Why is cyanide poisonous? I know cyanide inhibits complex IV in the electron transport chain, but I don't understand why this makes it so toxic. If cyanide is bound to the complex, can't the electrons just bypass it and reduce oxygen directly? That way fewer protons would be pumped out of the matrix, but most still would, and aerobic respiration would just be less efficient, rather than stopping entirely. Essentially what I'm asking is what causes the electrons to get "stuck"?

Cyanide binds cytochrome oxidase so as to prevent the binding of oxygen. Electron transport is reduced to zero, and none of the oxygen breathed in can be used. Cytochrome oxidase is one of a superfamily of proteins which act as the terminal enzymes of respiratory chains, so if the cyanide is binding to the cytochrome then it inhibits the process and making it dangerous. Another example I would give rotenone which binds competitively.