Cellular energy systems
For the human cells to work properly they need energy, reaching them primarily with the blood in the form of glucose. The cells also need oxygen to be able to metabolize the glucose into carbon dioxide and water, thus creating energy. That is why oxygen is essential to us all. The lungs, heart and blood stream are responsible for transporting oxygen from the lungs to the cell.
Glucose, the most important energy source of the cells, is transported by the blood from the liver. Carbohydrates are digested in the intestines and broken down, resulting in glycogen. Glycogen is stored in the liver and in the muscles. When the cell needs energy glycogen is released and reaches the cells via the blood stream. There are two different hormones that control the level of glycogen in the blood, insulin and glucagon. Insulin tells the body it needs to store more energy for future use and glucagon tells the body that it is time to release more energy to the cells.
By providing the body with oxygen the cells are able to use the glucose. The body uses the energy to keep our muscles working, for temperature regulation, to keep our nervous system working or to help create new cells.
In the cells we find a membrane-enclosed organelle called the mitochondria. The mitochondria is called the power plant of the cell because it generates the ATP (Adenoside – tri – phosphate), used as energy by the cell. It is the site of the major cellular energy producing processes, including the citric acid cycle, oxidative phosphorylation and fatty acid oxidation.
The mitochondria contain outer and inner membranes. The inner membrane, the cristae, is formed by large folds to provide a larger inner membrane surface. Every mitochondrion contains more than 70 enzymes and other chemical substances like vitamins and minerals, involved in different energy creating reactions. They are the site of various chemical processes where energy is released step by step. The set of reactions involved in the ATP production is known as the citric acid circle or Kreb´s circle, named after the late German biochemist Hans Krebs (1900-1981). His identification of the citric acid circle awarded him the Nobel Prize in 1953.
Many of the steps in the Kreb´s circle release electrons that pass through the cytochrome, a membrane-bound heme protein that contains heme groups and carry out electron transport. To complete the oxidative phosphorylation, generating energy, there is a need of oxygen.
The cells store the energy in compounds called ATP, adeonosine triphosphate, and ADP, adenosine diphosphate, used in metabolism to create energy.