Carbohydrates are constituted of bonds between carbon, hydrogen and oxygen and provide a source of fast energy to the human body. Carbohydrate molecules, such as sugars, burn quickly. Just think of a marshmallow that was kept over the fire too long and then, as a result of this, dissolved in flames. This happens because glucose is very quickly transformed in energy and fire is a very visible form of energy (providing both heat and light).
Glucose and other carbohydrates provide energy to be used in the short term. Fat, which is built to store extra glucose molecules, provides energy for the long term. This is the reason why you’re only burning fat after more than twenty minutes of aerobic exercise. Before that, your body burns the glucose that is instantaneously available in your cells. Only after that primary glucose supply is depleted, your body begins to deconstruct the fat molecules, transforming them into glucose to be burned.
The most important carbohydrate molecule is glucose, a monosaccharide. It can be absorbed directly from food that contains carbohydrates (such as bread, pasta, candy and fruit). When the carbohydrates in the food that you have eaten are digested, the smallest possible resulting molecule of this digestion is glucose. The glucose molecules are excreted by the cells of the intestine and are absorbed by the blood. Through the blood stream, the molecules are transported through the entire body. Your body, however, also makes its own glucose when it is breaking down proteins and fats.
Glucose is transformed in usable energy through a process that is called glycolysis. The end product of this process is pyruvate. This pyruvate ends up in the Krebs-cycle which produces the energy-rich molecule adenosine triphosphate (or ATP). This process occurs in the mitochondria (energy providing organelles, the ‘factories’) of every cell in your body. The energy delivered by glucose can thus be measured by the number of ATP molecules. One could say that ATP basically functions as the ‘coin’ of living organisms. ATP is ‘spent’ when you use energy.
When there is more fuel in your body than necessary to cover the energy needed for that moment, your body stores some of this fuel, extracted from carbohydrates, in the form of glycogen. Plants store their excess fuel in the form of starch or use it to make more cellulose (structural molecules, one could say the ‘building blocks’). Glycogen, starch and cellulose are all long chains of glucose and are called polysaccharides.