You absorb glucose, a simple sugar found in plants, directly into your bloodstream, where it acts as your body's fuel source. Without the ability to convert glucose into energy rapidly and efficiently, you would not be in good health. It's very important that your energy-metabolism system work efficiently.
The metabolism process begins with digestion. Monosaccharides are absorbed into the bloodstream upon entering the small intestine. To control blood sugar, your body has three hormones: glucagon, insulin and epinephrine. Insulin, which your pancreas secretes when your blood sugar levels increase, helps along the transfer of glucose into your cells. Glucose metabolism is focused heavily in the muscles and liver, which receive more glucose than your other organs do because they have specific insulin receptors on their surface so that insulin can bind to them, thereby encouraging glucose entry and use in these cells.
Upon entering the liver and muscles, the glucose is changed into glycogen by the process of glycogenesis. Glycogen stays in your liver and muscles until your glucose levels are low and you need energy. At this time, the epinephrine and glucogon hormones are released to stimulate the conversion of glycogen to glucose in a process called glycogenolysis. Once the glucose enters your cells, it is burned for energy and produces heat. This process also creates adenosine triphosphate, or ATP, a molecule that releases energy when needed by the cell. The energy is used for many metabolic processes and is considered the universal energy currency for metabolism.
There are two types of glucose metabolism: aerobic metabolism, which is with oxygen, and anaerobic metabolism, which is without oxygen. During aerobic metabolism, the oxygen used comes from tiny bodies inside the cell called the mitochondria in order to be fully oxidized. Muscle cells use up the most energy during this process, and because they contain mitochondria, the process is simple and quick when oxygen is present. This process includes the Krebs cycle, which produces carbon dioxide and ATP. This chemical provides cells with the energy required for the synthesis of proteins from amino acids and the replication of DNA.
Anaerobic Metabolism and Glycogen Depletion
During strenuous exercise or moments of extreme exertion, the oxygen level in muscle cells may fall too low. If this happens, the process continues with anaerobic metabolism. The downside of burning glucose for energy without oxygen is the production of lactic acid. The substance has been known to make muscles ache when "lactic acid buildup" occurs. Another event that occurs when the glycogen has been used up in the muscles and liver is "hitting the wall." This is a natural defense mechanism for the body and prevents permanent damage during extreme exertion by slowing the key systems needed to maintain muscle contraction. When the body slows and oxygen becomes available, lactate reverts back to pyruvate. This allows aerobic metabolism to resume and for the body to recover from the strenuous event.