Chemical Reactions & the Metabolic Map

Introduction to Metabolism

Metabolism refers to the totality of chemical reactions that occur within a living organism to maintain life. These reactions are organized into metabolic pathways, where the product of one reaction serves as the substrate for the next. Metabolic pathways are essential for energy production, synthesis of macromolecules, and cellular maintenance.

• Metabolic Pathways: Series of enzyme-catalyzed reactions converting substrates through intermediates to final products.

• Enzymes: Biological catalysts that speed up metabolic reactions without being consumed.

• Intermediates: Compounds formed between the initial substrate and the final product in a pathway.

Example: Glycolysis is a metabolic pathway that breaks down glucose into pyruvate, generating ATP and NADH.

Metabolic Classification of Organisms

Energy and Carbon Sources

Organisms are classified based on how they obtain energy and carbon for growth. The two main energy sources are sunlight and chemical compounds, while carbon can be derived from inorganic or organic sources.

• Phototrophs: Obtain energy from sunlight.

• Chemotrophs: Obtain energy from chemical compounds.

• Autotrophs: Use inorganic carbon (e.g., CO2) as their carbon source.

• Heterotrophs: Use organic compounds as their carbon source.

Combining these categories gives four major metabolic types:

Additional info: This classification helps understand ecological roles and energy flow in ecosystems.

Anabolism and Catabolism

Types of Metabolic Reactions

Metabolism is divided into two broad categories: anabolism and catabolism.

• Anabolism: The set of metabolic pathways that construct molecules from smaller units. These reactions require energy input.

• Catabolism: The set of metabolic pathways that break down molecules into smaller units, releasing energy.

Example: Synthesis of proteins from amino acids is an anabolic process, while the breakdown of glucose during cellular respiration is catabolic.

Macromolecules and Their Subunits

• Carbohydrates are broken down into sugars (monosaccharides).

• Proteins are broken down into amino acids.

• Fats are broken down into fatty acids and glycerol.

• Nucleic acids are broken down into nucleotides.

Energy, Chemical Reactions, and Covalent Bonds

Energy in Chemical Reactions

Chemical reactions involve the making and breaking of covalent bonds, which store and release energy. The direction of a reaction (anabolic or catabolic) determines whether energy is consumed or released.

• Anabolic reactions (e.g., photosynthesis) require energy input to form new covalent bonds.

• Catabolic reactions (e.g., cellular respiration) release energy by breaking covalent bonds.

Example: The formation of carbonic acid from carbon dioxide and water:

This reaction is catalyzed by the enzyme carbonic anhydrase in cells.

Redox (Oxidation-Reduction) Reactions

Definition and Importance

Redox reactions involve the transfer of electrons between molecules. These reactions are fundamental to energy transfer in biological systems.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Redox reactions are always coupled; when one molecule is oxidized, another is reduced.

Mnemonic: "LEO the lion says GER" — Lose Electrons = Oxidation; Gain Electrons = Reduction.

Redox Agents

• Oxidizing agent: Accepts electrons and is reduced.

• Reducing agent: Donates electrons and is oxidized.

Example: In the reaction between sodium and chlorine:

• Sodium is oxidized (loses electrons) and acts as the reducing agent.

• Chlorine is reduced (gains electrons) and acts as the oxidizing agent.

Redox Reactions in Glucose Metabolism

Cellular Respiration and Photosynthesis

Cellular respiration and photosynthesis are key metabolic processes involving redox reactions.

• Cellular Respiration: Organic compounds (like glucose) are oxidized to produce carbon dioxide, water, and energy (ATP).

• Photosynthesis: Carbon dioxide is reduced to form glucose, using energy from sunlight.

Overall equation for cellular respiration:

Overall equation for photosynthesis:

Electron Carriers

• NAD+ (Nicotinamide adenine dinucleotide): A key electron carrier in redox reactions. It is reduced to NADH during catabolic reactions, storing energy for later use.

• FAD (Flavin adenine dinucleotide): Another electron carrier, reduced to FADH2.

Example: During glycolysis and the citric acid cycle, electrons are transferred from glucose to NAD+, forming NADH.

Summary Table: Metabolic Pathways and Energy Flow

Additional info: Understanding these processes is fundamental for studying energy flow and matter cycling in biological systems.