Most organisms need oxygen to breathe in order to survive. Oxygen is essential for cellular respiration, a metabolic process in which cells convert glucose and oxygen into energy, carbon dioxide, and water. This energy is vital for various biological functions, including growth, reproduction, and maintaining homeostasis. While some organisms, such as anaerobic bacteria, can survive without oxygen, the majority of multicellular life forms, including humans and animals, depend on a continuous supply of oxygen for their survival. The presence of oxygen in Earth's atmosphere is a result of photosynthesis conducted by plants and phytoplankton.