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Introduction to metabolism review

Key Terms

MetabolismThe sum of all chemical reactions that take place inside a cell
AnabolismThe synthesis of complex molecules from simpler ones
CatabolismThe breakdown of complex molecules into simpler ones
Exergonic reactionA reaction that releases energy
Endergonic reactionA reaction that consumes energy
ATPAdenosine triphosphate, the primary energy carrier in living things


Metabolism is a set of chemical reactions that interconnect in a series of pathways. It is a balancing act between the building and breakdown of molecules in the body.
Type of metabolismProcessEnergeticsExample
AnabolismBuilds complex molecules from simple onesEndergonicProduction of new body tissues
CatabolismBreaks down complex molecules into simpler onesExergonicDigestion of food
Anabolic pathway: small molecules are assembled into larger ones. Energy is typically required.
Catabolic pathway: large molecules are broken down into small ones. Energy is typically released.
Anabolic vs catabolic pathways. Image from OpenStax Biology, CC BY 4.0


Catabolism: Cellular respiration

Cellular respiration is a catabolic process during which glucose is broken down to release usable energy for a cell.
The overall reaction for cellular respiration is:
C6H12O6 + 6O2 6CO2 + 6H2O + energy
As in all catabolic processes, cellular respiration releases energy which can then be harnessed and used by other reactions in the cell.

Anabolism: Photosynthesis

Photosynthesis is an anabolic process during which plants use energy from sunlight to convert carbon dioxide gas and water into sugar molecules.
The overall reaction for photosynthesis is:
6CO2 + 6H2O + energy C6H12O6 + 6O2
Photosynthesis, like all anabolic processes, consumes energy as glucose is built. Some of this glucose is used by the plant itself, or it can be used as a food source for other organisms.

Common mistakes and misconceptions

  • Many metabolic reactions occur at the same time. Thousands of metabolic reactions are being carried out in your body at the same time. They are all coordinated with one another and regulated carefully to keep an organism functioning properly.

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