Metabolism

Cell metabolism refers to the complete set of life-sustaining chemical reactions that occur within living organisms. These reactions enable cells to extract energy from nutrients, synthesise essential biomolecules, eliminate waste, and maintain the structural and functional integrity required for survival. Metabolism encompasses both the intracellular reactions termed intermediary metabolism and the broader physiological processes such as digestion and transport between tissues.

Fundamental Roles and Organisation

The principal functions of metabolism are the conversion of food-derived energy into usable cellular energy, the synthesis of building blocks for macromolecules such as proteins, carbohydrates, lipids, and nucleic acids, and the disposal of metabolic wastes. Metabolic reactions are catalysed by enzymes, which accelerate reaction rates and couple energy-requiring processes to energy-releasing ones. This coupling ensures that energetically unfavourable reactions proceed in a coordinated and regulated manner.
The metabolic capacities of organisms determine which substances they can use as nutrient sources. While hydrogen sulphide is toxic to animals, certain prokaryotes are capable of using it as a nutrient. The total energy expenditure of these biochemical reactions is measured as the organism’s basal metabolic rate.
A notable feature of metabolism is its evolutionary conservation. Core pathways such as the citric acid cycle and glycolysis are found in organisms as diverse as bacteria and large mammals. Their persistence reflects their early origin and functional efficiency. Conversely, disruption of metabolic processes is characteristic of several diseases, including metabolic syndrome, type II diabetes, and many cancers, where altered metabolic fluxes support abnormal cellular growth and proliferation.

Major Classes of Biomolecules

Metabolic pathways act upon four primary classes of biomolecules—amino acids, lipids, carbohydrates, and nucleic acids—either breaking them down for energy or synthesising them for structural or regulatory functions.
Amino acids and proteinsProteins consist of linear chains of amino acids joined by peptide bonds and perform a range of roles including enzymatic catalysis, structural support, cell signalling, transport across membranes, and DNA regulation. When necessary, amino acids can be degraded to provide carbon skeletons for entry into the citric acid cycle, supplying energy during periods of metabolic stress or glucose shortage.
LipidsLipids form the most structurally diverse group of biomolecules. They are crucial for membrane architecture, energy storage, and signalling. Comprising largely non-polar aliphatic chains with small polar regions, lipids dissolve in organic solvents but not in water. Triglycerides—glycerol bound to three fatty acids—serve as major energy stores. Phospholipids and sphingolipids form membrane bilayers, while steroids constitute important signalling molecules.
CarbohydratesCarbohydrates serve both structural and energy-related functions. These aldehydes or ketones with multiple hydroxyl groups can adopt linear or ring forms. Glucose, fructose, and galactose are key monosaccharides that can be polymerised into polysaccharides such as glycogen, starch, cellulose, and chitin. Carbohydrates are widely used for energy storage and transport as well as for building structural frameworks in cells and tissues.
Nucleotides and nucleic acidsNucleic acids—DNA and RNA—are polymers of nucleotides composed of a phosphate group, a ribose or deoxyribose sugar, and a nitrogenous base. DNA stores and transmits genetic information, while RNA supports gene expression through roles in transcription, translation, and catalysis. Nucleotides also function as metabolic regulators and coenzymes, participating in group-transfer reactions essential for cellular chemistry.

Coenzymes and Energy Transfer

Many metabolic reactions involve the transfer of functional groups, which requires specialised coenzymes acting as carriers. These intermediates are produced and consumed continuously and are regenerated through cyclical enzymatic reactions.
A central coenzyme is adenosine triphosphate (ATP), the primary energy currency of cells. Although present in limited quantities, ATP is regenerated so frequently that an adult human cycles through roughly his or her own body weight in ATP daily. ATP links catabolism and anabolism: while catabolic pathways generate ATP, anabolic pathways consume it. ATP also facilitates phosphorylation, a key regulatory mechanism in metabolism.
Other essential coenzymes include NAD⁺/NADH and NADP⁺/NADPH. Derived from vitamin B₃, these molecules function as hydrogen carriers. NAD⁺ primarily supports catabolic processes by accepting electrons, whereas NADPH powers anabolic reactions requiring reducing equivalents. Vitamins serve as precursors for many coenzymes and are therefore indispensable in diet.

Mineral Elements and Cofactors

Inorganic elements are vital to metabolic function, often acting as cofactors for enzymes. Common ions such as sodium, potassium, calcium, and chloride play roles in osmotic balance, nerve conduction, and structural integrity. Trace elements such as iron, zinc, and copper support redox reactions, enzyme stability, and electron transport. Approximately 99% of the human body is composed of carbon, hydrogen, oxygen, nitrogen, calcium, phosphorus, sodium, chlorine, and potassium, reflecting their central biochemical roles.

Catabolic Processes

Catabolism comprises reactions that break down complex molecules into simpler ones, releasing energy that is captured in ATP, GTP, NADH, or FADH₂. Cellular respiration is the archetypal catabolic process, incorporating glycolysis, the citric acid cycle, and oxidative phosphorylation. These pathways convert glucose and other nutrients into carbon dioxide, water, and usable energy. All cells carry out glycolysis, and most organisms supplement this with aerobic respiration when oxygen is available. Photosynthetic organisms additionally convert light energy into chemical energy to synthesise organic molecules.

Anabolic Processes

Anabolism encompasses the biosynthesis of macromolecules required for cell growth, repair, and maintenance. Because anabolic reactions are energetically unfavourable, they must be coupled with the energy released by catabolic processes. Examples include the synthesis of proteins from amino acids, nucleic acids from nucleotides, and polysaccharides from monosaccharides. Gluconeogenesis, the formation of glucose from non-carbohydrate sources, illustrates how anabolic pathways rely on unique enzymes to overcome irreversible steps found in catabolism.

Amphibolic Processes

Certain pathways, such as the citric acid cycle, perform dual roles and are therefore described as amphibolic. They supply energy through the oxidation of acetyl-CoA while simultaneously generating precursor molecules for biosynthesis. The glyoxylate cycle, seen in plants and some bacteria, represents a modified form that preserves carbon skeletons during growth on non-carbohydrate substrates.

Regulation of Metabolism

Metabolic regulation ensures that pathways operate efficiently and in coordination with cellular needs. Key regulatory mechanisms include:

• Rate-determining steps, which set pathway flux and are often controlled by feedback inhibition.
• Covalent modification of enzymes, such as phosphorylation, enabling rapid and reversible activity changes.
• Allosteric regulation, where small molecules bind to enzymes and alter their activity through non-covalent interactions.
• Flux regulation, determined by substrate availability, enzyme concentration, and transport across membranes.

Advanced analytical techniques, including carbon-13 metabolic flux analysis assessed by nuclear magnetic resonance or mass spectrometry, allow quantitative examination of metabolic activity by mapping isotope distribution in amino acids and other metabolites.
Through its integrated network of reactions, cell metabolism provides the foundation for growth, adaptation, reproduction, and survival in all forms of life.