by Morley Evans
Glucose is the body's fuel. Without glucose, or without being able to convert it into energy rapidly and efficiently, we cannot survive in good health. So it's very important that our energy-metabolism system works efficiently. Here is a very simple explanation of how we convert glucose into energy.
• In response to the rise in blood-glucose levels (say) after a meal, the pancreas releases insulin which "mops up" the glucose and carries it to cells that need extra energy.
• The glucose enters the cell by special molecules in the membrane called “glucose transporters”.
• The cells that need glucose have specific insulin receptors on their surface so that insulin can bind to them, encouraging glucose entry and utilization in the cells.
• Once inside your cells, the glucose is burned to produce heat and adenosine triphosyphate, (ATP) a molecule that stores and releases energy as required by the cell.
• The metabolism of glucose into energy may occur either in combination with oxygen (aerobic metabolism) or without it (anaerobic metabolism). The oxygen used comes from the mitochondria - tiny bodies inside the cell. However, red blood cells do not have mitochondria, so they change glucose into energy without the use of oxygen.
• Glucose is also converted to energy in muscle cells - who are probably the most important energy "customers". These muscle cells do contain mitochondria so they can process glucose with oxygen. But even if oxygen-levels in the muscle-cell mitochondria fall too low, the cells can proceed to convert glucose into energy without oxygen. Unfortunately, turning glucose into energy without oxygen produces the by-product lactic acid. And too much lactic acid makes your muscles ache.
Our food-intake and energy-needs do not always coincide. For example, we may have lots of food, but no energy needs. Or we may need lots of energy but no food in sight! Fortunately, glucose can be stored as glycogen (an "energy reserve") for later use. Without such a glucose storage system, we would either have to eat constantly or risk a dangerous lack of energy.