Some heavy metals such as copper are essential for our biological functioning. Excessive amounts of these metals become, nonetheless, toxic, leading to cellular and tissue damage. There are also non-essential metals such as aluminum and barium which can be toxic even at very low doses. Toxic amounts of heavy metals have been shown to affect cell membranes, their organelles (e.g. mitochondria, nuclei), and enzymes with important roles in metabolism, detoxification, and damage repair. In addition, metal ions interact with cell components such as DNA and nuclear proteins. Damage to the DNA leads to changes in the cycle of the cells and can cause carcinogenesis (formation of cancer) and apoptosis (cell death).