Anabolism and Catabolism Definition and Examples

Anabolism and catabolism are the two broad types of biochemical reactions that make up metabolism. Anabolism builds complex molecules from simpler ones, while catabolism breaks large molecules into smaller ones.

Most people think of metabolism in the context of weight loss and bodybuilding, but metabolic pathways are important for every cell and tissue in an organism. Metabolism is how a cell gets energy and removes waste. Vitamins, minerals, and cofactors aid the reactions.

Anabolism Definition

Anabolism or biosynthesis is the set of biochemical reactions that construct molecules from smaller components. Anabolic reactions are endergonic, meaning they require an input of energy to progress and are not spontaneous. Typically, anabolic and catabolic reactions are coupled, with catabolism providing the activation energy for anabolism. The hydrolysis of adenosine triphosphate (ATP) powers many anabolic processes. In general, condensation and reduction reactions are the mechanisms behind anabolism.

Anabolism Examples

Anabolic reactions are those that build complex molecules from simple ones. Cells use these processes to make polymers, grow tissue, and repair damage. For example:

• Glycerol reacts with fatty acids to make lipids:
  CH₂OHCH(OH)CH₂OH + C₁₇H₃₅COOH  →  CH₂OHCH(OH)CH₂OOCC₁₇H₃₅ 
• Simple sugars combine to form disaccharides and water:
  C₆H₁₂O₆ + C₆H₁₂O₆   →  C₁₂H₂₂O₁₁ + H₂O
• Amino acids join together to form dipeptides:
  NH₂CHRCOOH + NH₂CHRCOOH →  NH₂CHRCONHCHRCOOH + H₂O 
• Carbon dioxide and water react to form glucose and oxygen in photosynthesis:
  6CO₂ + 6H₂O  →  C₆H₁₂O₆ + 6O₂

Anabolic hormones stimulate anabolic processes. Examples of anabolic hormones include insulin, which promotes glucose absorption, and anabolic steroids, which stimulate muscle growth. Anabolic exercise is anaerobic exercise, such as weightlifting, which also builds muscle strength and mass.

Catabolism Definition

Catabolism is the set of biochemical reactions that break down complex molecules into simpler ones. Catabolic processes are thermodynamically favorable and spontaneous, so cells use them to generate energy or to fuel anabolism. Catabolism is exergonic, meaning it releases heat and works via hydrolysis and oxidation.

Cells can store useful raw materials in complex molecules, use catabolism to break them down, and recover the smaller molecules to build new products. For example, catabolism of proteins, lipids, nucleic acids, and polysaccharides generates amino acids, fatty acids, nucleotides, and monosaccharides, respectively. Sometimes waste products are generated, including carbon dioxide, urea, ammonia, acetic acid, and lactic acid.

Catabolism Examples

Catabolic processes are the reverse of anabolic processes. They are used to generate energy for anabolism, release small molecules for other purposes, detoxify chemicals, and regulate metabolic pathways. For example:

• During cellular respiration, glucose and oxygen react to yield carbon dioxide and water
  C₆H₁₂O₆ + 6O₂  →  6CO₂ + 6H₂O
• In cells, hydroxide peroxide decomposes into water and oxygen:
  2H₂O₂  →  2H₂O + O₂

Many hormones act as signals to control catabolism. The catabolic hormones include adrenaline, glucagon, cortisol, melatonin, hypocretin, and cytokines. Catabolic exercise is aerobic exercise such as a cardio workout, which burns calories as fat (or muscle) is broken down.

Amphibolic Pathways

A metabolic pathway that can be either catabolic or anabolic depending on energy availability is called an amphibolic pathway. The glyoxylate cycle and the citric acid cycle are examples of amphibolic pathways. These cycles can either produce energy or use it, depending on cellular needs.

Sources

• Alberts, Bruce; Johnson, Alexander; Julian, Lewis; Raff, Martin; Roberts, Keith; Walter, Peter (2002). Molecular Biology of the Cell (5th ed.). CRC Press.
• de Bolster, M. W. G. (1997). "Glossary of Terms Used in Bioinorganic Chemistry". International Union of Pure and Applied Chemistry.
• Berg, Jeremy M.; Tymoczko, John L.; Stryer, Lubert; Gatto, Gregory J. (2012). Biochemistry (7th ed.). New York: W.H. Freeman. ISBN 9781429229364.
• Nicholls D. G. and Ferguson S. J. (2002) Bioenergetics (3rd Ed.). Academic Press. ISBN 0-12-518121-3.
• Ramsey K. M., Marcheva B., Kohsaka A., Bass J. (2007). "The clockwork of metabolism". Annu. Rev. Nutr. 27: 219–40. doi:10.1146/annurev.nutr.27.061406.093546