Aerobic glocolysis and the Krebs cycle are limited to the epithelium or superficial fibers that still have mitochondria. The thickness of the lens cortex, in which newer fibers are present and which still contains organelles, is approximately 100 microns. ATP activity is higher in the epithelial cells and the newer fibers of the cortex near the equator and is lower near the poles. There is no such activity in the lens nucleus, and the fibers in the nucleus are not capable of protein synthesis.