Chemicals of Life
- Cellular Respiration
Thinkquest Team 22016
What is ATP? ATP is a molecule composed of an adenosine group attached to 3 phosphate groups. When a phosphate is broken loose from the ATP molecule (forming ADP: adenine diphosphate) a large amount of energy is released. This energy can be used by the cell to run its chemical reactions.
But as energy is released when a phosphate is cut off of ATP it takes the same amount of energy to put the phosphate back on and form ATP again. This is where cellular respiration comes in:
Glycolysis is the splitting of Glucose. The energy stored in glucose molecules is released when it is split during glycolysis. Glycolysis can occur with or without oxygen. During glycolysis 2 ATP are used up to begin the reaction, but at the end 4 ATP's are produced, thus a net gain of 2 ATP's. The final product of Glycolysis is pyruvic acid.
There are two main processes in Anaerobic Respiration:
1. Alcoholic Fermentation
When oxygen is absent or the necessary mechanisms for aerobic respiration is not provided organisms can carry on alcoholic fermentation. There is no additional ATP yield. The only purpose of fermentation is to regenerate the supply of NAD+, these are energy carrier that accepts some of the hydrogen's that come from the splitting of glucose and 2 electrons ( these electrons and hydrogen's will be used later on in cellular respiration ). During fermentation NADH loses the 2 electrons and the hydrogen and becomes NAD+. The pyruvic acid molecules accept the hydrogen's and the 2 electrons to form ethanol and carbon dioxide. Yeast carry on alcoholic fermentation all the time. The regeneration of NAD+ allows glycolysis to continue so more ATP can be produced.
2. Lactic Acid fermentation
Is the same as alcoholic fermentation but instead of ethanol and carbon dioxide being produced lactic acid is produced instead. It serves the same purpose, to regenerate the supply of NAD+ so glycolysis can continue. This happens sometimes in muscle cells doing heavy exercise when the muscle cells are not receiving enough oxygen. Lactic acid build up and cramping occurs.
Glycolysis releases only a minute amount of the potential energy stored in glucose. This is achieved through aerobic breakdown of pyruvic acid during the Krebs Cycle.
During the Krebs Cycle the 3 carbon molecule pyruvic acid is broken down further. Through a series of steps that are too complex to go into now, pyruvic acid is broken down into 3 carbon dioxide molecules and 4 molecules of NAD+ accept 8 electrons and a hydrogen, and another hydrogen acceptor FAD is reduced to FADH2. All this takes place in the matrix of the mitochondria.
The hydrogen and electrons removed from the Krebs cycle then go into the electron transport chain. The electron transport chain occurs along the inner membrane of the mitochondria. The electrons and the hydrogens are released from the energy carriers and the electrons are passed along to a series of electron acceptors with a higher affinity for electrons. Each time an electron is passed along a small amount of energy is released. That energy is used to pump the hydrogen ions across the mitochondria's inner membrane. As hydrogen ions build up along the outer part of the membrane a concentration gradient forms, forcing the hydrogen ions to the other side of the membrane. The hydrogens pass through a channel surrounded by ATP synthesis enzymes where ATP can be produced as a result of hydrogen ions flowing across this channel. The hydrogens then combine with oxygen to form water. By the end of the cycle, a total of 36 ATP molecules have been manufactured.
Aerobic respiration is a much more efficient process than anaerobic respiration, as anaerobic only produces about 2 ATP molecules per glucose molecule, compared to the possible 36 of aerobic. On the other hand however, aerobic is much faster than anaerobic.