Anabolism and Catabolism: Definition and Examples

How Anabolism and Catabolism Work Together for Metabolism

Anabolic exercise helps build muscle strength and endurance.
Anabolic exercise helps build muscle strength and endurance. Hans Berggren / Getty Images

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 body building, 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.

Key Takeaways: Anabolism and Catabolism

  • Anabolism and catabolism are the two broad classes of biochemical reactions that make up metabolism.
  • Anabolism is the synthesis of complex molecules from simpler ones. These chemical reactions require energy.
  • Catabolism is the breakdown of complex molecules into simpler ones. These reactions release energy.
  • Anabolic and catabolic pathways typically work together, with the energy from catabolism providing the energy for anabolism.

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 mechanism behind anabolism.

Anabolism Examples

Anabolic reactions are those which build complex molecules from simple ones. Cells used these processes to make polymers, grow tissues, 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 + H2
  • 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 the 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 citric acid cycle are examples of amphibolic pathways. These cycles can either produce energy or use it, depending on cellular needs.

Sources

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  • Berg, Jeremy M.; Tymoczko, John L.; Stryer, Lubert; Gatto, Gregory J. (2012). Biochemistry (7th ed.). New York: W.H. Freeman. ISBN 9781429229364.
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