What Is Metabolism?

How Do We Actually Turn Food into Energy?

We hear the word used all the time, read diet books on how to boost ours, and lament its slowing as we age. But what exactly is metabolism?

The Cell

Metabolism is the collection of reactions that take place within the cells of an organism. These reactions that build up and tear down materials are going on constantly in each and every cell in your body.

Anabolic reactions of metabolism (anabolism) build complex molecules from simple substances and typically require an input of energy in order to take place. Catabolic reactions, or catabolism, break down complex molecules into simpler ones often resulting in a release of energy.

In the metabolic process, food molecules (most commonly glucose), are broken down in our cells and the energy released is transformed into ATP, the currency of cellular energy.

What is ATP?

Adenosine triphosphate (ATP) is an organic nucleotide that is the universal unit of energy used in all living cells. This molecule is produced and broken down in the metabolic reactions of all living systems. ATP store stores and transports the energy we need to do just about everything that we do.

Adenosine Phosphates

Nucleotides can have different numbers of phosphate groups attached to them, and the specific name of the nucleotide reflects its number of phosphate groups:

  • Adenosine monophosphate (AMP) has one phosphate group.
  • Adenosine diphosphate (ADP) has two phosphate groups.
  • Adenosine triphosphate (ATP) has three phosphate groups.
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Why Are the Phosphate Groups Important?

These molecules can transport energy because phosphate bonds contain a lot of potential energy, the energy that is released when the bonds are broken.

How Is ATP Made?

ATP is made both during the anabolic reactions of photosynthesis and the catabolic reactions of cellular respiration.

Photosynthesis is the process of making food from sunlight energy, carbon dioxide, and water. The self-sufficient organisms that can do this neat trick are called autotrophs and include plants, some bacteria, and some tiny single-celled animal-like creatures called protists.

Heterotrophs, like us, are organisms that cannot make organic compounds from inorganic sources. We must obtain our food by consuming other organisms. Heterotrophs include animals, fungi, and some protists and bacteria.

In addition to being produced by autotrophs during photosynthesis, ATP is produced by both autotrophs and heterotrophs during a catabolic process known as cellular respiration.

Cellular Respiration

Cellular respiration is a series of reactions that make ATP by completely breaking down glucose into inorganic molecules of carbon dioxide and water. These reactions happen in several stages and include:

  • glycolysis
  • synthesis of Acetyl-CoA
  • Krebs cycle
  • the electron transport chain

Additional Cell Biology Resources

For more information on cell biology and metabolism see Science Prof Online and the book Ultrametabolism by Dr. Mark Hyman provides a fascinating insight into how diet affects metabolism.


  • Bauman, R. (2005) Microbiology.
  • Park Talaro, K. (2008) Foundations in Microbiology.

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