Introduction to Metabolism

Overview of Metabolism

Metabolism encompasses all the chemical reactions that occur within a living organism. These reactions are essential for maintaining life, enabling organisms to grow, reproduce, maintain their structures, and respond to environmental changes.

• Metabolism is the sum of all chemical reactions in an organism.

• It is an emergent property of life, arising from the orderly interactions between molecules within the cell.

• Metabolic pathways transform matter and energy, allowing organisms to function and survive.

Metabolic Pathways

Types of Metabolic Pathways

Metabolic pathways are sequences of chemical reactions, each catalyzed by a specific enzyme. These pathways can be classified as either catabolic or anabolic.

• Catabolic pathways break down complex molecules into simpler compounds, releasing energy. Example: Cellular respiration, where glucose is broken down to carbon dioxide and water.

• Anabolic pathways build complex molecules from simpler ones, consuming energy. Example: Synthesis of proteins from amino acids.

Forms and Transformations of Energy

Energy in Biological Systems

Energy is the capacity to cause change or do work. In biological systems, energy exists in various forms and can be transformed from one form to another.

• Kinetic energy: Energy associated with motion. Example: Movement of molecules.

• 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. Example: Water behind a dam.

• Chemical energy: Potential energy available for release in a chemical reaction. Example: Energy stored in glucose.

Energy can be converted from one form to another, such as potential energy to kinetic energy during a dive.

Thermodynamics in Biology

Laws of Thermodynamics

Thermodynamics is the study of energy transformations. Biological systems obey the laws of thermodynamics:

• First Law (Conservation of Energy): Energy cannot be created or destroyed, only transferred or transformed.

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

Organisms are islands of low entropy in an increasingly random universe, maintaining order through constant energy input.

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.

• The change in free energy () during a chemical reaction determines whether the reaction is spontaneous.

• Equation:

• Reactions with a negative are spontaneous and can perform work.

• Systems tend toward greater stability (lower free energy).

Exergonic and Endergonic Reactions

• Exergonic reactions: Release free energy; spontaneous; is negative. Example: Cellular respiration.

• Endergonic reactions: Require energy input; nonspontaneous; is positive. Example: Synthesis of glucose during photosynthesis.

ATP: The Cell's Energy Currency

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 stores energy in the bonds between its phosphate groups.

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

• Equation:

Energy Coupling

• Cells use the energy released from exergonic reactions (like ATP hydrolysis) to power endergonic reactions.

• ATP hydrolysis is often coupled to cellular work, such as muscle contraction, active transport, and biosynthesis.

Enzymes and Catalysis

Role of Enzymes

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

• Most enzymes are proteins.

• The substrate is the reactant that an enzyme acts on.

• The active site is the region on the enzyme where the substrate binds.

• Enzyme-substrate complex: The temporary association between enzyme and substrate during a reaction.

Mechanisms of Enzyme Action

• Enzymes orient substrates correctly, strain substrate bonds, provide a favorable microenvironment, or directly participate in the reaction.

• Enzyme activity is affected by temperature, pH, and the presence of inhibitors or activators.

Enzyme Regulation

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

• Noncompetitive inhibitors bind elsewhere, changing the enzyme's shape and reducing activity.

• Allosteric regulation involves regulatory molecules binding to a site other than the active site, affecting enzyme function.

• Feedback inhibition occurs when the end product of a pathway inhibits an earlier step, preventing overproduction.

Coenzymes and Cofactors

Enzyme Helpers

• Cofactors are nonprotein enzyme helpers; may be inorganic (e.g., metal ions) or organic.

• Coenzymes are organic cofactors, often derived from vitamins (e.g., vitamin C).

• Coenzymes are essential for enzyme function in many metabolic pathways.

Summary Table: Exergonic vs. Endergonic Reactions

Key Terms and Concepts

• Metabolism: All chemical reactions in an organism.

• Catabolic pathway: Breaks down molecules, releases energy.

• Anabolic pathway: Builds molecules, consumes energy.

• ATP: Main energy currency of the cell.

• Enzyme: Protein catalyst that speeds up reactions.

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

• Free energy (): Energy available to do work.

• Entropy (): Measure of disorder.

• 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.

Examples and Applications

• Lactose intolerance: Caused by lack of the enzyme lactase, which breaks down lactose in milk.

• Phenylketonuria (PKU): Genetic disorder caused by inability to convert phenylalanine to tyrosine due to a missing enzyme.

• Scurvy: Disease caused by vitamin C deficiency, affecting collagen synthesis (a coenzyme function).

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