What makes certain plants poisonous to humans?

Plants produce toxic compounds primarily as a defense mechanism against insects, herbivores, and pathogens. These chemicals are called secondary metabolites because they aren't directly involved in the plant's growth or reproduction but serve protective purposes. Common toxic compounds include alkaloids like strychnine and nicotine, glycosides like those in foxglove, oxalates found in rhubarb leaves, and terpenes in certain conifers. These substances interfere with human biology in various ways depending on their chemistry. Alkaloids often disrupt nerve signaling or protein synthesis, cardiac glycosides interfere with heart muscle function, and oxalates cause tissue damage by binding calcium. The concentration and specific combination of these compounds determine how dangerous a plant is, which is why some plants are only mildly irritating while others can be lethal in small doses.