Describe the light-dependent reactions of photosynthesis.

The light-dependent reactions of photosynthesis occur in the thylakoid membrane of the chloroplast in a plant cell. Embedded in this membrane are two chlorophyll-protein complexes called photosystem 1 and photosystem 2. Energy from the sun's light raises two electrons from each chlorophyll molecule to a higher energy level. These 'excited' electrons pass along the electron transport chain in a series of redox reactions with the electron carrier molecules in the membrane, losing energy. This energy is used for the synthesis of ATP via photophosphorylation. The electrons lost from photosystem 2 must be replaced in order for the chain to continue. Within the thylakoid space an enzyme catalyses the splitting of water (photolysis) to give oxygen, hydrogen ions and electrons. These electrons replace those lost from photosystem 2. Oxygen diffuses out of the cells as a waste product. The hydrogen ions combine with the electrons which have passed through the electron transport chain and NADP to form reduced NADP, which is then used in the light-independent reactions to form GALP and later, glucose.