Metabolism and Energy Production

Metabolism Overview

  • Metabolism: The sum of all chemical reactions that occur in the body, consisting primarily of:

    • Catabolic: breaking down molecules

    • Anabolic: building up molecules)

Energy and ATP

  • ATP (Adenosine Triphosphate): The primary molecule involved in energy exchanges within the body.

    • Synthesized through a process known as phosphorylation.

  • Energy cost of being alive includes:

    • Routine maintenance of cellular functions.

    • Replacement of intracellular and extracellular components.

    • Vital functions, such as growth, secretion, and contraction.

Energy Production from Food

  • Ingestion and Digestion: Breakdown of complex macromolecules into monomers (e.g., monosaccharides, amino acids, fatty acids).

  • Monomers are transported to cells and converted into ATP.

Stages of Energy Utilization

  1. Digestion: Breakdown of nutrients into absorbable forms in the GI tract.

  2. Cellular Processing: Conversion of nutrients into energy within cells (glycolysis).

  3. Oxidative Breakdown: In mitochondria, catabolic pathways release energy, forming CO₂ and H₂O.

Carbohydrate Metabolism Overview

  • Carbohydrates:

    • Sources: Starch, sugars from fruits, honey, and milk; fiber (both soluble and insoluble) from vegetables.

  • Uses: Glucose is the primary fuel for ATP production; excess glucose is stored as glycogen or fat.

Steps of Carbohydrate Metabolism (Cellular Respiration)

  1. Glycolysis: Conversion of glucose into pyruvic acid; occurs in the cytoplasm.

    • Reaction: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

    • Key Outputs: Net gain of 2 ATP, 2 NADH; significant energy remains in pyruvic acid.

  2. Formation of Acetyl CoA:

    • From 2 molecules of pyruvic acid.

    • Outputs: 2 Acetyl CoA, 2 NADH.

  3. TCA Cycle (Krebs Cycle):

    • Takes place in the mitochondria.

    • Each cycle generates: 1 ATP, 3 NADH, and 1 FADH₂ per Acetyl CoA molecule.

  4. Electron Transport Chain:

    • Electrons from NADH and FADH₂ transport through various complexes, allowing hydrogen ions to create a gradient for ATP synthesis.

    • Final electron acceptor is oxygen, forming water.

Energy Yield

  • Total net ATP production:

    • Glycolysis: 2 ATP

    • Pyruvate oxidation: 2 NADH (equivalent to 4 ATP)

    • Krebs Cycle: 6 NADH (equivalent to 18 ATP) + 2 FADH₂ (equivalent to 4 ATP)

    • Total: 38 ATP (36 in eukaryotes due to energy cost of NADH transport into mitochondria).

Glycogenesis and Glycogenolysis

  • Glycogenesis: The process of synthesizing glycogen from glucose for storage

    • stimulated by insulin.

  • Glycogenolysis: The breakdown of glycogen to glucose

    • stimulated by glucagon.

Hormonal Regulation of Blood Glucose Levels

  • Insulin: Lowers blood sugar by increasing glucose uptake and stimulating glycogenesis.

  • Glucagon: Raises blood sugar by promoting glycogenolysis and decreasing lipogenesis.

Gluconeogenesis

  • Formation of glucose from non-carbohydrate sources (e.g., glycerol, amino acids) in the liver, prevents hypoglycemia when glucose is low.