Carbohydrates, such as the sugar glucose, have potential energy stored in their chemical bonds. Carbohydrate metabolism allows organisms to release that potential energy to power the machinery of life. At the cellular level, the sugar is broken down through interrelated chemical processes that maximize the energy released. Glycolysis and the Krebs cycle, or the citric acid cycle, work together in aerobic glucose metabolism, also called cellular respiration, but they need an extra step, the bridge stage, to move between the two chemical processes.
Glycolysis literally means sugar breaking. It is the process of breaking sugar into two pyruvate molecules and transforming a net of two adenosine diphosphate molecules, or ADP, into adenosine triphosphate, or ATP molecules. The last bit is important because ATP is the energy currency of the cell. All living organisms use glycolysis, but many organisms, called aerobic organisms, can use oxygen and the byproducts of glycolysis to get 18 times more energy out of a sugar molecule. The bridge stage is the chemical transformation that connects the two.
The beginning of the bridge stage starts at the end of glycolysis. In the first step of glycolysis, two ATP are consumed and two additional free phosphorous atoms are added to glucose, which then splits into two three-carbon units. In the second stage of glycolysis, four ATP are produced and the three-carbon molecules are transformed into pyruvate. Pyruvate molecules have three carbon, three hydrogen, and three oxygen atoms and an extra electron, they are negatively charged.
Pyruvate can’t be directly fed into the Krebs cycle, so it is transformed during the bridge stage into acetyl coenzyme A, or acetyl-CoA. Coenzyme A is a complex compound, consisting of an ADP attached to a protein-like chain, that delivers carbon atoms to the Krebs cycle. At the end of the chain is a highly reactive sulfur-hydryl group that can bind and carry specific carbon compounds.
Bridge Stage Details
Several things happen during the bridge stage to produce acetyl-CoA. The pyruvate bonds with thiamine pyrophosphate, or TPP, another coenzyme that splits off a carbon dioxide molecule. The extra electron from the original pyruvate molecule is donated to nicotinamide adenine dinucleotide, or NADH, a coenzyme that accepts electrons during cellular respiration and then powers electron transport phosphorylation, another metabolic process that produces ATP. The resulting acetyl group is released by TPP, transferred to lipoamide, another intermediary compound, and finally bonds to the sulfur group of coenzyme A, creating acetyl-CoA. The bridge step is complete, and the carbon moves on to the Krebs cycle.
- Jupiterimages/Photos.com/Getty Images