Photosynthesis
Photosynthesis is a biological process used by plants, algae, and certain bacteria to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's metabolic activities. This process plays a central role in the carbon cycle and is fundamental to life on Earth, supporting nearly all food chains by producing oxygen as a byproduct and storing solar energy in organic compounds. Understanding photosynthesis has been a major area of biological research for over two centuries.
Overview
The process begins when light energy is absorbed by chlorophyll molecules located within the thylakoid membranes of chloroplasts. This energy is used to split water molecules in a reaction known as photolysis, releasing oxygen as a waste product. The hydrogen ions and electrons produced are then used to drive the synthesis of ATP and NADPH, which power the subsequent dark reactions of the Calvin cycle. Carbon dioxide from the atmosphere is then fixed into glucose and other organic compounds during the Calvin cycle, completing the photosynthetic process.
Light reactions
The light-dependent reactions occur in the thylakoid membranes and require direct light to proceed. Photosystem II absorbs photons and uses their energy to split water molecules, generating oxygen, protons, and electrons. The electrons travel through an electron transport chain, ultimately reducing NADP+ to NADPH while pumping protons into the thylakoid lumen. This proton gradient powers ATP synthase, which produces ATP from ADP and inorganic phosphate. The process is remarkably efficient at converting light into stored chemical energy.