The light reactions of oxygenic photosynthesis use light energy to drive electrons through a series of enzymes in the thylakoid membrane in order to make a proton gradient, which drives ATP synthesis. The reducing power of those electrons is also stored in the form of NADPH. The ATP and NADPH molecules are common biochemical currency used for a variety of other reactions within the cell. They are primarily used to energetically fund the formation of sugar from carbon dioxide in the light-independent reactions. For the light reactions, water is used to supply the electrons, producing oxygen as a fortunate byproduct.
There are two photosystems (PSI and PSII) that perform the energy-requiring steps. These complexes are also associated with antenna complexes to help them capture the light they need to perform their reactions. Photosystem II (PSII) resides at the beginning of the photosynthetic electron transfer chain. It uses light energy to pull electrons from two water molecules and transfer them to a chemical called plastoquinone (PQ). In this reaction O2 is produced and along with the reduced form of PQ (aka plastoquinol, PQH2). The PQH2 diffuses within the thylakoid membrane to the cytochrome b6f complex (Cyt b6f). Cyt b6f is a proton pump which uses some of the energy from the electron transfer reactions to create a proton (H+) gradient across the thylakoid membrane before passing electrons along to plastocyanin (PC). PC transfers electrons to Photosystem I (PSI) which requires an additional input of light energy before transferring electrons to Ferredoxin (Fd). Finally Ferredoxin-NADP+ Reductase (FNR) stores the electrons on NADPH. The H+ gradient created across the thykaloid membrane (acidic thylakoid lumen vs. basic stroma) provides the driving force for ATP synthesis by ATP synthase.
If this seems like a lot of steps to make two chemicals (ATP and NADPH), you are right. However, each electron transfer step allows these enzymes to do useful work, like making that H+ gradient. Multiple electron transfer steps also provides the opportunity for intricate regulation of the system. The light reactions are the engine of the cell, and all of these components can be finely tuned depending on environmental conditions and the needs of the cell.
The Z-scheme graphs all of the electron transfer cofactors of the light reactions in a way that shows their relative redox potentials.
Here’s a video lecture on the light reactions from the Khan Academy:
The Photosynthetic Z-Scheme by Govindjee
For more information about different aspects of the light reactions click the links below:
Cyclic Electron Transfer