Whether you're jogging, sprinting or lifting weights, your body derives its muscle power from the oxidation of glucose and other energy-rich organic compounds. One key phase in this process is the Krebs cycle, a chemical pathway that begins and ends with a compound called oxaloacetate. Many of your cells, such as those in your brain, use this pathway at all times; others, such as your muscle cells, do so only when oxygen is readily available.
Pyruvate
All the cells of your body degrade glucose through a pathway called glycolysis. The end products of this pathway include a compound called pyruvate. Organelles inside your cells called mitochondria take up the pyruvate and oxidize it, removing one carbon atom in the form of CO2; the remaining two carbon atoms in a molecule of pyruvate are hitched to a molecule of coenzyme A to make acetyl-CoA. Acetyl-CoA is the raw material the mitochondria feed into the Krebs cycle.
Pathway
Glycolysis by itself is not very efficient; much of the energy stored in the glucose remains stored in the pyruvate end product. Oxidizing this pyruvate via the Krebs cycle yields significant dividends for the cell, but it's only feasible if oxygen is available. Each "turn" of the Krebs cycle begins by adding the two carbon atoms on acetyl-CoA to a molecule of oxaloacetate, thereby generating a molecule of citrate. The cycle regenerates oxaloacetate by removing two carbons from the citrate in the form of carbon dioxide; the energy extracted during the cycle powers the synthesis of an energy-rich compound called ATP. The Krebs cycle and several other associated processes are collectively called cellular respiration.
Anaerobic Exercise
Many cells in your body always use the Krebs cycle to make ATP and cannot do without oxygen even for a short period of time -- your brain cells are the most notable example. Your muscle cells ordinarily use the Krebs cycle, as well. During vigorous exercise, however, your bloodstream can't keep pace with your muscles' increasing demand, and your muscles can no longer obtain sufficient oxygen for cellular respiration and the Krebs cycle. They fall back on glycolysis and a closely related process called anaerobic fermentation. Later on, your liver absorbs the lactate generated by anaerobic fermentation and converts it back to pyruvate to make more glucose.
Biosynthesis
Not only is the Krebs cycle important for providing energy, it also has many uses in biosynthesis, as well. Intermediates in the cycle serve as the starting-point for the synthesis of many useful compounds inside your cells. Succinyl-CoA, for example, is an intermediate in the cycle that serves as a precursor for synthesizing heme groups such as those found in hemoglobin, the protein that carries oxygen in your bloodstream. Alpha-ketoglutarate, another intermediate in the cycle, is important in the biosynthesis of amino acids such as glutamate and glutamine. Your cells need molecules like these to carry out their tasks, so the Krebs cycle is an essential part of the biochemistry that keeps your body operational.
References
- "Lehninger Principles of Biochemistry"; David L. Nelson, et al.; 2008
- Kimball's Biology Pages: Cellular Respiration
Member Comments