In the context of biology, needing energy means more than trying to stay awake after a long day. Each cell in the body requires energy to preform its functions. In the body, the energy is found in a molecule called adenosine triphosphate, or ATP. This molecule is obtained by breaking down nutrients, such as glucose, through cellular respiration. This process has three main steps that begin with glycolysis.
Glycolysis Begins Cellular Respiration
Glycolysis is the first step of the cellular respiration process. The reaction to break down a single glucose molecule has about 10 steps. The molecule is considered a simple sugar and is made up of carbon, hydrogen and oxygen atoms. Through a series of complex reactions, glucose is broken down and transformed into the energy-containing molecule ATP, the enzyme NADH and the biochemical pyruvate.
A total of four ATP, two NADH and two pyruvate are produced through glycolysis. However, during the reaction series two ATP molecules are used, making the net yield only two ATP. At this point, the glucose molecule is completely broken down and the session of glycolysis ends. The pyruvate molecules move on to the next steps of cellular respiration, while a new glycolysis cycle begins elsewhere in the cell.
Pathway to Aerobic Respiration
When oxygen is present, the pyruvate molecules will follow the pathway of aerobic respiration. Through yet another series of complex chemical reactions, the pyruvates are oxidized to form acetyl coenzyme A. The acetyl coenzyme A moves to the inner part of a cell's mitochondria. Here the citric acid cycle begins and produces two additional ATP and binds hydrogen atoms to the NADH molecules. The addition of the hydrogen atoms establishes a gradient and provides energy to the electron transport chain. Through electron transport, 32 ATP are produced.
Pathway to Anaerobic Respiration
If no oxygen is available, cellular respiration occurs via the anaerobic pathway. Through this route, pyruvate cannot be oxidized to form acetyl coenzyme A. Instead the pyruvate undergoes a series of reactions to yield ethanol or lactate and NAD+. The NAD+ is needed to start the glycolysis process again. At the end of the anaerobic cycle, the yield is only two ATP. The ethanol or lactate left behind is considered waste to the cell and if not removed will kill the cell.