Metabolism: Overview and Pathways

Definition and Pathway Structure

Metabolism encompasses all of an organism's chemical reactions. These reactions are organized into metabolic pathways, which are series of reactions that alter a substrate through multiple steps before producing a final product.

• Metabolic Pathways: Consist of multiple steps, each catalyzed by a specific enzyme.

• Substrates are converted into products through a sequence of enzyme-mediated reactions.

• Example: The breakdown of glucose in cellular respiration involves several enzymes and intermediate products.

Catabolic and Anabolic Pathways

Metabolic pathways are classified into two main types based on their function:

• Catabolic Pathways: Release energy by breaking down complex molecules into simpler ones. Example: Cellular respiration breaks down glucose into carbon dioxide and water, releasing energy.

• Anabolic Pathways: Consume energy to build complex molecules from simpler ones. Example: Synthesis of DNA or proteins from nucleotides or amino acids, respectively.

Comparison Table:

Energy in Biological Systems

Definition and Types of Energy

Energy is the ability to perform work. In biological systems, work often involves the transfer of energy that causes an overall change.

• Potential Energy: Stored energy due to position or structure. Examples: Chemical energy in bonds, gravitational energy.

• Kinetic Energy: Energy of motion. Examples: Moving objects, muscle contractions.

Comparison Table:

Thermodynamics in Biology

System vs. Surroundings

Thermodynamics is the study of energy transfers between bodies of matter. In biology:

• System: The specific portion of matter being studied (e.g., a cell, an organism).

• Surroundings: Everything outside the system.

• Biological systems exchange both energy and mass with their surroundings.

Example: A plant (system) exchanges energy (sunlight in, heat out) and mass (CO2 in, O2 out) with its environment.

First Law of Thermodynamics

The First Law of Thermodynamics states that energy can be transferred and transformed, but it cannot be created or destroyed. This is also known as the Principle of Conservation of Energy.

• The total amount of energy in the universe remains constant.

• Example: Energy from sunlight is converted to chemical energy in plants, which is then transferred to animals through consumption.

Equation:

Where is the change in internal energy, is heat added to the system, and is work done by the system.

Entropy and the Second Law of Thermodynamics

Entropy is a measure of disorder or randomness in a system. The Second Law of Thermodynamics states that every energy transfer increases the entropy of the universe.

• 100% efficient energy conversion is impossible because some energy is always lost as heat.

• The natural tendency of systems is to move toward a state of maximum entropy (disorder).

• Example: As energy is transferred along the food chain, some is lost as heat, increasing entropy.

Equation:

Where is the change in entropy.

Summary Table: Laws of Thermodynamics

Practice and Application

• Catabolic pathways break down molecules and release energy; anabolic pathways build molecules and consume energy.

• Potential energy is stored (e.g., in chemical bonds); kinetic energy is energy of motion (e.g., muscle contraction).

• Thermodynamics explains how energy and mass are exchanged in biological systems and why energy conversions are never 100% efficient.

• Entropy increases with every energy transfer, explaining the loss of usable energy as heat in biological processes.

Additional info: These notes are based on foundational concepts in general biology, particularly as they relate to metabolism and energy transformations in living systems.