The difference between NADH and NADPH is that NADPH has an extra phosphate group. While NADH is used primarily to power cellular reactions such as aerobic cellular respiration and glycolysis, during which molecules are broken down, NADPH is used to power photosynthesis.
NAD, or nicotinamide adenine dinucleotide, is the basic coenzyme or molecule powering many cellular reactions. In cellular respiration, or the process of producing energy in the cells, this compound combines with two hydrogen atoms. Building up molecules in this way is called anabolism. The compound takes one hydrogen atom and the electron from the other hydrogen atom to turn into high-energy NADH. NADH then travels with the extra electrons and drops them off, releasing energy in a process called catabolism, when a molecule breaks down to produce energy.
NADP, or nicotinamide adenine dinucleotide phosphate, undergoes a similar process, but the coenzyme contains a phosphate group. The NADP compound combines with an extra hydrogen atom and electron to form NADPH. This molecule transports electrons to power photosynthesis, or the process of turning sunlight, carbon dioxide and water into food for plants. It is also used during the Calvin Cycle, a process that captures carbon dioxide molecules to aid in the formation of carbohydrates.