What is the difference between the light-dependent and light-independent reactions of photosynthesis?

The light-dependent reactions of photosynthesis require light energy, while the light-independent reactions do not.

During the light-dependent reactions, light energy is absorbed by pigments such as chlorophyll and converted into chemical energy in the form of ATP and NADPH. This process occurs in the thylakoid membranes of the chloroplasts and involves the splitting of water molecules into oxygen, protons and electrons. The oxygen is released as a by-product, while the protons and electrons are used to create a proton gradient that drives the synthesis of ATP.

The light-independent reactions, also known as the Calvin cycle, take place in the stroma of the chloroplasts and do not require light energy. Instead, they use the ATP and NADPH produced during the light-dependent reactions to fix carbon dioxide into organic molecules such as glucose. This process involves a series of enzyme-catalysed reactions that convert carbon dioxide into a three-carbon compound called glyceraldehyde-3-phosphate (G3P), which can be used to synthesise glucose and other organic molecules.

Overall, the light-dependent and light-independent reactions of photosynthesis work together to convert light energy into chemical energy in the form of organic molecules. While the light-dependent reactions require light energy to generate ATP and NADPH, the light-independent reactions use these molecules to fix carbon dioxide and produce organic compounds.