Metabolism is the complete set of chemical reactions and associated processes through which cells and organisms acquire, transform, store and use matter and energy. It includes catabolism, which generally breaks molecules down and can release usable energy, and anabolism, which builds cellular components and generally requires energy input. Metabolism is not a single pathway but a highly interconnected network involving carbohydrates, lipids, amino acids, nucleotides and numerous cofactors and intermediate metabolites.
Cells continuously adjust metabolic activity according to nutrient availability, energy demand, hormonal signals, oxygen availability, developmental state and cellular conditions. Metabolism therefore provides both the material and energetic basis for growth, maintenance, movement, reproduction and repair. Although often associated with digestion or calorie expenditure in everyday language, the biological meaning is much broader and encompasses essentially all regulated chemical transformations occurring in living cells.
ATP is a central energy-transfer molecule in metabolism. Cells use ATP to couple energy-releasing processes to energy-requiring processes, including biosynthesis, active transport and mechanical work. Cellular respiration extracts energy from nutrients through coordinated pathways. Glycolysis occurs in the cytosol and converts glucose into pyruvate while producing ATP and reduced electron carriers. Under aerobic conditions, pyruvate can be converted to acetyl-CoA, which enters the citric acid cycle. The citric acid cycle generates additional reduced electron carriers and other metabolic intermediates. Electrons are subsequently transferred through the mitochondrial electron-transport chain in aerobic human cells, with energy used to establish a proton gradient that drives ATP synthesis through oxidative phosphorylation. These pathways are tightly connected and their products feed other biochemical processes.
Carbohydrate metabolism includes pathways for glucose utilisation, storage and production. Glycolysis converts glucose into pyruvate and can provide ATP rapidly. When oxygen-dependent mitochondrial metabolism cannot meet the circumstances or when cellular conditions favour it, pyruvate can be converted to lactate, allowing continued regeneration of NAD+ required for glycolysis. Glycogenesis stores glucose as glycogen, particularly in liver and skeletal muscle, while glycogenolysis releases glucose units from glycogen. Gluconeogenesis synthesises glucose from non-carbohydrate precursors and is particularly important during fasting. The liver plays a major role in maintaining blood glucose, whereas skeletal muscle primarily uses its glycogen locally. Hormones including insulin, glucagon and adrenaline coordinate carbohydrate metabolism according to nutritional and physiological conditions.
Lipid metabolism provides both energy and essential cellular materials. Fatty acids can undergo beta-oxidation, producing acetyl-CoA and reduced electron carriers that can support ATP generation. Triacylglycerols are major energy-storage molecules because their reduced carbon content permits high energy density. During fasting or prolonged energy demand, stored fatty acids are mobilised and oxidised. The liver can convert acetyl-CoA into ketone bodies under particular metabolic conditions, providing an alternative fuel for several tissues.
Lipid synthesis is also important because cells require fatty acids, phospholipids and sterols for membranes and other functions. Cholesterol serves as a membrane component and precursor for steroid hormones and bile acids. Lipid metabolism therefore encompasses energy storage, energy production, membrane construction and signalling rather than simply the accumulation or removal of body fat.
Amino-acid metabolism serves both nutritional and metabolic purposes. Proteins are broken down into amino acids, which can be incorporated into new proteins or used in other biochemical pathways. Unlike carbohydrates and lipids, amino acids contain nitrogen, so their degradation requires management of nitrogen-containing products. In humans, excess nitrogen is largely converted into urea in the liver and excreted through the kidneys. Carbon skeletons remaining after removal of amino groups can enter pathways involved in energy production or biosynthesis.
Some amino acids are essential because humans cannot synthesise sufficient quantities of them and therefore must obtain them through diet. Amino acids also act as precursors for neurotransmitters, hormones, nucleotides and other molecules. Protein metabolism is therefore integrated with both energy balance and the synthesis of numerous specialised biological compounds.
Metabolic pathways are strongly compartmentalised. Glycolysis occurs primarily in the cytosol, while many reactions of aerobic energy metabolism occur within mitochondria. Fatty-acid synthesis and oxidation are separated into different cellular contexts, helping prevent uncontrolled simultaneous operation of opposing pathways. Organelles, membranes and transport systems therefore influence which metabolites can encounter particular enzymes. Metabolic flux describes the rate at which material passes through a pathway, and flux can change rapidly in response to substrate availability, enzyme activity and regulatory signals. Cells can redirect intermediates between competing pathways according to demand. This network behaviour means that changing one metabolic reaction can influence many others. Metabolism is consequently best understood as a regulated system of material flows rather than a fixed sequence of independent reactions.
Hormonal regulation coordinates metabolism across tissues and changes with nutritional state. After food intake, insulin promotes processes associated with glucose utilisation, storage and biosynthesis, while reducing several processes associated with mobilisation of stored fuels. During fasting, lower insulin and increased glucagon signalling favour processes that maintain circulating fuel availability, including glycogen breakdown and gluconeogenesis. Adrenaline can rapidly mobilise energy stores during acute physiological demand. Other hormones and signalling pathways integrate metabolism with growth, stress, reproduction and circadian rhythms. The liver, skeletal muscle, adipose tissue, brain and other organs have distinct metabolic roles and fuel preferences. Whole-body metabolism therefore emerges from communication among tissues rather than from one central metabolic organ or pathway.
Metabolic dysfunction contributes to many diseases. Diabetes involves abnormal regulation of glucose metabolism, with mechanisms differing among disease types and individuals. Obesity involves complex interactions among energy intake, expenditure, adipose biology, hormones, genetics, behaviour and environment rather than a single metabolic defect. Inherited metabolic disorders can result from defects in enzymes, transporters or cofactors and may cause accumulation of toxic intermediates or deficiency of essential products. Mitochondrial disorders can impair energy production, particularly affecting tissues with high energy demands. Cancer cells can also display altered metabolic patterns that support rapid growth and survival, although tumour metabolism remains diverse and context-dependent. Metabolic measurements, including glucose, lipids, lactate, ketone bodies and numerous other metabolites, can provide information about physiological or pathological states.
