Background
Cells need glucose--also known as blood sugar--as their source of energy. Once within a cell, glucose enters a series of reactions that accumulate chemical energy. Chemical energy is energy stored within bonds in molecules. The most important molecule for capturing and storing chemical energy is adenosine triphosphate, or ATP, according to the authors of "Molecular Cell Biology." Cells metabolize glucose into ATP. Specialized compartments of the cell, mitochondria, create most ATP molecules. Mitochondria are cellular power plants, and glucose is a fuel. One of the final products, ATP molecules, give cells energy, just as steam gives people heat.
Different Cells
Almost all living beings use glucose as an important cellular fuel. After food digestion, complex sugars such as lactose, sucrose or starch turn into simple sugars, one of them glucose. All human cells use glucose as their energy source. For red blood cells, it's the only source of energy.
Liver removes two-thirds of glucose from human blood. Liver cells convert most of that glucose into glycogen, a storage molecule of glucose.
Muscle cells have a large storage of glycogen, too, almost three-fourths of all glycogen in the human body. Muscles use glucose as one of their major fuels for contractions.
Brain cells use glucose almost as their only source of energy. They use about 120g (4 oz.) of glucose daily. Glucose provides energy for the transmission and propagation of nerve impulses, signals within the brain.
Fatty cells need glucose to create triglycerides, one of the main elements of fats.
Glucose Metabolism
Human cells digest glucose in presence of oxygen. Glucose goes through a series of changes that create two molecules of ATP and smaller metabolic products. They enter mitochondria, which help transform glucose further into water, carbon-dioxide and 28 molecules of ATP. This process is known as citric acid cycle, cellular respiration or breathing, because cells "breathe in" the oxygen and "breathe out" carbon dioxide. One glucose molecule creates as much as 30 molecules of chemical energy carriers.
However, in absence of oxygen, as it happens in muscles after extensive training, glucose has a different fate. It creates lactic acid instead of carbon dioxide and only two ATP molecules.
Importance
Cells use glucose-derived ATP as a common energy currency for many of their essential processes. These processes are synthesis and degradation of proteins, transport of molecules in and out of the cell and also within the cell, and cell movement. Cells need ATP for duplication of DNA, which happens during cell division. Without cell division, there would be no new cells to replace dead cells, no tissue regeneration, and no growth. Enzymes need ATP to function. Communication between cells also requires energy stored within ATP. In all, ATP as the product of glucose metabolism provides chemical energy for normal functioning and survival of cells.
References
- "Biochemistry"; Jeremy M. Berg Ph.D., John L. Tymoczko Ph.D., Lubert Stryer M.D. ; 2002
- "Molecular Cell Biology"; Harvey Lodish, Arnold Berk, Paul Matsudaira, Chris A. Kaiser, Monty Krieger, Matthew P. Scott, S. Laurence Zipursky and James Darnell; 2004
- "Molecular Biology of the Cell"; Bruce Alberts Ph.D., Alexander Johnson Ph.D., Julian Lewis Ph.D., Martin Raff M.D., Keith Roberts Ph.D., Peter Walter Ph.D., December 2007
