Converting Glucose and Oxygen Into ATP

Animal's are multicellular organisms and while we may not see the cells, every cell is "intelligent" and highly structured. Body cells which make up tissues and in turn organs, need to perform many processes in order to serve their function. The reason they are able to carry out these functions and work properly is because of energy. They need energy to survive and carry out their regular cellular functions. This energy is made from glucose and oxygen, which are taken in by humans through eating and breathing. The digestive, respiratory, and cardiovascular systems help to transport glucose and oxygen to cells. When an animal eats or absorbs nutrients, the food is broken down into molecules of glucose in the digestive system and then absorbed into other body cells. When an animal breathes or takes in oxygen, the oxygen is passed into the blood stream where it is transported throughout the body. 

Once the oxygen and glucose have been transported to a cell, the cell needs to transform these chemicals into energy that the cell can use. The energy that the body's cells use to do these regular processes is called ATP. ATP stands for adenosine triphosphate. ATP is made from glucose (sugar molecules) and oxygen in the mitochondria of a cell, which are the power factories of the cell because they produce the energy. The process that occurs in and directly around the mitochondria, which transforms the glucose and oxygen into ATP is called cellular respiration. Cellular respiration consists of three parts: glycolysis, citric acid cycle, and oxidative phosphorylation.

The first part of cellular respiration is the process of glycolysis. One molecule of glucose is chemically split into two molecules of pyruvate, two molecules of NADH, and two molecules of ATP. This entire process occurs in the cytosol of the cell, not inside the mitochondria.

Next is the citric acid cycle. The two molecules of pyruvate, which were produced in glycolysis, now enter the mitochondria, where they get converted into acetyl CoA, which is then oxidized into eight NADH, two FADH2, and two ATP. 

Finally, the ten NADH and two FADH2 from the last two steps go into the inner membrane of the mitochondria to undergo oxidative phosphorylation. This is where the electron transport chain takes place. In the electron transport chain, a hydrogen gradient is produced across the inner mitochondrial membrane. This hydrogen gradient produces the energy necessary to power a structure called ATP synthase, which takes ADP and adds a phosphate group to make ATP. The process of producing ATP by use of a hydrogen gradient is called chemiosmosis. This process produces around twenty-eight ATP. 

Together, these three processes will produce a total of thirty-two ATP. They are known collectively as cellular respiration. The process requires oxygen and glucose to work. The majority of the ATP produces comes from the electron transport chain in part three of the process. This process also produces a small amount of H2O, that comes from the combination of oxygen with the hydrogen from the hydrogen gradient.