During apoptosis, in contrast to necrosis, the cell and eventually the fragments of the cell all maintain their ion-gradients and energy levels. When tadpoles lose their tails, the tail seems to shrink day after day, that is because the cells are being recycled. Leaves fall from trees in the autumn, because apoptosis shuts the leaves down where they connect to the branch. These are programed cell deaths, and they are good for the organism to develope as it should. In the womb, our hands start off as paddles, and the skin separates into fingers, as the paddles are remodeled by apoptosis.
Some cells go away, and new ones are made somewhere else.
In necrosis, damage from toxic compounds or oxidation damage often harms the mitochondria or membrane ion pumps and causes the energy levels in the cells to drop. Lower glutathione levels make the mitochondria even sicker and less able to make ATP and on and on. When mitochondria are damaged in the cell electron transport does not work well, and more oxygen reacts at the wrong place in the chain, leading to formation of superoxide. However, lots of H2O2 can build up, and react with metal ions to create hyroxy radicals .OH that react and damage proteins and membranes. Catalase gets rid of H2O2 to make water, but it can become overwhelmed.