 When you're studying chemistry, it's usually by one equation at a time. In biological chemistry, or biochemistry, an important reaction may be represented by one equation when in reality it occurs over many steps in long sequences called chains or cascades. This is really important for a few reasons. Carefully controlled reaction cascades mean that the cell can stop or at least pause at many places and carefully meet out the products and the reactants and the energy that's released in the reaction. We're not going to get into the very depths of the respiration cascades but we're going to take a look at some of the key steps. Cellular respiration occurs inside the cell. There are three major parts of the process and they each occur in different parts of the cell. Part one occurs in the cytosol which is the semi-fluid part of the cytoplasm. Part one is called glycolysis. Part two occurs inside the mitochondria. This part is called the Krebs cycle named after Hans Krebs who identified the steps that occur in this pathway and how they cycle round. Part three occurs in the inner membrane of the mitochondria. This is called the electron transport chain. Because each part of cell respiration is separate, if necessary cells can pause the process in each location. Conveniently even undergoing one of the parts allows the cell access to some of the chemical energy. Glycolysis releases some energy in a cell-ready energy molecule called ATP. If the cell is ready the products of glycolysis feed into the Krebs cycle. The process of the Krebs cycle accesses some more energy and packages it into ATP too. The products of the Krebs cycle can then flow into the electron transport chain and the electron transport chain enables controlled access to even more energy which can also be packaged into ATP. We'd better now have a look at this ATP. ATP stands for adenosine triphosphate. We can break it down by looking at the component parts. Adenosine is this molecule here. Phosphates hold a lot of energy in their bonds. Adenosine can hold one phosphate which would make it adenosine monophosphate. It can hold two, adenosine diphosphate and three, adenosine triphosphate. A lot of organisms use ATP and ADP as their major go-between for energy because they're convenient molecules that can be broken and put back together again over and over and over and allow that controlled energy release to the cell. They do this many, many, many times over and very fast. A working animal muscle cell can recycle its ATP at a rate of about 10 million molecules per second.