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 each step is catalyzed by a specific enzyme. Metabolism is broadly divided into two categories: anabolism (building up molecules) and catabolism (breaking down molecules).

• Anabolic reactions: Require energy to synthesize complex molecules from simpler ones.

• Catabolic reactions: Release energy by breaking down complex molecules into simpler ones.

• Enzymes: Biological catalysts that speed up metabolic reactions.

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

Example: The breakdown of glucose during cellular respiration is a catabolic process that releases energy.

Metabolic Classification of Organisms

Energy Sources: Phototrophs vs. Chemotrophs

Organisms are classified based on how they obtain energy and carbon. The two main energy sources are sunlight and chemical compounds.

• Phototrophs: Obtain energy from sunlight.

• Chemotrophs: Obtain energy from chemical compounds.

Carbon Sources: Autotrophs vs. Heterotrophs

Organisms are further classified by their carbon source:

• Autotrophs: Use inorganic carbon (such as CO2) as their carbon source.

• Heterotrophs: Use organic carbon compounds as their carbon source.

Metabolic Classification Table

Macromolecules and Their Subunits

Major Biological Macromolecules

Cells are composed of four major types of macromolecules, each built from specific subunits:

• Carbohydrates: Built from sugars (monosaccharides).

• Proteins: Built from amino acids.

• Fats (Lipids): Built from fatty acids.

• Nucleic acids: Built from nucleotides.

Example: Starch is a carbohydrate polymer made from glucose subunits.

Energy and Chemical Reactions

Covalent Bonds and Energy Storage

Chemical energy in biological systems is stored in covalent bonds. The formation and breaking of these bonds are central to metabolic reactions.

• Anabolism: Formation of covalent bonds, requires energy input.

• Catabolism: Breaking of covalent bonds, releases energy.

Example Reaction:

• Formation of carbonic acid from carbon dioxide and water:

Redox (Oxidation-Reduction) Reactions

Definitions and Principles

Redox reactions involve the transfer of electrons between molecules. These reactions are always coupled: when one molecule is oxidized (loses electrons), another is reduced (gains electrons).

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• LEO the lion says GER: Lose Electrons = Oxidation; Gain Electrons = Reduction.

• Oxidizing agent: Accepts electrons (is reduced).

• Reducing agent: Donates electrons (is oxidized).

Example: In the reaction between sodium and chlorine, sodium is oxidized (loses electrons) and acts as the reducing agent.

Redox Reactions in Glucose Metabolism

Cellular Respiration Overview

Cellular respiration is a highly regulated process where organic compounds (such as glucose) are oxidized to produce energy, carbon dioxide, and water.

• Glucose: The primary fuel molecule for most cells.

• Oxygen: Acts as the final electron acceptor in aerobic respiration.

• Energy: Captured in the form of ATP (adenosine triphosphate).

Example: During glycolysis and the citric acid cycle, glucose is gradually oxidized, and electrons are transferred to carriers such as NAD+ and FAD.

Electron Carriers and Energy Intermediates

NAD+ and NADH

Electron carriers such as NAD+ (nicotinamide adenine dinucleotide) play a crucial role in cellular respiration by accepting and donating electrons.

• NAD+: Oxidized form; accepts electrons and hydrogen to become NADH.

• NADH: Reduced form; carries electrons to the electron transport chain.

Example: NADH generated during glycolysis and the citric acid cycle is used to produce ATP in the mitochondria.

Summary Table: Anabolic vs. Catabolic Reactions

Additional info: The metabolic map shown in the first image is a comprehensive diagram of interconnected metabolic pathways, including glycolysis, the citric acid cycle, and amino acid metabolism. These pathways illustrate how energy and matter flow through cells, supporting life processes.