In this lesson, we will explore the relationship between ATP and food. The foods that we eat contain raw materials that our cells need to build new structures during growth and repair, and also atoms and molecules that will participate in many chemical reactions as our cells go about their usual work. But the foods we eat also contain energy, chemical energy that is stored within the bonds in the food molecules. Our cells need that energy to do their work. But they cannot use the energy directly from food. Instead, the energy in food molecules is transferred to an energy-storing molecule called adenosine triphosphate or ATP. All cells then use ATP directly for their energy needs. The major pathway by which ATP is produced in our cells is a process called cellular respiration. Through this complex mechanism, chemical energy in the molecules of the food we eat is first removed by breaking down the food molecules. The energy that is released during this breakdown is then used to form ATP in a process called phosphorylation. ATP stores the energy and it is this stored or potential energy that the cells can use directly to do their work. The foods we eat contain varying amounts of energy, measured in calories. But not all of that energy in the foods can be used for ATP production. Through cellular respiration, carbohydrates, which includes fruits and vegetables, sweets and starchy foods, provide on average about 4 calories of energy per gram. So do proteins such as those found in meat, fish, chicken, cheese and nuts, for example. Fat, however, provides over twice as much energy. Nine calories of energy are derived from each gram of fat. The process of cellular respiration begins with complex organic molecules and, in the process of extracting their energy, they are broken down to simpler molecules. For example, cellular respiration of glucose, a carbohydrate, involves three sets of reactions. The first set, glycolysis occurs in the cell cytoplasm and produces a small amount of ATP, 2 ATP. Glycolysis is said to be anaerobic because it does not require oxygen to occur. If oxygen is present though, the breakdown process continues inside the mitochondria with two sets of reactions called the citric acid or Krebs cycle and the electron transport chain. These reactions do require the use of oxygen so they are called aerobic reactions. But by using oxygen, much more ATP is produced. In theory, these reactions should produce about 36 more ATP for an overall theoretical total of 38 ATP per molecule of glucose. In reality though, research suggests that these reactions are less efficient than once believed and some of the energy does not go into ATP. So it is believed that the overall yield is probably lower, more likely in the range of 30 to 32 ATP per molecule of glucose. Still, in the end, the original glucose has been broken down to carbon dioxide, water, and lots of ATP that can keep ourselves hard at work keeping us alive. To underscore how important ATP production is, you can now see why we have a respiratory system. We breathe in oxygen that is needed to maximize ATP production and we exhale carbon dioxide that is made from the breakdown reactions in cellular respiration.