Metabolism in Living Organisms: Importance and Evolution of Anaerobic Processes

Metabolism in living organisms is the foundation of the existence of protein bodies, and this mode of existence consists in the continuous self-renewal of the chemical components of living things. Metabolism is the complete set of chemical activities carried out in the cells and organs of every living organism, converting food into energy, building the molecules of the body, and eliminating waste products. It is the single trait shared by all life, from bacteria such as Escherichia coli to the most complex creatures.

Metabolism in living organisms: definition and essence

Metabolism is the sum of all chemical reactions that sustain life, encompassing two opposing but linked activities: the breakdown of substances (catabolism) and the construction of new ones (anabolism). The term Metabolism, from the Greek word for "change," describes how organisms take something from their surroundings and constantly return something to it. This exchange of matter and energy powers growth, reproduction, movement, and the maintenance of internal balance, or homeostasis. Because the same core pathways appear across species, metabolic processes are among the most evolutionarily conserved features of biology.

What is life

On parchment darkened by time, on clay tablets and sheets of papyrus, one finds the thoughts of ancient sages about what life is and how it arose.

Writers and poets have devoted countless lines to depicting and explaining life. For many centuries scholars have persistently and methodically probed its mysteries. From the definitions of life alone one could compile a thick book.

Metabolism as the principal sign of life

The one universal sign of life, present in every organism from the simplest microbes to the most highly organized creatures, is metabolism — the constant self-renewal of the components of the body. Many statements of the ancients contained accurate observations reflecting the diversity of life's manifestations, yet they lacked the essential thing: a common trait characteristic of every form of life. Modern achievements in biochemistry and physiology have fully confirmed that this trait is metabolism. All other properties of life — irritability (the capacity to respond to environmental stimuli), growth, development, and reproduction — are merely different expressions flowing from its basic property, self-renewal.

Self-renewal of living organisms

Self-renewal of living organisms consists of two processes occurring simultaneously within the body: the destruction of existing organic matter and the creation of new. The substances of any organism's body continuously break down, and at the same time new substances, similar to those destroyed, arise within it. These two sides of an organism's life activity — the destructive and the creative — are inseparably linked and form a single process of life.

Destruction and creation in living and non-living nature

Destruction and creation are observed everywhere, including in non-living nature, where they are tied to the action of water, wind, and glaciers. For example, through weathering a granite cliff gradually turns into rubble and even into sand. From this material new hard rocks may later form, but they will no longer be the former granite. Destruction within a living organism is entirely different: here the breakdown of matter is the source from which new organic matter arises, and it is therefore the basic condition for the organism's survival. If breakdown ceases, the formation of new living matter ceases at the same time, and death sets in.

Dissimilation and assimilation

The destruction and breakdown occurring in the body of a living organism are called dissimilation, while the opposite process — the formation of new matter — is called assimilation. The essence of life lies in dissimilation and assimilation, or, as it is put, in the exchange of matter and energy. For an organism always receives something from its surroundings and constantly gives something back. This is the qualitative distinction between the living and the non-living, since no non-living body carries out such an exchange of matter.

Catabolism and anabolism: modern terminology

In modern biochemistry, dissimilation is termed Catabolism and assimilation is termed Anabolism, and the two together constitute Metabolism. Catabolism breaks large molecules down into smaller ones and releases stored chemical energy; Anabolism uses that energy to assemble complex molecules such as Proteins and DNA from simpler building blocks. Enzymes — protein catalysts — drive both directions, lowering the energy barrier of each reaction so that pathways proceed at controlled, rapid rates within cells.

• Catabolism: breakdown of Carbohydrates, Lipids, and Proteins to release energy and simpler units.
• Anabolism: synthesis of new tissue, storage compounds, and molecules such as Nucleic Acids and RNA using captured energy.

These pathways are organized and controlled through compartmentalization: many oxidative reactions take place inside the Mitochondria, while others, such as glycolysis, occur in the cytosol. The direction of a reaction depends on chemical equilibrium and Gibbs free energy — reactions proceed toward states of lower free energy, and the cell links energy-releasing steps to energy-requiring ones to keep building order out of disorder.

Dissimilation as oxidation and the release of energy

Dissimilation is ultimately the combination of the organic substances of the living body with oxygen — that is, oxidation — through which the potential energy hidden within them is released. For this reason dissimilation is sometimes compared to combustion, but the two are not identical. Combustion is also oxidation, yet it proceeds comparatively quickly, and almost all the chemical energy of the burning body passes directly from its latent state into heat. Living cells instead capture that energy gradually, avoiding uncontrolled combustion and storing it in usable form.

The energy released during dissimilation may appear as the energy of movement, drive various chemical reactions in which it passes from one form to another, be stored "in reserve," or even be converted into electrical energy; some processes run faster and others slower. Only in the end do all forms of energy pass into heat — here, as everywhere, the law of conservation of energy holds, a principle whose foundations trace to work by Émilie du Châtelet. Assimilation, the transformation of the matter of food into the body of the organism, occurs at the expense of the energy freed during dissimilation. Such is the essence of an organism's self-renewal.

Metabolic energy and the law of conservation of energy

The energy of metabolism obeys the laws of thermodynamics: energy is neither created nor destroyed but only converted from one form to another, and every conversion ends partly as heat. In biology this energy is measured in calories, and the amount of energy a food yields reflects how completely it can be oxidized. Cells do not release this energy all at once; they capture it in small, controlled steps and store it in a chemical carrier that can be spent wherever the cell needs it.

ATP — the universal energy currency of the cell

Adenosine triphosphate (ATP) is the universal energy currency of every living cell, carrying chemical energy in the bonds between its phosphate groups. When a cell needs energy — for muscle contraction, active transport, or building molecules — it breaks a phosphate bond of ATP, releasing energy for the task. Because ATP is constantly spent and remade, the same molecules cycle through the body many times each day, linking the energy-releasing reactions of Catabolism to the energy-consuming reactions of Anabolism.

ATP synthesis and cellular respiration

ATP synthesis takes place chiefly through cellular respiration, the multi-stage oxidation of glucose and other fuels inside the cell. Glucose is first split during glycolysis in the cytosol; the products then enter the Mitochondria, where the Krebs cycle (also called the Citric Acid Cycle) oxidizes them fully. Electron carriers such as NAD+ (Nicotinamide Adenine Dinucleotide), built from Vitamin B3 (Niacin), collect electrons during these redox reactions and deliver them to the Electron Transport Chain, which powers the bulk of ATP production. The Krebs cycle is named for Hans Krebs, and the study of fatty-acid oxidation traces to Franz Knoop, whose work underpins modern understanding of Lipid breakdown.

Aerobic and anaerobic metabolism

Metabolism can proceed with or without oxygen, and the two modes yield very different amounts of energy. Aerobic metabolism uses oxygen to oxidize fuels completely, producing a large amount of ATP; anaerobic metabolism operates without oxygen and yields far less. When muscles work harder than the oxygen supply allows, they fall back on anaerobic pathways and produce Lactic acid, which can lower the pH of the Blood. This connection between energy production and acidity ties cellular respiration directly to the body's Acid Base Balance.

Types of metabolism

Metabolism is commonly divided by the class of nutrient it processes — carbohydrate, protein, and lipid metabolism — each handled by its own pathways and regulatory organs. Digestion prepares these nutrients through three stages: mechanical and chemical breakdown of food, absorption of the resulting building blocks, and cellular processing into energy or new tissue. The Pancreas and the Liver are the central organs coordinating these pathways.

Carbohydrate metabolism and the regulation of glucose

Carbohydrate metabolism centers on Glucose, the primary fuel of the body and the near-exclusive fuel of the brain. Excess glucose is stored as Glycogen in the Liver and muscle, and released again when needed. Two hormones from the Pancreas regulate blood glucose: Insulin lowers it by promoting uptake and storage, while Glucagon raises it by releasing stored glycogen. When this control fails, the result is Diabetes, one of the most common metabolic disorders. In germinating grain the same chemistry appears in miniature: the starch of the grain is converted into sugar and becomes food for the awakening cells of the embryo, just as the concentration of stored energy in the kernel of corn fuels early growth.

Protein metabolism and amino acid metabolism

Protein metabolism breaks dietary Proteins down into Amino acids, which the body reassembles into its own proteins or uses for energy. Transamination reactions transfer amino groups between molecules, and the nitrogen stripped from surplus amino acids is converted into a harmless waste through the Urea Cycle in the Liver, after which the Kidneys excrete it. Proteins remain the foundation of life: the protoplasm of cells, muscles, tendons, ligaments, vessel walls, internal organs, horns, hooves, claws, skin, and hair of animals are all built from various proteins.

Although these proteins differ greatly in appearance and properties, they share a single elemental composition — carbon, hydrogen, oxygen, and nitrogen — and a molecular structure peculiar to proteins alone. The protein molecule continuously restores itself during assimilation: as protein breaks down, it creates new protein similar to itself, and in this the basic property of self-renewal is expressed.

Lipid metabolism and beta-oxidation of fatty acids

Lipid metabolism supplies the most energy-dense fuel and builds the membranes of every cell. Fats are broken down by beta-oxidation, a mitochondrial process that strips fatty acids two carbons at a time to feed the Krebs cycle. Lipids also serve as the carriers of cholesterol through the Blood and as the structural basis of biological membranes. Because fat molecules are volatile, Fat-related food smells strongly shape human food preferences, and olfactory perception guides the appetite that drives nutrient intake. Sensory programming during Early childhood exposure can set long-lasting food preferences, linking developmental critical periods to lifelong metabolic and health outcomes.

Basal metabolism (basal metabolic rate)

Basal metabolic rate is the amount of energy the body spends at complete rest simply to stay alive — powering the heart, brain, kidneys, and the constant turnover of tissue. It represents the largest share of daily calorie burning for most people and sets the baseline rate at which the body processes calories. Metabolic activity also varies throughout the day, rising with movement, digestion, and temperature and falling during rest.

Factors affecting the rate of metabolism

Basal metabolic rate is shaped by several measurable factors, and understanding them dispels common myths about "fast" and "slow" metabolism. Body weight is influenced far more by total energy balance than by small differences in resting rate, so the idea that a naturally slow metabolism alone causes weight gain is largely a myth.

• Age: metabolic rate generally declines with aging as lean tissue decreases.
• Body composition: muscle burns more energy at rest than fat.
• Environmental factors: temperature and activity level raise or lower demand.
• Genetics and hormones: inherited traits and hormone levels set individual baselines.

Metabolic disorders and diseases

Metabolic disorders arise when the balance between anabolism and catabolism breaks down, often because a single enzyme is missing or faulty. Many are inherited metabolic diseases with genetic origins, detectable before birth through genetic testing and amniocentesis. Well-documented examples include Phenylketonuria (PKU), Tay-Sachs disease, Niemann-Pick disease, and Hemochromatosis, an iron-overload condition affecting Hemoglobin-related iron handling. Obesity, with its many risk factors, and Diabetes represent metabolic risks that are common in the wider population. Beyond inherited disease, cancer cell metabolism reprograms energy pathways to fuel rapid growth, and understanding these shifts has opened applications in cancer treatment and biotechnology, including genetic manipulation to intervene in disease. This work draws on reference sources such as StatPearls Publishing, indexed by the NCBI at the National Library of Medicine and the National Institutes of Health, and peer-reviewed collections like Essays in Biochemistry.

The importance of metabolism for health and nutrition

Metabolism ties directly to health because it depends on a steady supply of essential nutrients, and its balance keeps the internal environment stable. Vitamins and minerals act as cofactors and coenzymes: Vitamin B3 is built into NAD+, iron is required for Hemoglobin, and trace elements such as boron, manganese, copper, zinc, bromine, arsenic, cobalt, and strontium — even in vanishingly small amounts — enhance an organism's vital activity and intensify metabolism. This principle underlies the method of micro-fertilization used to raise crop yields, in which minute quantities of substances containing these elements improve plant nutrition.

Metabolism also maintains the body's Acid Base Balance, defined as the regulation of hydrogen ion concentration measured on the pH scale. When too many Hydrogen ions accumulate, the result is Acidosis; when too few, Alkalosis — each with respiratory and metabolic causes of clinical significance. The body defends its pH through buffer systems in the Blood, chiefly the Bicarbonate buffer, supported by regulation from the Lungs (which expel carbon dioxide) and the Kidneys (which adjust bicarbonate and excrete acid). Water, formed from hydrogen and oxygen, is central to all of this: it fills the protoplasm of cells, mediates the interaction of proteins and other substances, and enables metabolism and the organism's exchange with its surroundings.

Conclusion

Metabolism is the defining feature of life: the ceaseless coupling of Catabolism and Anabolism through which every organism renews itself, converts food into ATP, builds Proteins and DNA, moves, grows, reproduces, and maintains homeostasis. From the germinating wheat grain to the human brain running on glucose, the same conserved chemistry links energy, matter, and health.