Living things use oxygen in a process called cellular respiration, which is the process by which organisms break down food molecules (like glucose) to release energy in the form of ATP (adenosine triphosphate). This energy is essential for all life processes, from muscle movement and nerve signaling to cell growth and repair.

Here's how it works:

1. Oxygen is inhaled: Living things take in oxygen from their environment, whether it's the air for land animals or the water for aquatic organisms.

2. Oxygen enters the cells: Oxygen travels through the circulatory system (blood for animals) and reaches individual cells.

3. Inside the mitochondria: Within each cell, oxygen is used in the mitochondria, the powerhouses of the cell.

4. Electron transport chain: In the mitochondria, oxygen acts as the final electron acceptor in a series of reactions called the electron transport chain. This chain releases energy that is used to produce ATP.

5. Carbon dioxide is produced: As a byproduct of cellular respiration, carbon dioxide (CO2) is produced. CO2 is then transported back to the lungs (for land animals) or gills (for aquatic animals) to be exhaled.

Oxygen's role is crucial:

* Energy production: Without oxygen, the electron transport chain cannot function, and very little ATP is produced.

* Waste removal: Oxygen helps to remove waste products like carbon dioxide from the body.

* Maintaining cell function: Oxygen is essential for the proper functioning of many cell processes, including protein synthesis and DNA replication.

In short, oxygen is essential for life as we know it. It allows living organisms to efficiently extract energy from food, which powers all of the activities necessary for survival and growth.