The atmospheric gas essential for aerobic respiration is oxygen. This gas plays a crucial role in the energy production process within cells, allowing organisms to convert glucose into usable energy in the form of ATP (adenosine triphosphate). In aerobic respiration, glucose is broken down in the presence of oxygen, resulting in carbon dioxide and water as byproducts. Without sufficient oxygen, organisms rely on anaerobic respiration, which is less efficient and produces different byproducts, such as lactic acid or alcohol, depending on the organism.