Metabolism is the totality of chemical reactions that sustain life within cells and organisms. These reactions transform nutrients, capture and use energy, construct cellular components, and break down substances for recycling or removal. Metabolism comprises both degradative and synthetic processes, organized into interconnected pathways rather than isolated reactions. It provides the chemical basis for growth, reproduction, repair, and the maintenance of homeostasis, and is a central subject of biochemistry. (openstax.org)
Catabolism and anabolism
Metabolism is conventionally divided into catabolism and anabolism. Catabolic pathways break down compounds into simpler products, often releasing free energy that cells capture in usable forms. Anabolic pathways synthesize complex compounds from smaller precursors and generally require energy and reducing power. Examples include the breakdown of sugars and fats, and the synthesis of proteins from amino acids. These processes are coupled: degradation supplies both energy and building materials for biosynthesis. (openstax.org)
A metabolic pathway is a sequence of reactions in which the product of one step becomes a substrate for another. Pathways may be linear, branched, or cyclic, and their intermediate compounds are called metabolites. Some pathways serve both breakdown and biosynthesis and are described as amphibolic. For example, the citric acid cycle participates in fuel oxidation while supplying intermediates used to make other cellular substances. Consequently, metabolism is better understood as a network with shared intermediates than as two separate sets of reactions. (openstax.org)
Enzymes and energy coupling
Most metabolic reactions are accelerated by enzymes, biological catalysts that recognize particular substrates. Enzymes lower the activation-energy barrier, allowing reactions to occur rapidly under cellular conditions. They do not change a reaction’s equilibrium or make an energetically unfavorable transformation favorable by themselves. Reaction direction depends on Gibbs free energy, including the effects of reactant and product concentrations. (ncbi.nlm.nih.gov)
Cells drive unfavorable reactions by coupling them to favorable ones. Adenosine triphosphate (ATP) is a major intermediary in this coupling. Its hydrolysis can be linked to biosynthesis, movement, or transport across membranes, while energy-yielding processes regenerate ATP from ADP and phosphate. The favorable energy change belongs to the overall hydrolysis reaction; breaking a chemical bond alone requires energy. Other carriers, including NADH and NADPH, transfer electrons between reactions. NADH commonly supports ATP production, whereas NADPH frequently supplies reducing power for biosynthesis. (ncbi.nlm.nih.gov)
Central metabolic pathways
Glycolysis converts one molecule of glucose into two molecules of pyruvate, with a net production of two ATP and two NADH. It occurs in the cytosol and does not directly require oxygen. In aerobic metabolism, pyruvate can be converted into acetyl-CoA, which enters the citric acid cycle. Fatty-acid breakdown and the degradation of some amino acids also feed into this central network. (ncbi.nlm.nih.gov)
During aerobic cellular respiration, electrons from reduced carriers pass through an electron-transport chain and ultimately reduce oxygen to water. Electron transfer establishes an electrochemical gradient across a membrane. ATP synthase uses this gradient to make ATP, a process called oxidative phosphorylation. In eukaryotic cells, these reactions occur at the inner membrane of the mitochondrion; in respiratory prokaryotes, they occur at the plasma membrane. (ncbi.nlm.nih.gov)
Fermentation provides another means of regenerating oxidized electron carriers, allowing glycolysis to continue without a respiratory electron-transport chain. Depending on the organism and pathway, products include lactate or ethanol. Anaerobic respiration differs from fermentation: it employs an electron-transport chain but uses a terminal electron acceptor other than oxygen. Central pathways also supply precursors for making sugars, lipids, amino acids, and nucleotides. (ncbi.nlm.nih.gov)
Regulation and compartmentalization
Metabolic activity changes with nutrient availability, energy demand, and environmental conditions. Rapid regulation occurs through substrate availability, binding of regulatory molecules to enzymes, and chemical modification of enzymes. In feedback inhibition, a pathway’s product inhibits an earlier reaction, limiting further production. Longer-term adjustments involve changes in gene expression and therefore in the abundance of enzymes and transport proteins. (openstax.org)
Compartmentalization helps separate reactions and control the movement of metabolites. Glycolysis and mitochondrial oxidation, for example, occupy different cellular locations connected by transport systems. In multicellular organisms, signals coordinate metabolism between tissues. Insulin promotes nutrient utilization and storage, while glucagon stimulates hepatic glucose production, including through glycogen breakdown and gluconeogenesis. Such coordination allows different organs to contribute to the maintenance of circulating fuel supplies. (ncbi.nlm.nih.gov)
Metabolic diversity
Organisms differ in their sources of energy, carbon, and electrons. Phototrophs obtain energy from light, whereas chemotrophs obtain it from chemical reactions. Autotrophs use inorganic carbon, usually carbon dioxide, to build organic compounds; heterotrophs obtain carbon from organic substances. These classifications describe separate properties: using light does not necessarily imply using carbon dioxide as the carbon source. (openstax.org)
Photosynthesis illustrates light-driven metabolism, while diverse microorganisms obtain energy by oxidizing organic or inorganic compounds. Some can change metabolic modes as conditions change. Despite this diversity, many organisms share central pathways and molecular carriers, reflecting the conservation of important biochemical mechanisms across evolutionary lineages. (openstax.org)
Study and measurement
Metabolism is investigated through enzyme assays, measurements of substrate consumption and product formation, and analysis of cellular metabolites. Metabolomics examines many metabolites together to characterize biochemical states and responses to genetic or environmental changes. Techniques include mass spectrometry and nuclear magnetic resonance spectroscopy. Metabolite abundance and reaction rate are distinct quantities: a concentration measurement describes a pool of material, whereas metabolic flux describes its movement through reactions over time. (ebi.ac.uk)