Metabolism
Overview
Metabolism (Korean: 대사; Hanja: 代謝) is the totality of all chemical reactions occurring within an organism to sustain life. It encompasses catabolism, which breaks down nutrients to obtain energy; anabolism, which uses that energy to synthesize cellular components; and the processes that excrete waste products. The rate and pathways of metabolism are finely regulated by various factors, including genetic factors, hormones, diet, sleep, exercise, and gut microbiota.
Main content
Definition and history
The concept of 'metabolism' was established in the mid-19th century, centered on German physiologists. Theodor Schwann and Justus von Liebig, among others, attempted to explain material transformations within living organisms chemically, and in the 20th century, as enzymology and biochemistry developed, key pathways such as glycolysis, the citric acid cycle, and oxidative phosphorylation were successively elucidated. Hans Krebs won the 1953 Nobel Prize in Physiology or Medicine for identifying the citric acid cycle (TCA cycle), and this cycle is also called the 'Krebs cycle' after him.
Catabolism and anabolism
- Catabolism: It breaks down large molecules such as carbohydrates, fats, and proteins into smaller molecules and releases energy. Glycolysis, beta oxidation, protein degradation, and glycogen breakdown are representative examples.
- Anabolism: It consumes ATP and NADPH to synthesize amino acids, lipids, nucleic acids, proteins, and so on. Protein synthesis, fatty acid synthesis, gluconeogenesis, and photosynthesis belong here.
The two processes proceed simultaneously using different pathways and are balanced through hormones, allosteric regulation, and feedback inhibition.
Energy currency ATP
ATP (adenosine triphosphate) is the cell's energy currency, releasing about 7.3 kcal/mol of free energy when one phosphate bond is broken. When one molecule of glucose is completely oxidized, up to about 30–32 molecules of ATP are produced throughout the entire process of cellular respiration. ATP is used in almost all cellular activities, including muscle contraction, active transport, and biosynthesis, and its intracellular concentration is regenerated and consumed repeatedly on a timescale of seconds.
Major metabolic pathways
1. Glycolysis: In the cytoplasm, glucose is broken down into pyruvate, producing 2 ATP and 2 NADH. It is the most universal pathway and can proceed without oxygen.
2. Citric acid cycle (TCA cycle): In the mitochondrial matrix, acetyl-CoA is oxidized to produce NADH, FADH2, CO2, and a small amount of ATP.
3. Oxidative phosphorylation: This is the key step that produces ATP in large quantities using the electron transport chain and the chemiosmotic gradient.
4. Fatty acid oxidation and ketone body production: These become important energy sources during fasting or a low-carbohydrate state.
5. Urea cycle: It converts toxic ammonia produced by protein breakdown into urea for excretion in urine.
6. Photosynthesis: In plants and algae, this is the carbon assimilation pathway that converts light energy into chemical energy.
Metabolic regulation and hormones
Insulin lowers blood glucose after meals and promotes storage, while glucagon induces glycogen breakdown and gluconeogenesis during fasting. Cortisol breaks down proteins and fats under stress to raise blood glucose, and thyroid hormones regulate overall metabolic rate. Adrenaline promotes immediate breakdown reactions in response to sudden energy demand.
Basal metabolic rate
Basal metabolic rate (BMR) is the minimum energy expenditure required to sustain life in a state of complete rest, accounting for about 60–70% of total daily energy expenditure. Muscle mass, body composition, age, sex, thyroid function, and genetic factors have a major influence, and as people age, BMR tends to decrease due to reduced muscle mass.
Recent trends
In 2024–2025, metabolic research is rapidly expanding into single-cell-level precision analysis and clinical applications. With advances in single-cell metabolomics and spatial metabolomics, it has become possible to identify metabolic heterogeneity among cells within tissues, and research combining genome-scale metabolic models (GEMs) with machine learning to predict and design metabolic pathways is active. Clinically, GLP-1 drugs (semaglutide, tirzepatide) have changed the paradigm for treating obesity and metabolic diseases, and large-scale clinical trials verifying the metabolic effects of time-restricted eating and intermittent fasting are ongoing. Metabolic targeted therapy aimed at the Warburg effect in cancer, research on interactions between gut microbiota and host metabolism (microbiome), and stricter regulation of aging-related metabolic supplements such as NAD+ precursors are also major trends. In addition, as waist circumference, insulin resistance, and liver fat levels are emphasized as indicators for assessing metabolic health, management of 'metabolic syndrome' has emerged as a key agenda in preventive medicine.
Related topics
- [[Enzyme]]
- [[Mitochondria]]
- [[ATP]]
- [[Basal metabolic rate]]
- [[Glycolysis]]
- [[Gut microbiota]]
- [[Metabolic syndrome]]