A group of compounds derived from 2-methyl-1,4-naphthoquinone that prevent bleeding in mammals and birds. Vitamin K₁ (phylloquinone) is produced by green plants; a related form, vitamin K₂ (menaquinone), is produced by intestinal bacteria. Chemically synthesized forms include vitamin K₁, K₂, and menadione (vitamin K₃). All of these compounds are fat-soluble liquids at room temperature that become biologically inactive when exposed to light or alkali.