The gas that living beings need to breathe is oxygen (O2). Oxygen is essential for the survival of most organisms, as it plays a crucial role in cellular respiration. In this process, cells convert glucose and oxygen into energy (in the form of adenosine triphosphate, or ATP), along with carbon dioxide and water as by-products. While many terrestrial animals, including humans, rely on oxygen for respiration, some organisms, such as anaerobic bacteria, can survive without it. The oxygen in our atmosphere is primarily produced by photosynthetic organisms, like plants and algae, highlighting the interconnectedness of life on Earth.