Metabolism

A hummingbird with its wings blurred in motion hovers beside a red flower-shaped feeder, its long thin beak inserted into one of the feeder's yellow-rimmed ports. — 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 through a series of biochemical reactions. The flight muscles in birds are extremely efficient in energy production. (credit: modification of work by Cory Zanker)

Virtually every task performed by living organisms requires energy. Organisms require energy to perform heavy labor and exercise, but humans also use considerable energy while thinking, and even during sleep. Every organism’s living cells constantly use energy. Organisms import nutrients and other molecules. They metabolize (break down) and possibly synthesize into new molecules. If necessary, molecules modify, move around the cell and may distribute themselves to the entire organism. For example, the large proteins that make up muscles are actively built from smaller molecules. Complex carbohydrates break 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 to synthesize and break down molecules. Additionally, signaling molecules such as hormones and neurotransmitters transport between cells. Cells ingest and break down bacteria and viruses. 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.

The cellular processes that we 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? This chapter will discuss different forms of energy and the physical laws that govern energy transfer. This chapter will also describe how cells use energy and replenish it, and how chemical reactions in the cell perform with great efficiency.

Sections

Energy and Metabolism — metabolic pathways and their two major types, and how chemical reactions transfer energy.
Potential, Kinetic, Free, and Activation Energy — energy and the difference between kinetic and potential forms, free energy and activation energy, and endergonic versus exergonic reactions.
The Laws of Thermodynamics — entropy and the first and second laws of thermodynamics.
ATP: Adenosine Triphosphate — ATP’s role as the cell’s energy currency and how ATP hydrolysis releases energy.
Enzymes — the role of enzymes in metabolic pathways, how they function as molecular catalysts, and how various factors regulate them.

This chapter is adapted from Biology 2e, Chapter 6: Metabolism by Mary Ann Clark, Jung Choi, Matthew Douglas, and OpenStax, © OpenStax, licensed under CC BY-NC-SA 4.0. Access the original for free at openstax.org. Each section page records its own changes from the source. Changes: the chapter-opening photograph is the source’s, re-encoded for the web with its credit kept in the caption and its alt text rewritten to describe the hummingbird’s motion and the feeder rather than restate the caption’s point about flight energy.

Bulk Transport Energy and Metabolism