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Each time you inhale, you take oxygen from the atmosphere into your lungs. This oxygen is delivered via the circulatory system to your body's cells, where it is used by mitochondria to perform cellular

respiration.

This series of chemical reactions releases energy from food molecules and uses it to make adenosine triphosphate (ATP), a small molecule that serves as the primary energy currency of the cell. The energy stored within ATP's phosphate bonds is used to power most energy-requiring cellular reactions.

All organisms require energy to survive, but not all organisms live in oxygen-rich environments like humans do. There are fungi, invertebrates, bacteria, and archaea that live in places where oxygen is limited, such as beneath the soil, deep in the ocean, inside hydrothermal vents, and even within an animal's digestive tract. These organisms can still break down food molecules to obtain the energy they need using processes that do not require oxygen input.