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phytochelatin Any of a group of sulphur-rich peptides found in plants exposed to high concentrations of heavy metals (e.g. cadmium, zinc, lead, copper, mercury) and thought to play a role in detoxification. They are unusual peptides, with the general formula (γ-glutamic acid-cysteine)n-glycine (compare glutathione), where n is 2–11, and bind the metal ions in the cell cytoplasm. They may serve to shuttle the toxic metal ions to the cell vacuole, where they are sequestered by organic acids.