Metabolism & Nutrition

Overview of Metabolism & Nutrition

Metabolism encompasses all chemical reactions occurring within the body, including the transfer and transformation of energy. Nutrition provides the substrates required for these metabolic processes, supporting growth, maintenance, and cellular function.

• Metabolism: The sum of all chemical reactions in the body, including both energy-releasing and energy-consuming processes.

• Nutrition: The intake and utilization of nutrients necessary for metabolism, growth, and cellular repair.

• Three uses of nutrients:

  1. Energy for active processes

  2. Synthesis of structural or functional molecules

  3. Storage as fat or glycogen

Key Metabolic Terms

Understanding metabolism requires familiarity with several foundational terms and concepts.

• Catabolic vs Anabolic Reactions:

  • Catabolic: Breakdown of complex molecules into simpler ones, releasing energy. Example: AB → A + B

  • Anabolic: Synthesis of complex molecules from simpler ones, requiring energy. Example: A + B → AB

• Exergonic vs Endergonic Reactions:

  • Exergonic: Reactions that release energy.

  • Endergonic: Reactions that require energy input.

• Oxidation vs Reduction:

  • Oxidation: Electrons are removed from a molecule, decreasing its energy content. (ENERGY INVESTMENT)

  • Reduction: Electrons are added to a molecule, increasing its energy content. (ENERGY PAYOFF)

  • Mnemonic: OIL RIG – Oxidation Is Loss, Reduction Is Gain (of electrons)

Chemical Digestion & Absorption

Chemical digestion breaks down macromolecules into absorbable units through enzymatic hydrolysis.

• Carbohydrates digested into monosaccharides (e.g., glucose, galactose)

• Proteins digested into amino acids

• Lipids digested into fatty acid chains and monoglycerides

Energy Transfer: Oxidation-Reduction

Energy transfer in cells often involves oxidation-reduction (redox) reactions, which move electrons between molecules.

• Oxidation: Electrons are removed from a molecule, resulting in lower energy content.

• Reduction: Electrons are added to a molecule, resulting in higher energy content.

Example: NAD+ is reduced to NADH during cellular respiration, capturing energy for later use.

Equation:

Energy Transfer: ATP Generation

ATP (adenosine triphosphate) is the primary energy currency of the cell, generated through phosphorylation of ADP.

• Phosphorylation: Addition of a phosphate group to ADP to form ATP.

• Three methods to phosphorylate ADP:

  1. Substrate-level phosphorylation: Direct transfer of a phosphate group from a substrate to ADP (e.g., glycolysis, Krebs cycle).

  2. Oxidative phosphorylation: Electrons are transferred to oxygen via the electron transport chain, driving ATP synthesis.

  3. Photophosphorylation: Occurs in plants during photosynthesis (not covered in human metabolism).

Equation:

Role of ATP in Metabolic Reactions

ATP provides the energy required for cellular processes, including both catabolic and anabolic reactions.

• Cellular respiration: The process by which cells metabolize food molecules to capture energy for ATP production.

• Catabolic reactions: Break chemical bonds to release energy, which is used to synthesize ATP.

• Anabolic reactions: Use ATP to form new chemical bonds and build complex molecules.

Overview: Glucose Catabolism

Glucose catabolism is the primary pathway for ATP production in cells, involving several stages.

• Oxidation of glucose produces ATP and occurs in all cells.

• Stages of glucose catabolism:

  1. Glycolysis: Occurs in the cytoplasm; glucose is split into two pyruvate molecules.

  2. Pyruvate oxidation (acetyl-CoA formation): Pyruvate is converted to acetyl-CoA in the mitochondria.

  3. Krebs cycle (citric acid cycle): Acetyl-CoA is oxidized, producing NADH and FADH2.

  4. Electron transport chain (ETC): Electrons from NADH and FADH2 are transferred to oxygen, generating ATP.

Overall equation for cellular respiration:

Summary Table: Catabolic vs Anabolic Reactions

Additional info:

• Potential and kinetic energy are interconvertible; metabolic reactions often involve the transformation of one form to another.

• NAD+ and FAD are key electron carriers in cellular respiration.