Energy pathways in our cells occur in either the mitochondria or the intracellular fluid (cytoplasm). In the latter, monosaccharides such as glucose become engaged in an energy pathway called glycolysis. All cells can use glucose for energy; meanwhile fructose and galactose are used by the liver mainly. Glycolysis converts glucose to two molecules of pyruvate. In this process, two ATP molecules and heat energy will be generated. Since these ATP will be generated without the need for oxygen, glycolysis is often referred to as anaerobic energy metabolism.
Pyruvate has several options, depending on the type of cell and what is going on inside of that cell. If the cell lacks mitochondria, such as in RBCs, pyruvate is converted to lactic acid (lactate). This lactate enters the blood and can serve as fuel for certain other organs such as the kidneys. Meanwhile, astrocytes that create the blood-brain barrier produce lactate which neurons in our brain can use. The blood-brain barrier is a special molecular fence that separates the cerebral spinal fluid, which nourishes the brain and spine, from the general circulation. Perhaps the most famous source of lactic acid is muscle during intense exercise such as weight lifting or sprinting.
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