The main source of energy for living organisms is glucose, a simple sugar that serves as a primary energy source for cellular respiration. Glucose is produced during photosynthesis in plants, where sunlight is converted into chemical energy, stored in glucose molecules. In animals, glucose is derived from the digestion of carbohydrates. Once absorbed into cells, glucose undergoes glycolysis followed by aerobic or anaerobic respiration to generate adenosine triphosphate (ATP), the energy currency of the cell. ATP powers various cellular activities, including metabolism, muscle contractions, and active transport, making glucose vital for sustaining life across all ecosystems.