Introduction to Metabolism

Metabolism encompasses all chemical reactions that occur within living organisms to sustain life. These reactions transform matter and energy, following the laws of thermodynamics. Understanding metabolism is essential for grasping how cells obtain and use energy, build complex molecules, and regulate their internal environment.

Section 8.1: Metabolism and Thermodynamics

Definitions and Overview

• Metabolism: The sum of all chemical reactions in an organism.

• Catabolism: The breakdown of large molecules into smaller ones, releasing energy (e.g., cellular respiration).

• Anabolism: The synthesis of complex molecules from simpler ones, requiring energy (e.g., protein synthesis).

Examples of metabolic processes include cellular respiration, photosynthesis, and protein synthesis.

Key Questions in Metabolism

• How do organisms obtain energy?

• How do organisms acquire electrons for metabolic reactions?

• How do organisms obtain carbon?

Energy and Electron Sources

• Phototrophs: Use light as an energy source.

• Chemotrophs: Use chemical compounds as an energy source.

• Organotrophs: Obtain electrons from organic molecules (e.g., glucose).

• Lithotrophs: Obtain electrons from inorganic molecules (e.g., H2S, Fe2+).

Carbon Sources

• Autotrophs: Use inorganic carbon (CO2) to build organic molecules.

• Heterotrophs: Use organic carbon from other organisms.

Types of Reactions and Environments

• Aerobic: Require oxygen.

• Anaerobic: Occur without oxygen.

• Oxic: Oxygen-present environment.

• Anoxic: Oxygen-absent environment.

• Hypoxic: Low-oxygen environment.

Forms of Energy

• Kinetic Energy: Energy of motion (e.g., radiant, mechanical, electrical, thermal).

• Potential Energy: Stored energy (e.g., chemical bonds).

Energy Transformations in Cells

Chemical energy stored in molecules like glucose is used to generate ATP, which powers cellular processes.

First Law of Thermodynamics

• Energy can be converted from one form to another, but cannot be created or destroyed.

Second Law of Thermodynamics

• Entropy (disorder) tends to increase in a closed system.

• Living systems require energy input to maintain order; Earth is an open system receiving energy from the sun.

Section 8.2: Free Energy and Spontaneity

Gibbs Free Energy ()

• Determines whether a reaction occurs spontaneously.

• Negative indicates a spontaneous (exergonic) reaction.

• Positive indicates a non-spontaneous (endergonic) reaction.

Energy Dynamics in Cellular Respiration

• Equation: Glucose + O2 + ADP + Pi → CO2 + H2O + ATP

• Glucose is the electron donor (high energy); oxygen is the electron acceptor (low energy).

• Standard free energy change for glucose oxidation: kcal (or 2870.22 kJ).

• Theoretical ATP yield: ATP; actual yield is 30–32 ATP due to inefficiency.

Calculating Reduction Potential ()

• Find the electron donor and acceptor in a reduction potential table.

• Calculate as the difference between acceptor and donor potentials.

• Relationship:

Types of Metabolism in Prokaryotes

• Chemoorganotrophs: Use organic molecules as electron donors and various acceptors (O2, NO3-, SO42-, etc.).

• Chemolithotrophs: Use inorganic molecules as electron donors (NH3, H2, Fe2+, etc.).

Types of Reactions

• Endothermic: Absorb energy.

• Exothermic: Release energy.

• Endergonic: Require energy input (anabolic).

• Exergonic: Release energy (catabolic).

Section 8.3: ATP and Energy Coupling

ATP Powers Cellular Work

• Chemical work: Driving endergonic reactions (e.g., synthesis of macromolecules).

• Mechanical work: Movement (e.g., muscle contraction).

• Transport work: Pumping substances across membranes.

Energy Coupling

Cells couple exergonic reactions (e.g., ATP hydrolysis) to endergonic reactions to drive essential processes. ATP often transfers a phosphate group to a substrate, forming a phosphorylated intermediate that is more reactive.

Section 8.4: Enzymes and Metabolic Reactions

Enzyme Function

• Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• They do not change the overall energy released or required by the reaction.

• Enzymes are specific to their substrates.

Activation Energy

Activation energy is the energy required to reach the transition state of a reaction. Enzymes lower this barrier, increasing the reaction rate.

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature; too high or too low reduces activity.

• pH: Each enzyme has an optimal pH range.

• Substrate concentration: Increasing substrate increases reaction rate until the enzyme is saturated.

• Enzyme concentration: More enzyme increases the maximum rate of reaction.

Enzyme Inhibition

• Competitive inhibitors: Bind to the active site; increase Km, Vmax unchanged at high substrate concentration.

• Non-competitive inhibitors: Bind elsewhere; Km unchanged, Vmax decreases.

Cofactors and Coenzymes

• Cofactors: Inorganic ions (e.g., metals) that help enzyme function.

• Coenzymes: Organic molecules (often vitamin-derived) that assist enzymes, often by carrying electrons.

Vitamins

• Water-soluble: B and C; not stored, must be replenished regularly.

• Fat-soluble: A, D, E, K; stored in body fat, excess or deficiency can cause disease.

Deficiency Diseases

Lack of essential vitamins or nutrients can cause diseases, which are often reversible with proper diet.

Section 8.5: Regulation of Enzyme Activity

Allosteric Regulation

Enzyme activity can be regulated by molecules binding at sites other than the active site, allowing cells to control metabolic pathways efficiently.

Cooperativity

Binding of one substrate molecule increases the activity of other subunits in a multi-subunit enzyme (e.g., hemoglobin).

Feedback Inhibition

The end product of a metabolic pathway inhibits an earlier step, preventing overproduction and conserving resources.

Additional info: These notes cover the core concepts of metabolism, energy transformations, enzyme function, and metabolic regulation, as outlined in Chapter 8 of a General Biology textbook. They are suitable for exam preparation and foundational understanding of cellular metabolism.