The Function of Glucose
The carbohydrates we eat perform different functions. One type of carbohydrate is deoxyribose, which is a building block of genes. Other carbohydrates are converted to protein and used to build structures in the body. But the primary function of carbohydrates is to provide energy for the body. This requires the carbohydrates to be in the form of glucose.
Carbohydrates exist as monosaccharides, disaccharides and polysaccharides. These names refer to the size of the compound, with “mono” being the smallest unit; disaccharides consist of two and polysaccharides contain three or more monosaccharides. Glucose is a monosaccharide that the body converts to a form of energy used by the cells.
Even though glucose is the principal nutrient our bodies rely on for energy, the cells that are responsible for carrying out our life-sustaining processes need the energy to be in a different form. Cellular respiration is the name of a long series of chemical reactions that begin with glucose and end-up creating a chemical form of energy called ATP. ATP is the common currency used by every cell in the body, providing the energy to build structures, to produce hormones and other biochemicals, and to keep the body functioning. Cellular respiration includes three phases that are called glycolysis, the Krebs Cycle, and the electron transport system. Glucose begins a chain reaction in which it combines with other chemicals and changes into different compounds. At every step in the series, as one reaction stimulates the next through the three cycles, ATP is created.
The health and functioning of every cell in the body depends on energy provided by glucose, but the brain is especially dependent on a stable and constant supply to sustain its function. As the authors of the article Brain Energy Metabolism state, “Glucose is the obligatory energy substrate for the brain.” Glucose can also be incorporated into lipids and proteins, and it’s a precursor of certain neurotransmitters such as GABA, glutamate, and acetylcholine.
Given the importance of glucose for maintaining energy, a balanced supply must be available. When the intake of glucose exceeds the amounts immediately required for the body, the excess is stored as glycogen in the liver or as fat. On average, there is enough glucose circulating in the blood to provide 15 minutes of energy, so as the levels of glucose drop, the glycogen stored in the liver quickly converts into energy. If the supply of glycogen is depleted, then fat is converted into glucose; however, that process takes more time and is less efficient.
The pancreas secretes two hormones in direct response to the amount of glucose in the blood in order to maintain the level of “blood sugar.” Glucagon is the hormone that increases the level of glucose by accelerating the conversion of glycogen into glucose in the liver. Insulin is the hormone that decreases blood sugar level when it is too high. Insulin acts like a key that prompts glucose to leave the bloodstream, enter the cells, and be converted for storage.