The relationship between cellular respiration and photosynthesis is critical for energy flow in an ecosystem. Photosynthesis occurs in plants, algae, and some bacteria, converting sunlight, carbon dioxide, and water into glucose and oxygen. In contrast, cellular respiration, performed by all living organisms, breaks down glucose in the presence of oxygen to produce energy, carbon dioxide, and water. The by-products of photosynthesis serve as the reactants for cellular respiration, creating a cyclical relationship that sustains life and maintains atmospheric oxygen and carbon dioxide levels.