Metabolism in Living Organisms

Definition and Overview

Metabolism refers to the totality of an organism's chemical reactions, which are essential for maintaining life. These reactions involve the transformation of matter and energy within cells and can be categorized into two main types: those that build biomolecules and those that break them down.

• Anabolism: The set of metabolic pathways that construct molecules from smaller units. This process typically requires energy, often in the form of ATP, and is responsible for building biomolecules necessary for cell structure and function.

• Catabolism: The set of metabolic pathways that break down molecules into smaller units. These reactions release energy, which can be captured in the form of ATP or used directly in other cellular processes.

• ATP (Adenosine Triphosphate): The primary energy currency of the cell, produced and consumed during metabolic reactions.

Example: Cellular respiration is a catabolic process that breaks down glucose to produce ATP, while photosynthesis is an anabolic process that builds glucose from carbon dioxide and water.

Energy in Biological Systems

Forms and Functions of Energy

Energy is the capacity to do work or cause change. In biological systems, energy exists in various forms and is essential for driving cellular processes.

• Kinetic Energy: The energy of motion, such as the movement of molecules.

• Potential Energy: Stored energy, such as energy stored in chemical bonds.

• Thermal Energy: A specific type of kinetic energy associated with the random movement of particles; contributes to temperature.

• Chemical Energy: A specific type of potential energy stored in chemical bonds of molecules.

Energy can be converted from one form to another but cannot be created or destroyed, according to the First Law of Thermodynamics.

Energy Gradients and Work

Gradients, such as concentration, pressure, or electrical gradients, can be used to do work in biological systems. Work is defined as the ability to move matter against opposing forces.

• Concentration Gradient: Movement of solutes against their concentration gradient requires energy and is considered work.

• Air Pressure Gradient: Movement of air from high to low pressure can do work.

• Gravitational Gradient: Movement of objects due to gravity involves conversion of potential to kinetic energy.

• Chemical/Electrical Gradient: Movement of ions across membranes (e.g., during nerve impulses) involves electrical potential and can do work.

Example: Active transport of ions across a cell membrane against their concentration gradient requires ATP.

Free Energy and Thermodynamics

Gibbs Free Energy

Gibbs 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 process is spontaneous.

• Equation:

• = change in free energy

• = change in total internal energy (enthalpy)

• = temperature in Kelvin

• = change in entropy

Spontaneity:

• If , the reaction is spontaneous (no energy input required).

• If , the reaction is nonspontaneous (energy input required).

Example: Hydrolysis of ATP to ADP and inorganic phosphate is a spontaneous reaction ().

Entropy in Biological Systems

Definition and Role

Entropy (S) is a measure of disorder or randomness in a system. It also reflects the system's closeness to equilibrium and its ability to do work.

• Higher entropy means greater disorder and less ability to do work.

• As systems approach equilibrium, entropy increases and the capacity to do work decreases.

Example: Diffusion of molecules from high to low concentration increases entropy.

Summary Table: Types of Metabolic Pathways

Key Equations

• Gibbs Free Energy:

• Relationship between total energy and free energy:

• Change in free energy during a reaction:

Endergonic vs. Exergonic Reactions

Comparison

• Exergonic Reaction: Releases energy, spontaneous ().

• Endergonic Reaction: Requires energy input, nonspontaneous ().

Example: Synthesis of glucose from carbon dioxide and water during photosynthesis is endergonic.

Additional info: Some explanations and examples have been expanded for clarity and completeness.