics in Biology

Thermodynamics is the study of energy transformations. Biological systems can be described as:

• Isolated (closed) systems: Do not exchange energy or matter with surroundings.

• Open systems: Exchange energy and matter with surroundings. Organisms are open systems because they constantly exchange energy and matter with their environment.

The First Law of Thermodynamics

The first law of thermodynamics (law of energy conservation) states:

• Energy can be transferred and transformed, but it cannot be created or destroyed.

• Example: Chemical energy in food is converted to kinetic energy and heat in animals.

The Second Law of Thermodynamics

The second law of thermodynamics states:

• Every energy transfer or transformation increases the entropy (disorder) of the universe.

• Example: As animals use chemical energy, some is lost as heat, increasing entropy.

Free Energy and Metabolic Reactions

Free Energy (G)

Free energy (G) is the portion of a system’s energy that can perform work when temperature and pressure are uniform. The change in free energy () determines whether a reaction is spontaneous.

• Exergonic reactions: Release energy, are spontaneous, and have .

• Endergonic reactions: Require energy input, are nonspontaneous, and have .

Key points:

• Reactions with less free energy (lower G) are more stable and have less work capacity.

• Reactions with more free energy (higher G) are less stable and have greater work capacity.

Equations:

• Change in free energy:

Exergonic vs. Endergonic Reactions

Equilibrium and Metabolism

Equilibrium in Biological Systems

In a closed system, reactions eventually reach equilibrium () and can no longer do work. In contrast, living cells are open systems and never reach equilibrium, allowing continuous work and metabolism.

• At equilibrium, no net change occurs, and no energy is available to do work.

• Cells maintain disequilibrium by constantly exchanging materials and energy with their environment.

Example: A hydroelectric system with flowing water (open system) can do work, but once the water levels equalize (closed system), no work can be done.