In its simplest form, photosynthesis can be defined as the process by which a plant uses sunlight to combine carbon dioxide from the air and water from the ground to produce sugar. In other words, the solar energy is converted to chemical energy stored in the sugar. An a fortunate byproduct for us that is, is the production of oxygen. But as we'll soon see in photosynthesis, solar energy is not transferred directly to the sugar molecule, but rather first stored in two types of storage molecules called ATP and NADPH. A basic understanding of these molecules is essential for understanding photosynthesis. So we'll have to go over some basic chemistry. ATP is an essential molecule that contains a huge amount of energy, it has three parts to it. Adenine, this is the A, a type of sugar called ribose here in the center. And the chain of three phosphate atoms, hence the TP for triphosphate. Within the ATP, energy is stored in the connection between the second and third phosphate atoms. The way that the energy is stored is similar to what happens if I hold together two magnets, with the negative sides touching. They want to repel each other, but if I use enough force, they can be held together. The potential energy is now stored in this connection. If I break this connection, the energy is released in pushing the two magnets apart. In a similar vein within the ATP, energy is stored in the connection between the second and third phosphate atoms. If this connection is broken, energy is released that the cell can use. In this way, ATP is like a currency for energy. The more ATP the cell has, the more potential it has for doing things. Or in a different analogy, ATP is like a full battery waiting to power something. To release the energy, a phosphate is removed, yielding ADP, adenine diphosphate, a free phosphate atom and energy. In photosynthesis, plants utilize solar energy to make ATP by reattaching the free phosphate group to the ADP. Thus the sun's energy is been converted from light to stored energy in the ATP, which can then be used by the cell for other processes. The second storage molecule is NADPH and it serves as a proton carrier. To understand what a proton carrier is though, we need to go over some more very basic chemical principles about atomic structure. All atoms are made up of a nucleus which is surrounded by a cloud of moving electrons. The electrons have a negative charge and one of the parts of the nucleus, the protons have a positive charge. Hydrogen like all atoms can transfer an electron to a neighboring atom. When this happens, the hydrogen donor is made up solely of a single proton and this is a positive charge. We designate this is H+ or just call it a proton. NADPH is a rather complex molecule that serves as a proton carrier, used to transport protons to other molecules. It can do this because NADPH has two forms. NADP+ is positively charged because it lacks two electrons and one proton. NADPH carries two electrons and one proton. So NADP+ captures two electrons and a proton that can be stored for later use. In photosynthesis, NADP+ gets its electrons and protons from two different hydrogen atoms. In this process, one proton binds to make NADPH, and the second proton remains dissolved for future use. The NADPH can then when needed, release the bound proton reverting to NADP+. In this way, NADPH is a carrier molecule that serves as a taxi to transfer protons and electrons from a place where they are produced to a place where they're needed. So getting back to photosynthesis, plants need energy to convert carbon dioxide to sugars. This is done in two stages. This energy of course originates in sunlight which is converted in the first stage to the chemical coins ATP and NADPH, which can be stored for later use. In the second stage, the plant utilizes the ATP and NADPH to capture carbon from the air and to convert this into usable sugars. The first stage is called the light reactions and the second stage, the dark reactions.