Zinc is the second most abundant transition metal present in living organisms second only to iron. It is critical for the growth and survival of cells. In humans, zinc is primarily found in various organs and tissues such as the brain, intestines, pancreas and mammary glands. In prokaryotes, zinc can function as an antimicrobial, zinc oxide nano-particles can function as an antibacterial or antibiotic. Zinc homeostasis is highly controlled to allow for its benefits without risk of death via its high toxicity. Because of zinc's antibiotic nature, it is often used in many drugs against bacterial infections in humans. Inversely, due to the bacterial nature of mitochondria, zinc antibiotics are also lethal to mitochondria and results in cell death at high concentrations. Zinc is also used in a number of transcription factors, proteins and enzymes.