Chapter 6: Introduction to Metabolism

Overview of Metabolism

Metabolism refers to the totality of an organism’s chemical reactions, which together manage the material and energy resources of the cell. It is a fundamental property of life, arising from the orderly interactions between molecules within biological systems.

• Metabolism: The sum of all chemical reactions occurring in a living organism.

• Emergent property: Metabolism emerges from the interactions of molecules within the cell.

• Metabolic pathways: Series of chemical reactions that either build up or break down molecules, each step catalyzed by a specific enzyme.

• Example: Cellular respiration is a metabolic pathway that breaks down glucose into carbon dioxide and water, releasing energy.

Types of Metabolic Pathways

Metabolic pathways are classified based on whether they build complex molecules or break them down.

• Anabolic pathways: Build complex molecules from simpler ones; require energy input (e.g., synthesis of proteins from amino acids).

• Catabolic pathways: Break down complex molecules into simpler compounds; release energy (e.g., breakdown of glucose in cellular respiration).

• Example: Photosynthesis is an anabolic pathway, while cellular respiration is catabolic.

Energy and Its Forms

Definition and Types of Energy

Energy is the capacity to cause change or do work. It exists in various forms and can be transformed from one form to another.

• Kinetic energy: Energy associated with motion.

• Thermal energy: Kinetic energy associated with the random movement of atoms or molecules; transferred as heat.

• Potential energy: Energy that matter possesses due to its location or structure.

• Chemical energy: Potential energy available for release in a chemical reaction.

• Example: A diver on a platform has potential energy, which is converted to kinetic energy as they dive.

Thermodynamics in Biological Systems

First Law of Thermodynamics

The first law, also known as the principle of conservation of energy, states that energy cannot be created or destroyed, only transferred or transformed.

• Equation: (where is heat added, is work done)

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

Second Law of Thermodynamics

Every energy transfer or transformation increases the entropy (disorder) of the universe. Some energy is always lost as heat.

• Entropy: A measure of disorder or randomness.

• Example: Combustion in an engine releases heat, increasing entropy.

• Biological context: Organisms maintain order locally but increase entropy in their surroundings.

Free Energy, Stability, and Equilibrium

Gibbs Free Energy ()

Free energy is the portion of a system’s energy that can perform work when temperature and pressure are uniform.

• Equation:

• Processes with negative are spontaneous.

• Spontaneous processes can be harnessed to perform cellular work.

• Equilibrium: State of maximum stability; no net change in free energy.

Exergonic and Endergonic Reactions

Metabolic reactions are classified by their energy changes.

• Exergonic reaction: Proceeds with a net release of free energy; spontaneous ( is negative).

• Endergonic reaction: Requires energy input; nonspontaneous ( is positive).

• Example: Cellular respiration is exergonic; protein synthesis is endergonic.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

Adenosine triphosphate (ATP) is the primary energy carrier in cells. It consists of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups.

• ATP hydrolysis releases energy that can be used to drive endergonic reactions.

• Equation:

• ATP is regenerated by the addition of a phosphate group to ADP.

Enzymes and Metabolic Regulation

Role of Enzymes

Enzymes are biological catalysts that speed up metabolic reactions by lowering the activation energy barrier () without being consumed in the reaction.

• Active site: The region on the enzyme where the substrate binds.

• Enzyme-substrate complex: Temporary association between enzyme and substrate.

• Enzymes are highly specific for their substrates.

• Example: Sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

Factors Affecting Enzyme Activity

Enzyme activity is influenced by environmental conditions and regulatory molecules.

• Temperature: Each enzyme has an optimal temperature for activity.

• pH: Each enzyme has an optimal pH.

• Cofactors and coenzymes: Nonprotein helpers required for enzyme function; coenzymes are organic cofactors (often vitamins).

• Example: Vitamin C is a coenzyme for enzymes involved in collagen formation.

Enzyme Inhibition

Enzyme inhibitors can decrease or halt enzyme activity.

• Competitive inhibitors: Bind to the active site, competing with the substrate.

• Noncompetitive inhibitors: Bind elsewhere, changing the enzyme’s shape and reducing activity.

• Medicinal inhibitors: Antibiotics, HIV drugs, and cholesterol-lowering drugs act as enzyme inhibitors.

• Toxic inhibitors: Poisons such as nerve gas and certain mushroom toxins irreversibly inhibit enzymes.

Regulation of Enzyme Activity

Cells regulate metabolism by controlling enzyme activity and gene expression.

• Allosteric regulation: Regulatory molecules bind to sites other than the active site, affecting enzyme function.

• Feedback inhibition: The end product of a pathway inhibits an earlier step, preventing overproduction.

• Example: Isoleucine inhibits the first enzyme in its biosynthetic pathway.

Summary Table: Types of Metabolic Reactions

Key Terms and Concepts

• Metabolism: All chemical reactions in an organism.

• Catabolic pathway: Breaks down molecules, releases energy.

• Anabolic pathway: Builds molecules, requires energy.

• ATP: Main energy currency of the cell.

• Enzyme: Protein catalyst for biochemical reactions.

• Activation energy (): Energy required to start a reaction.

• Allosteric regulation: Regulation of enzyme activity by binding at a site other than the active site.

• Feedback inhibition: End product inhibits an earlier step in a pathway.

Additional info: Some context and definitions have been expanded for clarity and completeness, including the role of vitamins as coenzymes and the importance of feedback inhibition in metabolic regulation.