Chapter 8: An Introduction to Metabolism

Energy: Definitions and Types

Energy is a fundamental concept in biology, essential for driving cellular processes and sustaining life. It exists in various forms and can be classified based on its nature and behavior.

• Potential Energy: Stored energy due to an object's position or structure. Example: Chemical energy in bonds.

• Kinetic Energy: Energy of motion. Example: Movement of molecules during diffusion.

• Transformation: Energy can be converted from one form to another, such as chemical energy to kinetic energy.

Example: Glucose contains potential energy stored in its chemical bonds, which is released as kinetic energy during cellular respiration.

Thermodynamics in Biological Systems

Thermodynamics governs the flow and transformation of energy in biological systems. Two key laws are especially relevant to metabolism.

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed. In cells, chemical energy from food is converted to other forms, such as heat or work.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe. Some energy is lost as heat during metabolic processes.

• Entropy: Measure of disorder or randomness in a system.

• Enthalpy (H): Total energy of a system.

• Free Energy (G): Portion of a system's energy available to do work. Calculated as:

where is free energy, is enthalpy, is temperature, and is entropy.

Metabolism and Chemical Reactions

Metabolism encompasses all chemical reactions occurring in living organisms. These reactions are categorized based on energy changes and directionality.

• Exergonic Reactions: Release energy; products have less free energy than reactants. Example: Cellular respiration.

• Endergonic Reactions: Require energy input; products have more free energy than reactants. Example: Photosynthesis.

• Coupling of Reactions: Exergonic and endergonic reactions are often coupled in cells to drive necessary processes.

• Energy Transformation: Some energy is always lost as heat during these reactions.

ATP: Structure and Function

Adenosine triphosphate (ATP) is the primary energy currency of the cell. It stores and transfers energy for cellular activities.

• Structure: Composed of adenine, ribose, and three phosphate groups.

• Function: Hydrolysis of ATP releases energy used for cellular work.

• Role in Metabolism: ATP couples exergonic and endergonic reactions, enabling energy transfer.

Example: Muscle contraction and active transport across membranes require ATP hydrolysis.

Enzymes: Catalysts of Metabolism

Enzymes are biological catalysts that speed up chemical reactions without being consumed. They are essential for regulating metabolic pathways.

• Structure: Enzymes are proteins with a specific three-dimensional shape.

• Active Site: Region where substrate binds and reaction occurs.

• Activation Energy: Minimum energy required to start a reaction. Enzymes lower activation energy.

• Specificity: Each enzyme acts on a specific substrate.

• Catalysis: Enzymes facilitate the conversion of substrates to products.

Example: Amylase catalyzes the breakdown of starch into sugars.

Regulation of Enzyme Activity

Cells regulate enzyme activity to control metabolic pathways and respond to changing conditions.

• Enzyme Inhibition: Molecules can decrease enzyme activity.

• Competitive Inhibition: Inhibitor competes with substrate for the active site.

• Allosteric Regulation: Inhibitor or activator binds to a site other than the active site, altering enzyme function.

• Positive and Negative Regulation: Enzymatic activities can be increased or decreased as needed.

• Feedback Inhibition: End product of a pathway inhibits an earlier step, preventing overproduction.

Example: ATP acts as an allosteric inhibitor for enzymes in glycolysis when energy is abundant.