Light–dependent reaction Light is the source of energy for photosynthesis, and the first set of reactions which begin the process requires light – thus the name, light reactions, or light–dependent reactions. Photochemical reaction or Hill's reaction The light reactions rely on colored molecules called pigments to capture the energy of light.
The most important pigments are the green chlorophylls, but accessory pigments called carotenoids are also present, which are yellow or orange. In this reaction light plays the key role. The accessory pigments capture wavelengths of light that chlorophylls cannot, and then transfer the energy to chlorophyll, which uses this energy to carry out the light reactions. These pigments are arranged in the thylakoid membranes in clusters, along with proteins and electron carriers, to form light–harvesting complexes referred to as photosystems. Each photosystem has about two hundred chlorophyll molecules and a variable number of accessory pigments. In most plants there are two photosystems, which differ slightly in how they absorb light. At the center of each photosystem is a special chlorophyll molecule called the reaction center, to which all the other pigments molecules pass the energy they harvest from sunlight. When the reaction–center chlorophyll absorbs light or receives energy from its accessory molecules, a pair of electrons on it becomes excited. These electrons now carry the energy from light, and are passed to an electron acceptor molecule.
Sequential steps involved in light reactions. PS2 allows hydrogen to be pumped into the lumen which then powers the synthesis of ATP. The passing of electrons from PS1 powers creation of NADPH. Both products are used in the Dark Reaction or Calvin Cycle of Photosynthesis. The fate of these electrons depends on which photosystem they arose from. Electrons from photosystem I are passed down a short electron transport chain to reduce NADP to NADPH (which also gains an H ion). Electrons from photosystem II are passed down a longer electron transport chain, eventually arriving at photosystem I, where they replace the electrons given up by photosystem I' s reaction center.
Along the way, the energy released by the electrons is used to make ATP in a process called photophosphorylation. Like the mitochondrion, the chloroplast uses an electron transport chain, and ATP synthase to create ATP. The end result of excitation of both photosystems is that electrons have been transferred from chlorophyll to NADP+, forming NADPH, and some of their energy has been used to generate ATP. While photosystem I gains electrons from photosystem II, the electrons lost by photosystem II have not been replaced yet. Its reaction center acquires these electrons by splitting water. During this process, the electrons in water are removed and passed to the reaction center chlorophyll. The associated hydrogen ions are released from the water molecule, and after two water molecules are thus split, the oxygen atoms join to form molecular oxygen (O2), a waste product of photosynthesis.
The NADPH and ATP generated in the light reactions enter the stroma, where they participate in the dark reactions to synthesize sugars from CO2. Thus, the main stages of light reactions are: Activation of chlorophyll, Splitting of water (Photolysis), The electrons are used in converting ADP into ATP by adding one phosphate group