Metabolism refers to the sum total of all chemical reactions that take place within living organisms to maintain life. These biochemical processes allow cells to grow, reproduce, maintain structures, and respond to environmental changes. Pathways are rarely isolated; instead, they form intricate, interconnected networks where the end product of one reaction often serves as the starting substrate for another.
Adenosine triphosphate, or ATP, serves as the universal energy currency for all cellular activities. Structurally, it consists of an adenine base, a ribose sugar, and three phosphate groups. Energy is stored within the high-energy bonds linking these phosphate groups. When a cell requires energy, the terminal phosphate bond is cleaved through hydrolysis, converting ATP into ADP and releasing free energy to drive unfavorable biological reactions.
Carbohydrate breakdown begins with glycolysis, a ten-step anaerobic pathway occurring in the cytoplasm that converts glucose into pyruvate while generating a small amount of ATP. In the presence of oxygen, pyruvate enters the mitochondria, where the Krebs cycle and oxidative phosphorylation yield a large harvest of energy. Excess glucose can be stored as glycogen through glycogenesis, primarily in the liver and skeletal muscles.
Lipids provide a dense, highly concentrated source of stored energy. Through beta-oxidation, fatty acids are broken down in the mitochondrial matrix into two-carbon acetyl units that enter the Krebs cycle. When carbohydrate availability is severely restricted, the liver converts excess acetyl groups into ketone bodies, which serve as an alternative fuel source for the brain and other tissues during prolonged fasting or starvation.
Proteins are rarely used as a primary energy source, functioning instead as structural elements, enzymes, and hormones. Dietary proteins are digested into amino acids, which are then utilized to synthesize new body proteins. When amino acids are in excess or during starvation, they undergo deamination in the liver, where the amino group is removed to form urea, leaving a carbon skeleton that can enter energy pathways.
Basal metabolic rate (BMR) represents the minimum amount of energy required by the body to maintain basic physiological functions while at rest. Energy balance is achieved when total caloric intake equals total energy expenditure, which includes BMR, the thermic effect of food, and physical activity. A positive energy balance leads to weight gain as excess calories are stored as fat, while a negative balance results in weight loss.
Inborn errors of metabolism occur due to genetic defects that disrupt specific enzymatic pathways, such as phenylketonuria (PKU). Acquired metabolic disorders, like type 2 diabetes mellitus, involve cellular resistance to insulin, disrupting normal carbohydrate and lipid processing. Nurses must monitor blood glucose levels, assess nutritional status, and educate patients on dietary modifications to manage these conditions effectively.