Overview
This section explains how carbohydrate, protein, and lipid metabolic pathways are interconnected, allowing cells to extract energy from various food sources by integrating their catabolic processes.
Integration of Metabolic Pathways
- Carbohydrate, protein, and lipid catabolic pathways all feed into glycolysis and the citric acid cycle.
- Metabolic pathways are open systems; intermediates can enter or exit to connect with other pathways.
- Substances from one pathway can become reactants or products in another pathway.
Carbohydrate Metabolism Connections
- Glycogen is stored in liver and muscle cells and broken down to glucose-1-phosphate when ATP is needed.
- Glucose-1-phosphate is converted to glucose-6-phosphate and enters glycolysis.
- Dietary sugars like sucrose, fructose, and galactose are also funneled into glycolysis, yielding ATP similar to glucose.
Protein Metabolism Connections
- Proteins are broken down into amino acids, mainly recycled for new protein synthesis.
- Excess amino acids or those from starvation are converted (after removing the amino group) into intermediates of glycolysis or the citric acid cycle.
- The amino group is converted to ammonia, which mammals convert to urea for excretion.
- Amino acids can also be synthesized from metabolic cycle intermediates.
Lipid Metabolism Connections
- Cholesterol is synthesized from acetyl groups (not reversible) and is essential for membranes and hormone precursors.
- Triglycerides, made from glycerol and fatty acids, are energy storage molecules.
- Glycerol enters glycolysis after conversion to glycerol-3-phosphate.
- Fatty acids are broken down by beta-oxidation into acetyl CoA, which enters the citric acid cycle.
Evolutionary Perspective
- Early metabolic pathways evolved in response to nutrient availability and selection for efficient energy extraction.
- Glycolysis likely predates photosynthesis and originally helped exploit simple sugars in early Earth environments.
- Oxygenic photosynthesis allowed evolution of aerobic respiration, increasing the energy extracted from sugars.
Key Terms & Definitions
- Glycolysis — metabolic pathway that breaks down glucose to pyruvate, producing ATP.
- Citric Acid Cycle — series of chemical reactions used by aerobic organisms to generate energy.
- Glycogen — storage polysaccharide for glucose in animals.
- Beta-oxidation — process by which fatty acids are broken down to acetyl CoA in mitochondria.
- Acetyl CoA — molecule that enters the citric acid cycle and links various metabolic pathways.
- Urea — principal nitrogenous waste product formed from ammonia in mammals.
Action Items / Next Steps
- Review figures 7.18 and 7.19 for visual summaries of pathway connections.
- Be able to trace how carbohydrates, proteins, and fats enter cellular respiration pathways.
- Prepare to explain why metabolic cycles are considered open, interconnected systems.