Biology » Cell Metabolism » Energy and Metabolism

Introduction to Metabolism

By the end of this lesson and the next few, you should be able to:

  • Explain what metabolic pathways are and describe the two major types of metabolic pathways
  • Discuss how chemical reactions play a role in energy transfer

Life and the Need for Energy


A hummingbird needs energy to maintain prolonged periods of flight. The bird obtains its energy from taking in food and transforming the nutrients into energy. It achieves this through a series of biochemical reactions. The flight muscles in birds are extremely efficient in energy production. Image credit: modification of work by Cory Zanker

Virtually every task performed by living organisms requires energy. We need energy to perform heavy labor and exercise. But humans also use a great deal of energy while thinking, and even during sleep. In fact, the living cells of every organism constantly use energy. Furthermore, nutrients and other molecules are imported, metabolized (broken down) and possibly synthesized into new molecules, modified if needed, transported around the cell, and may be distributed to the entire organism.

For example, the large proteins that make up muscles are actively built from smaller molecules. Complex carbohydrates are broken down into simple sugars that the cell uses for energy. Just as energy is required to both build and demolish a building, energy is required for both the synthesis and breakdown of molecules. Additionally, signaling molecules such as hormones and neurotransmitters are transported between cells.

Pathogenic bacteria and viruses are ingested and broken down by cells. Cells must also export waste and toxins to stay healthy, and many cells must swim or move surrounding materials via the beating motion of cellular appendages like cilia and flagella.

What We Will Explore Under Metabolism

The cellular processes listed above require a steady supply of energy. From where, and in what form, does this energy come? How do living cells obtain energy, and how do they use it? The next few lessons will discuss different forms of energy and the physical laws that govern energy transfer. These lessons will also describe how cells use energy and replenish it, and how chemical reactions in the cell are performed with great efficiency.

Energy and Metabolism

Scientists use the term bioenergetics to discuss the concept of energy flow (see image below) through living systems, such as cells. Cellular processes such as the building and breaking down of complex molecules occur through stepwise chemical reactions. Some of these chemical reactions are spontaneous and release energy, whereas others require energy to proceed.


Most life forms on earth get their energy from the sun. Plants use photosynthesis to capture sunlight, and herbivores eat those plants to obtain energy. Carnivores eat the herbivores, and decomposers digest plant and animal matter. Image credit: OpenStax Biology

Just as living things must continually consume food to replenish what has been used, cells must continually produce more energy to replenish that used by the many energy-requiring chemical reactions that constantly take place. We refer to all of the chemical reactions that take place inside cells, including those that use energy and those that release energy, are the cell’s metabolism.

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  • Energy is required for the movement of cells, through beating of cilia or flagella. This can be paralleled to human motion, produced by muscle contraction. Cells also need energy to perform digestion, just as humans require energy to digest food.