Photosynthesis

The process of photosynthesis occurs in two main stages within the chloroplasts of plant cells. The first stage, the light-dependent reactions, takes place in the thylakoid membranes. Here, pigments like chlorophyll absorb photons from sunlight. This energy is used to split water molecules in a process called photolysis, which releases oxygen, protons, and electrons. The energy captured from light is converted into two short-term energy-storing molecules: ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). The second stage, known as the Calvin cycle or light-independent reactions, occurs in the stroma of the chloroplast. This cycle does not directly require light but uses the ATP and NADPH produced in the first stage. In the Calvin cycle, carbon dioxide from the atmosphere is ‘fixed’ into organic molecules and converted into glucose, a high-energy sugar that the plant can use for growth, metabolism, or storage as starch. This entire process is the foundation of nearly all life on Earth, providing the energy for most ecosystems and producing the oxygen in our atmosphere.