The leaves of this plant have cells that carry out photosynthesis. If we zoom in on this photosynthetic plant cell we can see the chloroplasts, where the reactions of photosynthesis occur. Photosynthesis consists of two primary steps: The light reactions, and the calvin cycle reactions. In this tutorial, we'll focus exclusively on the light reactions. The light reactions occur within the thylakoid of the chloroplast. Here, special pigments absorb light energy and transfer it to high energy electrons, eventually producing ATP and the electron carrier NADPH. Let's zoom into the thylakoids to take a closer look at how ATP is created. The light reactions use two photosystems, called photosystem I and photosystem II, which are both embedded in the thylakoid membrane. It's important to realize that these photosystems are named for the order in which they were discovered, not for the order in which they participate in the photosynthetic process. The light reactions actually begin at photosystem II. The first thing that happens is that photosystem II receives photons, or light energy. This light energy is transferred to a chlorophyll reaction center, causing electrons in the reaction center to become energized. These electrons become so energized that they escape photosystem II and move to a nearby electron acceptor molecule, located in the electron transport chain. Meanwhile, to replace the electrons leaving photosystem II, water is split, releasing oxygen, two hydrogen ions, and two electrons. The first set of electrons continues to move down the electron transport chain, releasing stored energy as it moves. This energy is used to create a hydrogen ion gradient. A protein in the electron transport chain pumps hydrogen ions from the stroma into the thylakoid space. This creates a high concentration of ions in the thylakoid space, relative to the low concentration of ions in the stroma. This gradient contains a large amount of potential energy, which is used by an enzyme called ATP synthase. The hydrogen ions flow down their concentration gradient, through a channel in ATP synthase, releasing energy in the process. ATP synthase uses this energy to add a phosphate to ADP, forming ATP. Let's zoom back out for a moment, and return to our chloroplast. Remember from the beginning of this tutorial, that the light reactions produce both ATP and NADPH. We've just seen how ATP is produced, but what about NADPH? Let's zoom back in to take a closer look. Notice these electrons over here where we left them at photosystem one. As photosystem I absorbs additional light energy, the electrons again become energized, escaping photosystem I and moving down the second electron transport chain. Electrons from the electron transport chain adjacent to photosystem II, replace those from photosystem I. And again, water is split to replace the electrons that have moved from photosystem II. At the end of this electron transport chain, the energized electrons and a hydrogen molecule are used to reduce NADP to NADPH. Let's zoom back out to review. The light reactions use light energy and water to produce ATP and NADPH. Oxygen gas is released as a by product. Together, the ATP and NADPH formed during the light reactions, are used by the Calvin cycle reactions, which are discussed more in depth in a separate tutorial. The important thing to remember from this tutorial, is that plants need both light and water to survive. Without these ingredients, the light reactions would shut down stalling photosynthesis, and causing the plant to die.