Metabolism encompasses every chemical reaction occurring within living organisms to sustain life. These reactions are organized into metabolic pathways, which are sequential chains of enzyme-catalyzed steps. Through these pathways, the body converts ingested nutrients into cellular energy and synthesizes necessary structural components.
The metabolic network divides neatly into two opposing yet complementary phases. Catabolism breaks down complex molecules into simpler units, liberating stored energy in the process. Anabolism uses that liberated energy to build complex macromolecules from simple precursors.
| Feature | Catabolism | Anabolism |
|---|---|---|
| Energy State | Exergonic (releases energy) | Endergonic (consumes energy) |
| Molecular Size | Large to small | Small to large |
| Primary Purpose | ATP generation and waste production | Tissue growth and repair |
| Classic Example | Glycolysis | Protein synthesis |
Chemical reactions inside body cells occur too slowly at normal temperatures to support life without biological catalysts. Enzymes are protein molecules that dramatically lower the activation energy required for metabolic reactions to proceed. They feature an active site where specific substrates bind.
Carbohydrates provide the body's most accessible and preferred fuel source, especially for the brain and red blood cells. Dietary polysaccharides are broken down into monosaccharides, primarily glucose, which enters the bloodstream for cellular uptake.
When carbohydrate stores run low, the body shifts toward lipid and protein metabolism to meet its energy demands. Triglycerides undergo lipolysis, splitting into glycerol and free fatty acids. Fatty acids then enter beta-oxidation inside the mitochondria, cleaving into two-carbon acetyl-CoA units that feed directly into the Krebs cycle.
Protein metabolism begins with dietary proteolysis into amino acids. Unlike carbohydrates and fats, excess amino acids are not stored for future energy use. Instead, their nitrogen group is removed through deamination in the liver, forming urea for renal excretion, while the remaining carbon skeleton is converted into glucose or ketone bodies.
Energy balance represents the equilibrium between energy intake from food and energy expenditure by the body. The largest component of daily energy expenditure is the Basal Metabolic Rate (BMR), which accounts for the minimal energy required to sustain vital organ function in a resting, awake state.
Disruptions in metabolic pathways result in serious pathological states that demand vigilant nursing assessment and intervention. Type 2 diabetes mellitus involves cellular resistance to insulin, impairing glucose uptake and causing chronic hyperglycemia. Inborn errors of metabolism, such as phenylketonuria (PKU), stem from genetic enzyme defects that block normal biochemical conversions.
Nurses must monitor serum electrolyte levels, track daily weights, assess nutritional intake, and watch for signs of metabolic acidosis in unstable patients. Patient education focuses on dietary compliance and understanding prescribed medication regimens.