Photosynthesis is a fundamental process that sustains life on Earth, providing oxygen and food through the conversion of light energy into chemical energy. The process occurs in two main stages: the light reactions and the dark reactions (Calvin–Benson cycle). The light reactions, taking place in the thylakoid membrane of chloroplasts, involve the splitting of water into oxygen, protons, and electrons, which are then used to generate ATP and NADPH. The dark reactions, occurring in the stroma, use these products to convert CO₂ into carbohydrates using the Calvin–Benson cycle. This cycle involves the fixation of CO₂ into a 3-carbon compound, which is then reduced to a 6-carbon compound and further metabolized into glucose. The efficiency of photosynthesis is crucial for maintaining global energy balance and supporting biodiversity. The article also discusses the evolution of photosynthesis, the structure and function of photosystems, and the regulation of the Calvin–Benson cycle by light reactions. Additionally, it highlights the importance of CO₂-concentrating mechanisms (CCMs) in plants, algae, and cyanobacteria to enhance carbon fixation efficiency under various environmental conditions.Photosynthesis is a fundamental process that sustains life on Earth, providing oxygen and food through the conversion of light energy into chemical energy. The process occurs in two main stages: the light reactions and the dark reactions (Calvin–Benson cycle). The light reactions, taking place in the thylakoid membrane of chloroplasts, involve the splitting of water into oxygen, protons, and electrons, which are then used to generate ATP and NADPH. The dark reactions, occurring in the stroma, use these products to convert CO₂ into carbohydrates using the Calvin–Benson cycle. This cycle involves the fixation of CO₂ into a 3-carbon compound, which is then reduced to a 6-carbon compound and further metabolized into glucose. The efficiency of photosynthesis is crucial for maintaining global energy balance and supporting biodiversity. The article also discusses the evolution of photosynthesis, the structure and function of photosystems, and the regulation of the Calvin–Benson cycle by light reactions. Additionally, it highlights the importance of CO₂-concentrating mechanisms (CCMs) in plants, algae, and cyanobacteria to enhance carbon fixation efficiency under various environmental conditions.